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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References

Abstract:  The use of edible coatings on fresh and processed food products as a means of extending shelf life by preventing or delaying spoilage, providing a partial barrier to moisture, oxygen, and carbon dioxide, improving the mechanical handling properties, and even as carriers of many functional ingredients, is rapidly growing. Edible coatings can be applied by different methods such as panning, fluidized bed, dipping, and spraying. This review presents and discusses some aspects of the application of edible coatings on food products using spraying, which is the most commonly used technique for applying food coatings and to obtain uniform layers.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References

The quality of food products depends on organoleptic, nutritional, and microbiological properties, all of which are subjected to dynamic changes during processing, storage, and marketing. These changes are mainly due to interactions between the food and its surrounding environment which can lead to deteriorative modifications of food quality, including water and gas losses (Debeaufort and Voilley 2009).

The possibility of applying an edible protective layer on the food surface has become popular among available techniques to preserve food quality and is referred to as edible coating. Although applications of edible coatings to preserve food quality is not new, recently considerable interest and advanced research activity has been witnessed on edible coatings, which have been driven by an increasing consumer demand for safer, high quality, and minimally processed foods (Gennadios and others 1997; Lin and Zhao 2007). For instance, edible coatings on fruits and vegetables could provide replacement and/or fortification of natural layers to prevent moisture losses and to control exchange gases involved in respiration processes such as oxygen, carbon dioxide, and ethylene (Pavlath and Orts 2009). Moreover, edible coatings may also improve mechanical handling properties and structural integrity of food products (Mellenthin and others 1982) and can be used as a vehicle for incorporating several ingredients.

Food quality parameters such as: water loss, respiration rate, texture, color, purge, microbial number, pH, total acidity, and soluble solids content, are measured (from the food product) to determine the effectiveness and quality of coating during storage (Lu and others 2010; Vu and others 2011; Xiao and others 2011). Direct measurements on the coating itself are also very valuable, even if they may be more difficult to carry out. Indeed, coating functionalities have been reported on water and gas barriers, uniformity, morphology, thickness, surface wettability, adherence (compatibility between coating and food surface), and mechanical and optical properties (Lin and Zhao 2007; Ribeiro and others 2007; Werner and others 2007; Wang and others 2009a; Velāsquez and others 2011). After its application, the coating remains on the product during storage and will be disintegrated or dissolved during cooking or the mastication process (Longares and others 2004). For consumer acceptance, organoleptic qualities of the final product must be investigated in detail. Exogenous flavor impact by the coating materials, unattractive surface appearance of coatings and others factors (color, aroma, taste, and texture) may affect consumer acceptance of the coated products (Lin and Zhao 2007).

Edible coatings can be applied by different methods such as panning, fluidized bed, dipping, and spraying. All these techniques exhibit several advantages and disadvantages and their performance depends principally on the characteristics of the foods to be coated and the physical properties of the coating (viscosity, density, surface tension, and so on). Spray coating is the most commonly used technique for applying food coatings (Debeaufort and Voilley 2009). A spray system increases the surface area of the liquid through the formation of droplets and distributes them over the food surface area by means of a set of nozzles. This technique offers as its main advantages uniform coating, thickness control, and the possibility of multilayer applications, such as using alternating sodium alginate and calcium chloride solutions (Martín-Belloso and others 2009; Ustunol 2009). Moreover, spraying systems do not contaminate the coating solution, allow coating solution temperature control, and can facilitate automation of continuous production.

Spray systems are also very important and widely used in several engineering processes and industrial applications. Indeed, in agricultural operations they allow application of pesticides for crop protection. In various food industries they are used in operations of spray drying, cooling, washing, humidifying, packaging, and coating. In all these applications, the physical properties of the used liquids (viscosity, surface tension, density, and so on) can be different and therefore the fluid behavior (drop generation) can be specific to each liquid. Thus, it is necessary to design different atomizers and more general spraying systems to respond to specific spray applications (Moita and others 2009; Tratnig and others 2009). Furthermore, final product quality, operating costs, and maintenance downtime are also parameters that affect spray nozzle selection, use, and application. In food industries, an appropriate application of spray technology offers the potential for significant enhancement of end products.

The main objective of this review is to present and discuss some aspects of the application of edible coatings on food products using the spray method. First, the potential use of edible coatings as a way to extend food product shelf life is briefly described. Then, different technological processes to apply an edible coating (fluidized-bed coating, pan-coating, dip-coating, and spray-coating) are presented and discussed, focusing on the spray-coating technology to generate an edible coating on a food surface. Also, the physics of droplets, spray formation, and the numerical methods for model are precisely dealt with. Important physical parameters for the control and quality of coatings like wetting and droplet impact are also included. Components of the spray system are detailed; nozzles are described and discussed from the point of view of their ability to generate spray. Finally, perspectives for spray coating and other possible techniques that could be used for food coating are discussed.

Edible Coatings: Importance and Applications

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References

Edible coatings are thin layers of edible materials formed directly onto the surface of the food that can be eaten as part of the whole product. They have been used for centuries to prevent moisture migration (for example, coating of oranges and lemons with wax was practiced in China in the 12th and 13th centuries) or to create a shiny surface for esthetic purposes (Kester and Fennema 1986; Guilbert and others 1995; Janjarasskul and Krochta 2010).

Edible coatings are made from biological materials like polysaccharides (starch, starch derivatives, cellulose, pectin, alginate), proteins (gelatin, casein, wheat gluten, zein, soy protein), and lipids (beeswax, acetylated monoglycerides, fatty alcohols, fatty acids). The minor components usually include polyols acting as plasticizers (such as glycerol or polyethylene glycol) or acid/base compounds used to regulate pH, such as acetic or lactic acid (Bravin and others 2006; Falguera and others 2011). Polysaccharides and proteins are polymers that could form cohesive molecular networks by strong interactions between molecules (hydrogen bonds, Van der Waals interactions, crystallization, or primary valence). Molecular cohesions impart good barrier properties to gases (O2 and CO2), and good mechanical properties (Wu and others 2002; Vargas and others 2008). However, in the case of polar polymers, that is, high soluble polymers in water (alginate, lambda-carrageenan, and so on), the matrix cohesion is low and, therefore, water and gas permeabilities are increased. This phenomenon can be explained by hydrogen bond formation between polymer and water (Kester and Fennema 1986; McHugh and others 1993; Park and others 1993). Some commercial edible coatings are available to be applied on a wide range of foods: Tal Pro-long®, Semprefresh F®, Nu-Coat Flo C®, BrilloshineC®, Snow-White®, and White-Wash® (Pavlath and Orts 2009).

The application of edible coatings has the potential to improve food quality and to prolong the shelf life of food products since they can provide selective barriers to oxygen, carbon dioxide, and flavor compounds. Among other functionalities, edible coatings can act as carriers of several active ingredients such as natural or chemical antimicrobial agents, antioxidants, flavors, enzymes, functional ingredients (probiotics), or nutritional substances (minerals and vitamins). Therefore, edible coatings can enhance safety and nutritional and sensory attributes of foods (Ribeiro and others 2007; Falguera and others 2011; Avena-Bustillos and McHugh 2012; Zhao 2012). Edible coatings can used as an additional method to improve unit operation efficiencies. For example, in frying pretreatments the application of hydrocolloid coatings allows to reduce oil content in deep-fat fried products, such as chicken breasts (Maskat and others 2005; Dragich and Krochta 2009), potato chips (Tavera-Quiroz and others 2011), and wheat flour dough (Suárez and others 2008). In osmotic dehydration processes (for example in papaya, strawberries, apples, among others), edible coatings can prevent large solute uptake without noticeably affecting water loss, because the coating serves as an extra barrier to mass transfer (Khin and others 2005; Matuska and others 2006; Mitrakas and others 2008; García and others 2010).

With the objective to improve the quality (texture, flavor, and appearance among others), edible coatings have been applied to a large variety of foodstuffs such as dry bakery products (Bravin and others 2006), plums (Eum and others 2009), quinces (Yurdugul 2005), apples (Lee and others 2003; Rojas-Graü and others 2008; Mehyar and others 2011), blueberries (Duan and others 2011), fresh meat (Antoniewski and others 2007), tilapia fish fillets (Ou and others 2002), and fish patties (López-Caballero and others 2005).

Nowadays, a new generation of edible coatings is under development by using nanotechnology processes, for example, nanocomposite edible films to improve mechanical and barrier properties, and active antimicrobials; nanoencapsulation of active compounds for helping to control their release under specific conditions; and nanolaminates to create multilayered systems that could be used to coat highly hydrophilic food systems (Azeredo and others 2009; Rojas- Graü and others 2009; Tunç and Duman 2011; Bilbao-Sainz and others 2011).

Different Technological Processes to Apply an Edible Coating

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References

In the food industry, typical methods for forming a coating include panning, fluidized bed, dipping, and spraying. In this section each method is presented and analyzed, and advantages and disadvantages are discussed.

Fluidized-bed processing

Over the past several decades, fluidized-bed coating has been a focus of research for a variety of key applications in the chemical, pharmaceutical, and food industries. Originally developed by the pharmaceutical industry, it is used to apply a very thin film layer onto dry particles of very low density and/or small size. Food industries initially rejected this application to specific problems because of high costs. However, nowadays its application is extended to a wide variety of food products of high markup, such as functional ingredients and additives including processing aids (leavening agents and enzymes), preservatives (acids and salts), fortifiers (vitamins and minerals), flavors (natural and synthetic), and spices (Dewettinck and Huyghebaert 1999; Chen and others 2009).

Typically, in a fluidized-bed coating process a coating material, either in the form of a solution or suspension, is sprayed through a set of nozzles onto the surface of fluidized powders to form a shell-type structure (Figure 1). Fluidized beds are categorized by three different configurations: top spray, bottom spray, and rotating-fluidized bed; but the conventional top-spray method has a greater possibility of success in the food industry compared to the other methods (Dewettinck and Huyghebaert 1999). Fluidization occurs when a flow of fluid upwards through a bed of particles reaches sufficient velocity to support the particles without carrying them away in the fluid stream. The bed of particles then assumes the characteristics of a boiling liquid, hence the term fluidization. The size of particles coated in the fluidized bed is larger than 100 μm because powders with smaller size either do not have a stable fluidization state in the conventional fluidized bed or form excessive agglomerates (Dewettinck and Huyghebaert 1998; Guignon and others 2002; Chen and others 2009). Fluidized-bed coating is used in the food industry to produce a large variety of encapsulated food ingredients and additives, such as puffed wheat, nuts, and peanuts. In the case of peanuts coated with whey protein, fluidized-bed coating also presents high-drying efficiency and allows to use a lower level of surfactant addition in comparison with dipping and panning processes (Lin and Krochta 2006; Solís-Morales and others 2009).

image

Figure 1. Diagram of top-spray fluidized bed coating. (a) Nozzle; (b) Solution coating; (c) Inlet air flux, (d) Fluidized particles; (e) Plenum; (f) Distributor plate.

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Panning

The panning process itself has been known since the 9th century, coming from Greek–Arabian cultures where it was applied to the coating of medicines. The 1st processes were nothing but stirring and movements in normal pots. The panning process consists of depositing the product to be coated into a large, rotating bowl, referred to as the “pan” (Figure 2). The coating solution is then ladled or sprayed into the rotating pan, and the product is tumbled within the pan to evenly distribute the coating solution over the surface of the food material. Forced air, either ambient or at elevated temperature, is applied to dry the coating (Pandey and others 2006; Dangaran and others 2009). During the panning process heat is produced by friction which must be removed with cold air. Panning is a method used by the pharmaceutical, confectionery, and chocolate industries. Extruded products are especially suitable for panning as they can be produced in a round or oval form and in different sizes which are relatively easy to coat. Other products, particularly small items such as nuts and raisins, are also coated by panning (Lee and others 2002; Geschwindner and Drouven 2009; Talbot 2009).

image

Figure 2–. Representation of a typical pan coating process. (a) Nozzle; (b) Spray zone; (c) Exhaust air (Pandey and others 2006; reprinted with kind permission from Springer Science+Business Media).

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Dipping

The dip application method involves submerging the product into a vat containing the coating solution. This method is advantageous when products require a total coating; it allows obtaining good uniformity around a complex and rough surface. After dipping the product and draining away the excess coating, it is dried either at room temperature or with the aid of a dryer. Several problems may occur using the dipping method, including coating dilution, build-up of trash or dirt, and microorganism growth in the dipping tank. Coating applications from this method are usually thick, which may pose problems with product respiration and storage characteristics (Grant and Burns 1994; Martín-Belloso and others 2009). Another disadvantage of the dipping method is that the solution can dilute the outer layer of the food surface and degrade its functionality. For example, the natural wax layer of fruits and vegetables could be removed after dipping (Lin and Zhao 2007).

Spraying

Spray coating is used to apply a more uniform thick or thin layer of coating over a food surface. Different from other systems, spray coating can work with large surface areas. Moreover, the bottom-product surface can also be coated in a separate operation after application of the initial coating and drying processes on the top side. Spraying makes it possible to deposit various kinds of aqueous solutions or suspensions, such as liquefied lipids or chocolate (Debeaufort and Voilley 2009). Spray applications are also suitable when dual or more successive applications are required, for example, to make a gel layer with alginate and calcium chloride solutions (Cutter 2006; Dangaran and others 2009).

Spray applications have been used in many food processes. A bovine gelatin was applied successfully to coat beef tenderloins, pork loins, salmon fillets, and chicken breasts. In all these applications the shelf life was increased and the color was preserved (Antoniewski and others 2007). In crackers, the spray-coating application with a polysaccharide-lipid-based solution confirmed the potential of edible packaging to become an integral part of the food and reduced the hydration kinetics in a high-moisture activity environment. In the spray technique, the atomization pressure is a critical parameter. In fact, in the case of the application on crackers, at the fixed height between the pneumatic nozzle and product of 40 cm, it is important to maintain the pressure value below 3.5 bars to avoid the destruction of the film-forming system. Also, film thickness of 30 μm was the most appropriate to ensure low water vapor permeability and desirable mechanical properties, so control of this parameter is crucial (Bravin and others 2006). It has been reported that the spray-coating process can form a thin film on food surfaces and can be reasonably well controlled (Grant and Burns 1994), despite the fact that the viscosity of the coating solutions must be carefully adjusted to suit the application with a specific spraying gum. Edible coatings based on a tapioca starch with green tea extracts have been shown to reduce the growth of aerobic microorganisms and yeasts when applied to fruit-based salads, romaine hearts, and pork slices (Chiu and Lai 2010). Chitosan solution applied as a preharvest spray or postharvest coating reduced decay in table grapes and affected the content of total phenolic compounds and the activities of antioxidative enzymes of the product (Meng and others 2008).

Advantage and disadvantages of the spray system against other processes

Although commercial coating application methods are very diverse, their selection depends upon the desired end product, desired thickness of the coating, solution rheology, and the drying technology in use. Both pan coating and fluidized-bed coating systems require intense tumbling, while dipping application may dilute the coating solution and result in significant residual coating material. Also, appropriate amounts of coating solution cannot be controlled easily by dipping, and a step to dry off surplus solution is needed in the dipping procedure, which requires more time and may hinder its industrial application. Hence, spray coating is a feasible technique for coating application systems, providing a uniform distribution of coating solution on the surface of food and facilitating factory-scale implementation.

For starch-methylcellulose film, spraying at 2 bar gave a significantly lower water vapor permeability value than spreading. Besides, tensile strength was significantly higher for sprayed (3.5 bar) samples than spread samples. However, deposition by spraying requires droplet–droplet contact and aggregation, which is a critical step for homogeneous film structure (Bravin and others 2006).

Among the advantages of spray coating, one is to form coatings combining hydrophobic and hydrophilic substances. Indeed, spray can generate a coating with 2 solutions, by applying an emulsion solution directly, formed before atomization (Figure 3a), or by forming a bilayer after 2 spray pulverizations (Figure 3b). The application of a bilayer has the disadvantage of requiring 4 steps (2 spray applications and 2 drying processes) for which the industry prefers the use of emulsified formulations (Martin-Polo and others 1992; Bosquez-Molina and others 2003).

image

Figure 3–. Schematic representation of coating formed by emulsion (a) and bilayer (b).

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Fundamental Principles of Atomization Spray Processes

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References

Gas-liquid 2-phase flows broadly occur in nature and the environment, such as the falling of raindrops and various spray processes. Droplets can be generated by vapor condensation and deposition or atomization. However, atomization is the most widely applied process for droplet generation (Liu 2000; Hede and others 2008). Atomization refers to the disintegration of a bulk liquid material, via an atomizer, into a collection of drops (a spray) in a surrounding gas or vacuum with or without a spray chamber.

Drop and spray formation

Roughly, the liquid-spray atomization process can be divided into 2 parts: 1st is jet-intact length, 2nd the liquid column breakup after the end of intact length. The process of atomization begins by forcing a liquid through a nozzle. The potential energy of the liquid (measured as liquid pressure for hydraulic nozzles or liquid and air pressure for 2-fluid nozzles) along with nozzle geometry causes the liquid to emerge as small ligaments (after the end of intact length). These ligaments then break up further into very small “entities”, which are usually called drops, droplets, or liquid particles (Figure 4). Numerous devices to generate spray flows have been developed and they are generally designated as atomizers, can be a nozzle that ejects the liquid and atomizing medium (gas or liquid), a centrifugal device, or an ultrasonic vibrator (Liu 2000; Schick 2008).

image

Figure 4–. Schematic of atomization process (reprinted with kind permission from Spraying Systems Co., Wheaton, IL, USA).

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The dominant forces involved in the atomization process are: a) inertial force attributed to undulations/perturbations, b) aerodynamic force attributed to drag/shearing effect, c) viscous forces attributed to dissipation of kinetic energy, and d) surface-tension forces which contribute to minimize surface energy. The 1st 2 forces (a and b) are disruptive in nature and the 3rd and 4th (c and d) are cohesive in nature (Rahman and others 2009). The competition between the cohesive and disruptive forces will set up on the liquid surface, leading to oscillations and perturbations in the liquid (Rayleigh–Plateau instability). In spray systems, the inertial forces are sufficient high to overcome the viscous forces. After the surface forces, inertial forces are higher than the viscous forces to form droplets. Under favorable conditions (such as low-viscous forces and high surface tension), the oscillations may be amplified to such an extent that the bulk liquid disintegrates into droplets. This initial breakup process is referred to as primary atomization. A population of larger droplets produced in the primary atomization may be unstable if they exceed a critical droplet size and thus may undergo further disruption into smaller droplets. This process is usually termed secondary atomization. Therefore, the final droplet size distribution produced in an atomization process is determined by the liquid properties in both the primary and secondary disintegration (Liu 2000; Gorokhovski and Herrmann 2008).

In certain applications, not only the mean-droplet diameter should have a suitable value, but also the droplet-size distribution must have a particular form for optimal operation (Babinsky and Sojka 2002). In edible-coating sprays, efficiency is known to depend on the width of the droplet-size distribution and small droplets are best for achieving a homogeneous coating.

Physical properties and adimensional numbers

In general, fluid motion is described by the Navier–Stokes equation derived from the basic principles of conservation of mass (Eq. 1) and momentum (Eq. 2).

  • image(1)
  • image(2)

where ρ is the density, ui is the fluid velocity, t is the time, and xj is the 3 components of space, p is the pressure in the flow, τij is the viscous stress tensor, and Fcap is the capillary forces.

For an assumed Newtonian flow, τij is written as follow (Eq. 3):

  • image(3)

where μ is the coefficient of dynamic viscosity of the fluid, λ is null only for mono-atomic gases, and δij is the Kronecker delta defined by Eq. 4.

  • image(4)

In addition to this equation, an equation of state and an equation for the conservation of energy can be added when necessary. Moreover, these equations can be adimensionalized after identification of the physical relevant variables. The atomization process (for steady nozzle, centrifugal device) characterized by the average droplet diameter, D, is governed by the following parameters:

  • L: characteristic dimension of the atomizer, for example, diameter of nozzle orifice

  • U: initial relative velocity of injected liquid and ambient gas

  • σ: surface tension

  • ρl, ρg: liquid and gas densities, respectively

  • μl, μg: dynamic viscosities of liquid and gas, respectively

After applying the Buckingham Π-theorem, the following relationship is obtained (Eq. 5):

  • image(5)

With 4 dimensionless numbers obtained, the 1st represents the Reynolds number (Eq. 6), which is the ratio of inertial force to viscous force:

  • image(6)

The 2nd dimensionless number represents the Webber number (Eq. 7), which is the ratio of inertial force to surface tension force:

  • image(7)

The 3rd and 4th dimensionless numbers are viscosity and density ratios for gas and liquid, respectively. The Reynolds and Weber numbers can be described for the liquid phase, using liquid properties and liquid velocity, or for the gas phase, using the gas properties and gas velocity.

Combining the Reynolds and Weber numbers to eliminate the velocity allows to obtain another important dimensionless group, the Ohnesorge number (Oh), denoting the relative importance of interfacial viscous stress and surface tension (Eq. 8). Low Ohnesorge numbers represent either a low-viscous or a high surface tension fluid.

  • image(8)

Typically values of the Reynolds and Weber numbers are presented in Table 1, drop solution coatings based on hydroxypropyl methylcellulose (HPMC) with different surfactants. For the same velocity (U = 1m/s) and drop diameter (50 μm), Re, We, and Oh values changed with the properties of liquid.

Table 1–. Physical properties of coating solutions based on HPMC with different surfactants and Re, We, Oh values (U = 1 m.s−1, D0= 50 μm).
LiquidDensity, kg/m3Surface tension, mN/mViscosity, mPa.sReWeOh
  1. Density, surface tension, and viscosity (Pastor 2010)

HPMC (1.5%w/v)- Sorbester 20 (3%w/v)1003.7324.95 1050.19201.150.28
HPMC (3%w/v)- Sorbester 80 (3%w/v)1004.0735 4012.55143.440.95
HPMC (4.5%w/v)- Sugin 471/PHK-40 (3%w/v)1009.1346.11503.36109.453.11
Water100072.31.050069.160.017

The Reynolds and Ohnesorge numbers can be used to characterize mechanisms of jet breakup, which are typically classified into 4 primary regimes according to the relative importance of inertial force, surface tension, and viscosity as presented in Figure 5 (Lin and Reitz 1998; Liu 2000). These have been named the Rayleigh regime (drop diameters are larger than jet diameter), the 1st wind-induced regime (drops have diameters of the order of jet diameter), the 2nd wind-induced regime (drop diameters are slightly smaller than jet diameter), and the atomization regime (drop diameters are smaller than jet diameter).

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Figure 5–. Break-up regimes of a liquid jet in a quiescent gas (a) Rayleigh breakup; (b) First wind-induced regime; (c) Second wind-induced regime; (d) Atomization regime.

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Other classifications of the regime breakup have been reported: breakup regime for shock-wave disturbances, relating We and Oh (Hsiang and Faeth 1995), and break-up regime of spiralling liquid jets (Wong and others 2004).

Modelling droplet-size distributions

Several methods for modelling droplet size distributions of sprays have been presented since 1930s. The older method is an empirical method based on experimental data in which a curve is used to fit the data collected for a wide range of atomizers and operating conditions (Babinsky and Sojka 2002). As an alternative to this empirical approach, several analytical approaches have been developed: for example, the maximum entropy (ME) and discrete probability function (DPF) methods.

The ME method views spray formation as a completely nondeterministic process that can be modeled using the principle of entropy maximization under several global constraints. The most likely droplet-size distribution is the one that maximizes the entropy function (Li and others 1991; Babinsky and Sojka 2002; Yongyingsakthavorn and others 2007). The DPF method divides the spray-formation process into deterministic and nondeterministic portions. Given a set of initial conditions (fluid physical properties and atomizer parameters) and a model of the break-up mechanism, it is postulated that the resulting droplet size is uniquely determined. A droplet size distribution is produced because the initial conditions fluctuate in a nondeterministic manner due to a variety of factors. The droplet-size distribution is computed by coupling a deterministic model that describes the formation of a single droplet to the DPF method (Liu and others 2006). Table 2 summarizes the advantages and disadvantages of empirical and analytical methods for modelling droplet size distributions of sprays.

Table 2–.  Advantages and disadvantages of empirical and analytical methods for modelling droplet-size distributions.
MethodProcedureAdvantagesDisadvantages
EmpiricalA curve is fit to data collected for a wide range of atomizer nozzles and operating conditionsIt can be used to model virtually any data setDifficulty of extrapolating the data to operating regimes outside the experimental range
Maximum entropy (analytical)First, a set of appropriate physical constraints is formulated. Then the distribution that maximizes the entropy of the system subject to the constraints is foundThe method is useful for processes dominated by secondary atomization where the break-up physics are highly stochastic in natureDetails of the break-up mechanism are ignored
Discrete probability function (analytical)A set of initial conditions and a gross fluid structure break-up model predicts the production of a unique drop of some diameter, and a drop size distribution is produced because the initial conditions fluctuateThe method can be adapted to a variety of atomizer configurations and operating conditions simply by selecting an appropriate instability model for break-upThe method is limited to primary atomization and requires a ligament formation sub-model

Modelling and simulation of spray

Computational fluid dynamics (CFD) represents a useful tool to obtain spray-flow characteristics. Indeed, CFD can be effectively utilized to understand flow physics, to interpret available experimental data, and to guide experimental work, as well as to execute pre-calculations for altered operating conditions. Moreover, CFD is cheaper than experimental measurements; therefore, various spray conditions may be obtained by changing liquid properties (viscosity, density, and surface tension), operating conditions (air pressure, liquid-flow rate, and so on), and physical condition (nozzle design and spray angle, among others) can be tested easily (Norton and Sung 2006; Gorokhovski and Herrmann 2008; Kuriakose and Anandharamakrishnan 2010; Sirignano 2010). The flow of the dispersive and continuous phases can be described by Navier–Stokes transport equations in 2D or 3D. Due to broad ranges of time and length scales present in atomization process, approximations in numerical methods (and therefore in the results) are inevitable in CFD.

There are 2 different ways in which the 2-phase spray flows are commonly represented in CFD: the Lagrangian model, where the paths taken by droplets or clusters of droplets are tracked through the domain and Eulerian model, where the spray is considered as a continuum across the whole flow domain (Langrish and Fletcher 2001; Jiang and others 2010).

In the Lagrangian model, the liquid is not considered a steady stream leaving the nozzle, but it is already in the form of droplets which then split on contact with air. These approaches, in which the liquid phase is always represented by a set of droplets, are particularly appropriate to describe the secondary breakup. However, they are not satisfactory in dense media where their extension to the case of primary breakup is a problem. Various existing Lagrangian models of atomization have been reported. Some are detailed briefly below:

  • • 
    Taylor analogy breakup (TAB) model. The TAB model has been proposed by O’Rourke and Amsden (1987). It treats the oscillation of distorting droplets with a spring-mass system analogy. The restoring force of the spring is represented by the surface tension, while external force is replaced by aerodynamic force. The TAB model works better in the particular case of breakup (called bag breakup) when We was low (usually for 12 < We < 40 to 100); extremely high-Weber-number sprays result in shattering of droplets, which is not described well by the spring-mass analogy (Liu and Reitz 1993; Zeoli and Gu 2006; Jiang and others 2010). Although this model focuses on splitting secondary breakup, it has often been used for the primary breakup of the jets. An alternative to the TAB model that is appropriate for high-Weber-number flows is the wave breakup model.
  • • 
    Wave model. The ‘‘wave’’ breakup model for atomization was developed by Reitz (1987), this model proposed that the atomization is caused by the unstable growth of Kelvin–Helmholtz waves at a liquid-gas interface, which occur when there is a shear motion of 2 fluids flowing alongside each other. Thus, this model is also called model KH. The liquid column is discretized numerically by liquid particles, called “blobs”, of diameter equal to that of the injection nozzle. In this approach, the breakup and the droplet size that results are related to the most amplified wave provided by the linear theory of instabilities. Indeed, the wave length and growth rate of this instability are used to predict details of the newly-formed droplets. The wave model is appropriate for high-speed injections (We > 100), where the Kelvin–Helmholtz instability is believed to dominate droplet breakup (Zeoli and Gu 2006; Jiang and others 2010).

At high velocity, drops produced by spray can be subdivided into droplets with a much smaller diameter. Therefore, a variety of secondary breakup models (after the initial spray production) have been proposed in the literature: Reitz–Diwakar model (Reitz and Diwakar 1986), Kelvin–Helmholtz-Rayleigh–Taylor (KH-RT) instability model developed by Patterson and Reitz (1998), and FIPA (Fractionnement Induit Par Acceleration) model (Habchi and others 1997).

In Eulerian-spray modelling approaches, the governing equations (1 and 2) have to be simplified and models need to be introduced which account for the effect of physical processes, such as turbulence. According to the way turbulence is modelled, 2 groups of field models can be distinguished, namely Reynolds-averaged Navier–Stokes simulations (RANS) and Large-Eddy simulations (LES). In the RANS approach, all turbulent fluctuations are removed from the governing equations by ensemble averaging and the only mean flow field is calculated, whereas the basic idea of the LSE approach is that the largest turbulent motions are resolved in time and space (van Maele and Merci 2008). The LES technique is beginning to emerge as a viable RANS alternative for industrial flows since unsteady flow dynamics may not be fully captured (lack of accuracy of the average model). LES may overcome this problem by using spatial filtering instead of time- or ensemble-averaging. In LES, an explicit account is taken of flow structures larger than the filter width, while the influence of unresolved scales is modelled using a subgrid-scale model (Jiang and others 2010; Martinez and others 2010). The most accurate and straightforward numerical approach to fluid-flow problems in the continuum limit is to solve the Navier–Stokes equations without averaging or approximation other than numerical discretizations whose errors can be estimated and controlled. This approach is the so-called direct numerical simulation (DNS), which is a simulation in computational fluid dynamics in which the Navier–Stokes equations are numerically solved without any turbulence model (Versteeg and Malalasekera 2007). The major disadvantage of DNS is that it is computationally too expensive, even for solving very simple flow configurations.

Spray Technology

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References

In Figure 6 a sketch of a basic industrial spray system is shown. It consists of a set of nozzles used in the formation of the droplets, a sprayer tank to facilitate liquid pressure, and heating jacket nozzle for temperature control of the liquid before injection. In the food industry, it is important to move food products with conveyor belts. Moreover, a data logger can be used to control physical parameters such as pressure, temperature (ambient air, liquids), flow rates, and liquid level for the sprayer tank. The air system participates in both the production of droplets in the nozzle and the circulation of the liquid flow.

image

Figure 6–. A sketch of the basic installation for spray coating in food engineering. (a) Air shut-off valve; (b) Air filter; (c) Air regulator and gauge; (d) Nozzles; (e) Food; (f) Conveyor belt; (g) Liquid regulator and gauge; (h) Liquid strainer; (i) Liquid shut-off valve; (j) Sprayer tank (nozzles and valves; reprinted with kind permission from Spraying Systems Co., Wheaton, IL, USA).

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Spraying nozzles

As seen previously, spraying nozzles play a critical role during the coating process. They are precision-machined components designed to yield very specific performance under very specific conditions. In designing spray systems there are often many options available to select an adequate nozzle; therefore, the selection process can become somewhat complicated. If dual or multiple nozzles are used in a conveyorized application, the overlapping liquid-distribution pattern of the nozzles needs to be considered because the process may depend strongly on the spray relative to local volume flux (Lefebvre 1989; Hagers 1997). Relevant characteristics which identify the performance of a nozzle are: liquid flow delivered as a function of nozzle-feed pressure, opening angle of the produced spray, nozzle efficiency, defined as the ratio between the energy of the spray and the energy employed by the nozzle, flow-distribution evenness over the target, and spray-droplet size distribution (Lefebvre 1989; PNR 2009). Table 3 shows the influence of operating pressure, temperature, and properties of the liquid (specific gravity, viscosity, and surface tension) on the spray characteristics.

Table 3–.  Influence of different parameters on spray characteristics.
Spray characteristicsIncrease in operating pressureIncrease in specific gravityIncrease in viscosityIncrease in fluid temperatureIncrease in surface tension
  1. Spraying Systems Co. (2000).

Pattern qualityImprovesNegligibleDeterioratesImprovesNegligible
Droplet sizeDecreasesNegligibleIncreasesDecreasesIncreases
Spray angleIncreases then decreasesNegligibleDecreasesIncreasesDecreases
CapacityIncreasesDecreasesFull/hollow cone – increases Flat – decreasesDepends on fluid sprayed and nozzle usedNo effect
ImpactIncreasesNegligibleDecreasesIncreasesNegligible
VelocityIncreasesDecreasesDecreasesIncreasesNegligible
WearIncreasesNegligibleDecreasesDepends on fluid sprayed and nozzle usedNo effect

Classification of spray nozzles

Spray nozzles can be classified into several different categories depending upon their operation method. Hydraulic and pneumatic atomizing nozzles are common application choices. Moreover, hydraulic nozzles are usually further classified by pattern type: hollow cone, full cone, flat spray, and solid stream.

  • • 
    Hollow cone. A hollow cone spray pattern (Figure 7a) consists of droplets concentrated onto the outer surface of a conical-shape volume, with no droplets contained in the inside of the conical-jet shape; it can be formed by tangential nozzle (the centrifugal force inside the whirl chamber provides the energy for liquid breakup) or deflection nozzle (the liquid jet passes to bounce off a deflecting surface). The so-called oil burner nozzles forms this pattern, and have been used in fruit coating-wax, but up to four nozzles must be installed to ensure complete coverage (Hall 2012).
  • • 
    Full cone. In a full-cone spray (Figure 7b) droplets are distributed into a volume which is limited by a cone, with its origin point at the nozzle orifice. Such a spray pattern is commonly encountered in a large variety of food processes (chocolate candies, meat, and so on), since it allows distributing the liquid flow in an even way onto a surface; it is obtained by different techniques: turbulence nozzle, deflection nozzle, and turbulence nozzle-air atomizer.
  • • 
    Flat spray. In a flat jet spray (Figure 7c) liquid droplets are sprayed in the shape of a flat-liquid layer, with different thicknesses according to the principle used to generate the spray. The vast majority of flat-spray nozzles employed in the industry work according to one of the following principles: line-flat jet, line-straight jet, or spoon-flat jet.
  • • 
    Solid stream. A solid stream spray pattern (Figure 7d) is a uniform stream of liquid. By using proper inlet chamber proportions and contours ahead of the orifice and/or by addition of internal flow-stabilizing vanes, these nozzles provide prolonged solid-stream integrity and delay breakup and droplet formation after leaving the nozzle orifice (Hagers 1997; Schick 2008; PNR 2009).
image

Figure 7–. Diagram of spray patterns with corresponding nozzles. (a) Hollow-cone nozzles; (b) Full-cone nozzles; (c) Flat-spray nozzles; (d) Solid stream (reprinted with kind permission from PNR UK/Group (a) and Spraying Systems Co., Wheaton, IL USA (b, c, d).

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Uniformity and symmetry of the spray pattern produced by atomizers are crucial parameters in most practical applications. The choice of the spray pattern can be carried out after consideration of the following recommendations: hollow cone nozzles tend to provide the smallest drop-size distributions obtainable among hydraulic spray styles (Schick 2008). If the surface is stationary, the choice of spray pattern is usually a full cone since this pattern will cover a larger area than other styles in general. In conveyorized applications, where either the target or the spray nozzle is moving relative to the other, any spray pattern may be employed depending upon other desired characteristics, for example, if higher impact is desired, then a flat-spray pattern would usually provide an advantage, but if droplet size uniformity at the target is needed, then a hollow-cone spray might be selected (Hagers 1997).

Pneumatic atomizers utilize compressed gas which is mixed with a liquid. These nozzles have been used to coat bakery products (Bravin and others 2006) and citrus (Hall 2012). Mixing may be internal or external to the nozzle itself. The 2 methods are presented below.

  • • 
    Internal mixture nozzles. In internal mix designs (Figure 8a), the gas and liquid are introduced into a mixing chamber inside the nozzle and later discharged through an exit orifice which is designed to provide a typically flat or round spray pattern (Hagers 1997; Hede and others 2008).
  • • 
    External mix nozzles. External mix pneumatic nozzles (Figure 8b) function by impacting a stream of liquid with a series of adequately placed air jets that break up the liquid and form a spray pattern usually conical or flat. External mix droplet sizes and ranges are typically larger than internal mix styles, and they are most often used in situations where the material being sprayed is very thick or would otherwise clog the inside of an internal mix-type nozzle (Hagers 1997; Hede and others 2008).
image

Figure 8–. Diagram of two-fluid nozzle designs. (a) Internal mixture nozzles; (b) External mixture nozzles (reprinted with kind permission from PNR UK/Group).

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Finally, other important aspects should be considered in the selection of the type of nozzle:

  • • 
    Control systems for air-atomizing systems are more complex than those for hydraulic systems. Operational costs are higher as well.
  • • 
    Compressed gas consumes energy at high rates and generally has high capital costs associated with implementation.
  • • 
    Maintenance and implementation costs for pneumatic systems are typically higher than those for hydraulic nozzle systems.

Droplet diameter measurements

Many useful measurement techniques for determining droplet size have been developed. Each of these methods has its advantages and limitations, and none of the methods is fully satisfactory. The measurement techniques for droplet sizing may be grouped conveniently into 4 primary categories: mechanical, electrical, optical, and acoustic methods (Liu 2000). However, to characterize sprays, optical methods are the most commonly used; they can be divided into 2 main categories: imaging (like photography, holography, and others) and nonimaging. Nonimaging method can also be subdivided into 2 classes, those that measure a large number of droplets simultaneously (ensemble) and those that count and size individual droplets one at a time (Choo and Kang 2004; Schick 2008). These different methods are presented below:

  • • 
    Optical imaging analyzers. These consist of a light source (typically a strobe light or laser) that is employed to illuminate the spray. This latter is recorded using a video camera. Then the image is scanned and the droplets are sized and separated into different classes. Main sources of error include blurring, depth of field variations, and improper sample size (Schick 2000).
  • • 
    Laser diffraction method (LDMs). The principle of a LDM is that particles of a given size diffract light through a given angle. The angle of diffraction is inversely proportional to particle size, and the intensity of the diffracted beam at any angle is a measure of the number of particles with a specific cross-sectional area in the optical path. For calculating particle sizes from light intensity sensed by detectors, 2 diffraction theories are commonly used: the Fraunhofer diffraction and the Mie theory (Kelly and others 2006; Di Stefano and others 2010). Both theories assume that the particles have spherical shape; in other words, the particle dimension is the optical spherical diameter. Fraunhofer theory is based on the approximation that the laser beam is parallel and the detector is at a very large distance compared with the size of the diffracting particle. The Mie theory is a solution of Maxwell equations describing propagation of light electromagnetic wave in space. This theory provides a solution for the case of plane wave on a homogeneous sphere of any size (Eshel and others 2004). The Mie theory requires that refractive indexes of suspending medium and particles must be known. It also requires that the imaginary component of the complex refractive index (relative measurement of the absorbency of the particle with respect to the irradiating light source) be known for the particle (Di Stefano and others 2010).
  • • 
    Phase-Doppler anemometry (PDA). PDA is an interferometric local technique for measuring velocity and size of individual droplets passing through the measurement volume. Measurement location in the flow field is defined by the probe volume, an intersection region of pairs of laser beams. The PDA system deduces the 2 measured quantities of particles penetrating the probe volume from the intensity of light scattered by the particles, which varies with time, and from a phase shift between pairs of scattered light signals, respectively. One important requirement to spherical liquid droplets when measuring their size with PDA is that they must exhibit homogeneous optical properties (Tratnig and others 2009; Liu and others 2010).

Distribution functions for drop-size distribution

The collected and recorded data by the drop-size analyzers are typically in the form of number count per class size. The data are arranged into a mathematical representation referred to as the droplet-size distribution function. The most commonly used distribution functions to fit the existing experimental data are the Rosin–Rammler (Eq. 9) and Nukiyama–Tanasawa (Eq. 10) distributions.

  • image(9)
  • image(10)

where D is the droplet diameter, inline image represents the distribution mean droplet diameter; m is a measure of particle-size distribution spread; b, p, and q are adjustable parameters, and a is a normalizing constant.

However, there are many other modified formulations such as the upper-limit, log-normal, and chi-squared functions (Semião and others 1996; Schick 2008).

From the distribution functions, more global parameters such as average radii can also be defined. Most of the time, an average as equation (11) is employed.

  • image(11)

Among the most commonly used diameters, the following are found:

  • • 
    Sauter Mean Diameter (SMD), also expressed as D32 (Eq. 12). The SMD is droplet diameter with the same volume-to-surface area ratio as the total droplets volume to the total droplets surface area. It is a means of expressing spray fineness in terms of surface area produced by the spray (Schick 2008).
  • image(12)
  • • 
    Volume median diameter (VMD) expressed as Dv0.5 (Eq. 13). The VMD droplet size, when measured in terms of volume, is a value where 50% of the total volume of liquid sprayed is made up of droplets with diameters larger than the median value and 50% with smaller diameters (Allen 1990; Schick 2008). The ratio VMD / SMD is a good indicator of the dispersion drop size.
  • image(13)

Important Parameters in the Control of Spray Coatings

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References

Knowledge of parameters, which allows controlling the coating process, is important for obtaining high-quality products and minimizing possible costs. Thickness and microstructure are 2 important parameters which should be taken into account for quality, effectiveness, and functionality of coatings since they directly affect their physicochemical and barrier properties (Cisneros- Zevallos and Krochta 2003; Lin and Zhao 2007; Vargas and others 2008).

The effectiveness of edible films and coatings for the protection of food depends primarily on controlling the spreading of the coating solution, which affects the thickness of the film. The thickness of edible films and coatings is an important parameter since it directly affects the biological properties and the shelf life of the coated food. When a drop of a liquid A is placed on a solid B (for example a fruit peel) the liquid can wet it totally or partially. The parameter that distinguishes these 2 states is the spreading coefficient, which can be calculated by equation 14:

  • image(14)

where SA/B is spreading coefficient, Esolid is the surface energy of the solid (food), θ is the Young contact angle, σSV, σSL, and σLV are the surface tension values of the solid-vapor, solid-liquid and liquid-vapor interfaces, respectively.

If SA/B > 0, the liquid drop spreads completely in order to lower its energy, so that the contact angle θ= 0 (Figure 9a). If SA/B < 0, the liquid drop does not spread but forms, at equilibrium, a spherical cap resting on the solid with a contact angle θ. For θ < 90° the liquid is said to be “mostly wetting” and for θ≥ 90°“mostly nonwetting” (Figure 9b). If θ= 180°, the liquid is perfectly nonwetting (Figure 9c). Practically, a large contact angle represents a hydrophobic surface, whereas a small contact angle implies a hydrophilic surface. The quantitative definitions for the relative terms “hydrophobic” and “hydrophilic” surfaces has been done, respectively, for surfaces exhibiting a water contact angle θ >65° and θ <65° (Vogler 1998; Ghanbarzadeh and others 2007).

image

Figure 9–. Liquid drop on a smooth solid substrate. (a) Total wetting, hydrophilic solid (θ= 0); (b) Partial wetting 0 < θ < 180°; (c) No wetting, hydrophobic solid (θ= 180°).

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The pioneering correlation between contact angle and interfacial tension is the Young equation (Eq. 15). It was developed for the case of an ideal solid surface, which is defined as smooth, rigid, chemically homogeneous, insoluble, and nonreactive (Karbowiak and others 2006; Marmur 2006). However, real solid surfaces always deviate from the ideal case and are often heterogeneous, deformable, and rough.

  • image(15)

It is difficult to estimate SA/B. Indeed, although σLV can be measured accurately, there are no direct methods for measuring σSV and σSL.

For low-energy solids, such as crystalline organic materials and most polymers, σSV can usually be estimated from the contact angle measurement of various liquid drops (pure liquids) deposited onto the surface. However, surface free energy determinations of solids using contact angle measurements, even using pure liquids, are not always straightforward and various problems have been reported, such as drop evaporation, porosity of the solid, chemical interactions between the liquid and the solid, metastable state of the shape of the examined drop, and more (Erbil 2006; van Oss 2006)

The main methods used for the determination of surface free energy are: the Zisman model (Fox and Zisman 1950), the Fowkes approach (Fowkes 1964), the multicomponent or “acid-base” theory (Fowkes 1987; van Oss and others 1988), and the “equation-of-state” approach (Neumann and others 1974). These methods have been used in various food products, such as: grapefruit (Hagenmaier and Baker 1993), apple-fuji (Choi and others 2002), strawberry (Ribeiro and others 2007), banana (Velásquez and others 2011), blueberry (Skurtys and others 2011), and quince (Ramírez and others 2012).

Finally, the wetting of a solid by a liquid is determined by the balance between the adhesion energy (Wa) of the liquid on the solid and the cohesive energy (Wc) of the liquid and can be calculated by Eq. 16. Adhesive energy causes the liquid to spread over the solid surface while cohesive energy causes it to shrink.

  • image(16)

Wa and Wc are expressed by Eqs. (17) and (18), respectively.

  • image(17)
  • image(18)

Thus, when the spraying system is employed to coat a food product, thickness and microstructure of the coating depend on the spreading coefficient, but also on mean droplet diameter and droplet impact.

Control of mean droplet diameter

Control of final droplet size depends on many factors such as spray-nozzle design, air and liquid properties (viscosity, density, and so on) and relation between Re and Oh. For specific applications, it is possible to take into account temperature and incoming air humidity.

  • • 
    Spray-nozzle design. Generally, full-cone nozzles produce the largest droplet size, followed by flat-spray and hollow-cone nozzles. The liquid flow rate and spray angle have a direct influence on droplet size (Hagers 1997; Schick 2008).
  • • 
    Air Properties. Of all the factors influencing the mean droplet size, air velocity is undoubtedly the most important. For low-viscosity liquids, the Sauter mean diameter (D32) is inversely proportional to the air velocity, which underlines the importance of arranging for a liquid to be exposed to the highest possible air velocity. However, in all spray installations, the air velocity is limited by the available pressure.
  • • 
    Liquid properties. Viscosity, density, and surface tension are the most important physical properties to generate controlled droplets. An increase in viscosity or in surface tension will typically increase D32, due to an increase in the amount of energy required to atomize the liquid. Furthermore, an increase in liquid viscosity causes a decrease of liquid-flow rate and, therefore, higher minimum pressure is necessary to maintain an adequate spray angle. On the other hand, if the surface tension increases, then a minimum operating pressure and decreasing spray angle is obtained (Rizkalla and Lefebvre 1975; Schick 2008). When nonNewtonian liquids are used (such as water-in-oil emulsions, with the aqueous phase between 10% and 50%), it has been shown that an increase of the aqueous phase concentration leads to a larger D32 (Sheng and others 2006). Liquid density also affects droplet diameter in a complex manner. In some cases the influence on D32 is fairly small, because an increase in liquid density produces a more compact spray that is less exposed to high-velocity air for the atomizing action. However, it can also improve atomization by reducing sheet thickness and by increasing the relative velocity for plain jet nozzles (Rizkalla and Lefebvre 1975).

Control of droplet impact

The fluid dynamic phenomenon which occurs when a single droplet impinges onto a solid surface depends on many parameters associated with interface and impact conditions (Liu 2000). A liquid droplet impinging on solid substrates (food) is a complex process that is associated with fluid dynamics, physics, and interfacial chemistry. Observations and interpretations of contact, wetting, rebounding, leveling, and especially the dynamic behaviors could help in controlling liquid-droplet impingement in a desired manner (Chan and Venkatraman 2007; Wang and others 2009b). Impingement has been reported to be related to impact velocity, its direction relative to the surface, droplet size, properties of the liquid (density, viscosity, viscoelasticity, and some other nonNewtonian effects), interfacial tension, roughness, and wettability of the solid surface (surface-free energy), and air entrapment (Mao and others 1997; Wang and Chen 2000; Yarin 2006; Andrade and others 2012).

Future Trends

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References

The development of biopolymer films has increased research activities on edible packaging, since such films consist of natural and biodegradable substances which may lead to replacement for plastics and can solve the waste disposal problem to a certain extent. Nowadays, biobased nanocomposites are a new alternative to conventional technologies for improving biopolymer barrier and mechanical properties. However, in spite of improvements of physical properties, these improvements are not sufficient for petroleum-based plastics to be replaced. Therefore, more research is required, not only in the development of optimum formulations for each polymer (combination of ingredients such as biopolymers, nanoclays, plasticizers, compatibilizers, or coupling agents), but also in application conditions of the coating solutions. It is worth pointing out that the large surface area and active surface chemistry of some nanoparticles could give rise to unwanted chemical reactions, moreover, there is a lack of understanding on how to evaluate the potential hazard of nanoparticles by the oral route and the impact of nanoparticles in waste disposal streams.

Seeking more feasible coating application systems that provide uniform distribution of a coating solution to the surface of foods is necessary in order to bring coating operations up to industrial scale. New trends in edible coatings must be focused on highly functional micro- or nanostructured, multilayered composite coatings, which will be developed by using techniques that, at present, have hardly ever been applied to food systems like atomic layer deposition (ALD). ALD is a technique that deposits ultra-thin films one atomic layer at a time. Reactants are introduced one by one, with pump/purge cycles in between, resulting in a self-saturating surface reaction limited to a single layer on the exposed surface. The result is the deposition of a 100% conformal film, with sequential cycles of these reactions enabling precise control of film thickness; however, these ultrathin films are flexible. ALD is a surface-controlled layer-by-layer process, which deposits with low-impurity content.

Less futuristic, there are also emerging new spray systems that can be used on an industrial scale to apply edible coatings: electrosprays and micro-sprays. Indeed, they provide the means to generate steadier and better-controlled spray flows. The liquid flowing out of a capillary nozzle, which is maintained at high-electric potential, is forced by the electric field to be dispersed into fine droplets. Electrospray systems have several advantages over mechanical atomizers. The size of electrospray droplets can range from hundreds micrometers down to several tens of nanometers. The size distribution of the droplets can be nearly monodisperse. Droplet generation and droplet size can be controlled to some extent via liquid-flow rate and voltage at the capillary nozzle. Nowadays, electrospray is used for micro- and nanothin-film deposition, micro-or nanoparticle production, and micro-or nanocapsule formation. Electrospraying is a single-step, low-energy, and low-cost material processing technology which can deliver products having unique properties (Jaworek 2007; Jaworek and Sobczyk 2008). These features make this technology very promising to be employed for edible coatings of food products.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References

Edible coatings can improve food quality and extend food shelf life. Despite significant benefits from using edible coatings for extending product shelf life and enhancing quality and microbial safety of foods, commercial applications on a broad range of edible coatings for foods are still very limited. In the past few years, research efforts have focused basically on searching for new coating materials, with the latest trends foresee on highly functional nanostructured and multilayered composite coatings.

To increase the use of edible coatings it is necessary to efficiently enhance their durability; this is achieved with the development of coating-application techniques such as spraying. This produces a uniform coating over a food surface, less cross-contamination, and is easier to perform in a production environment. In addition, this technique can control thickness, which is an important parameter that directly affects coating functionality. Proper spraying is achieved by controlling droplet diameters and distribution which, in turn, depend on fluid properties, nozzle design, and certain operating conditions.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Edible Coatings: Importance and Applications
  5. Different Technological Processes to Apply an Edible Coating
  6. Fundamental Principles of Atomization Spray Processes
  7. Spray Technology
  8. Important Parameters in the Control of Spray Coatings
  9. Future Trends
  10. Conclusions
  11. Acknowledgments
  12. References
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