R: Concise Reviews in Food Science
The Distribution of Fat in Dried Dairy Particles Determines Flavor Release and Flavor Stability
Dried dairy ingredients are utilized in various food and beverage applications for their nutritional, functional, and sensory properties. Dried dairy ingredients include milk powders of varying fat content and heat treatment and buttermilk powder, along with both milk and whey proteins of varying protein contents. The flavor of these ingredients is the most important characteristic that determines consumer acceptance of the ingredient applications. Lipid oxidation is the main mechanism for off-flavor development in dried dairy ingredients. The effects of various unit operations on the flavor of dried dairy ingredients have been investigated. Recent research documented that increased surface free fat in spray dried WPC80 was associated with increased lipid oxidation and off-flavors. Surface free fat in spray-dried products is fat on the surface of the powder that is not emulsified. The most common emulsifiers present in dried dairy ingredients are proteins and phospholipids. Currently, only an association between surface free fat and lipid oxidation has been presented. The link between surface free fat in dried dairy ingredients and flavor and flavor stability has not been investigated. In this review, some hypotheses for the role of surface free fat on the flavor of dried dairy ingredients are presented along with proposed mechanisms.
Dried dairy ingredients are utilized in various food and beverage applications for their nutritional, functional, and sensory properties. Lipid oxidation is the main mechanism for off-flavor development in dried dairy ingredients, and the distribution of fat may play a critical role in flavor and flavor stability. Some hypotheses for the role of surface free fat on the flavor of dried dairy ingredients are presented along with proposed mechanisms.
Bovine milk contains 88% water along with the macronutrients fat, protein, and carbohydrates. Due to the high water content and available macronutrients for microbial growth the shelf life of milk is relatively short. Dehydration is used to extend the shelf life of milk and milk-derived products. Milk and milk-derived products are generally dehydrated in 2 ways, by spray drying or by roller drying. The resulting product is a powder with very low moisture content. Spray drying is the most common method used in the dairy industry due to the intense heat treatment involved in roller drying. These dried dairy ingredients extend the shelf life and provide functional benefits and convenience.
Dried dairy ingredients are generally classified by their physical and compositional properties and can be divided into 2 general groups, milk powders and protein powders. All milk powders are defined as having less than 5% moisture (USDEC 2005). Nonfat dry milk (NFDM) and skim milk powder (SMP) are very similar in that both are produced from pasteurized skim milk and have less and 1.5% fat by weight (USDEC 2005). SMP must have at least 34% protein by weight, which can be regulated by addition of milk permeate, whereas NFDM does not have a legal definition in regards to protein (USDEC 2005). The cumulative thermal treatments for low-heat, medium-heat, and high-heat SMP and NFDM are 70 °C for 2 min, 70 to 78 °C for 20 min, and 88 °C for 30 min, respectively (USDEC 2005). Whole milk powder (WMP) is produced from pasteurized whole milk and has between 26% and 40% fat by weight (USDEC 2005). Buttermilk powder is produced from buttermilk during butter manufacture and contains greater than 4.5% milk fat (USDEC 2005). In 2012, over 1 million metric tons of dry milk products were produced in the United States with the majority as NFDM (USDA 2013).
Concentrated protein ingredients are available in the dairy industry due to advances in membrane filtration as well as ion exchange chromatography. The membrane filtration processes commonly used are microfiltration (MF), ultrafiltration (UF), nanofiltration, and reverse osmosis. Milk protein can be concentrated by UF of skim milk to remove lactose and minerals and produce milk protein concentrates (MPCs) with protein concentrations of 40% to 90% of the total solids. Using a combination of MF and UF, whey proteins can be concentrated to produce whey protein concentrate (WPC) with 25% to 89% protein or whey protein isolate (WPI) with greater than 90% protein of the total solids. Whey proteins that are removed prior to cheese making are called serum proteins and can be further concentrated to make serum protein concentrate (Nelson and Barbano 2005; Evans and others 2009, 2010). Production of WPC and WPI in the United States reached 230000 metric tons in 2012 while production of dry MPC reached 46000 metric tons (USDA 2013). Other dried dairy protein ingredients include caseins (rennet or acid) and caseinates.
Dried dairy ingredients are used in numerous applications due to their nutritional and functional properties (Kenny and others 2000; Foegeding and others 2002; Anema and others 2006; Davis and Foegeding 2007; Raikos 2010) but the most important factor in consumer acceptance of dried dairy ingredient applications is flavor (Caudle and others 2005; Drake 2006; Childs and others 2007). In order to characterize the flavor of dried dairy ingredients, flavor lexicons have been developed (Drake and others 2003; Carunchia Whetstine and others 2005; Drake and others 2009). Off-flavors resulting from lipid oxidation in dried dairy ingredients result in decreased consumer acceptance of dried dairy ingredient applications (Caudle and others 2005; Lloyd and others 2009b; Evans and others 2010). Raw milk quality has a substantial impact on the off-flavors in milk powders and is affected by animal feed, season, or microbiological quality (Celestino and others 1997; Coulon and Priolo 2002; Croissant and others 2007). Stapelfeldt and others (1997) reported that both water activity and storage temperature were important in reducing off-flavors in WMP, and Lloyd and others (2009a, 2009b) confirmed that temperature and oxygen levels were crucial to minimize lipid oxidation. Lipid oxidation is also a primary contributor to loss of shelf life in SMP although shelf stability is substantially longer than WMP (Drake and others 2006). Sources of lipid oxidation off-flavors in dried protein ingredients have been attributed to various unit operations such as starter culture, storage, bleaching, agglomeration, and instantization (Croissant and others 2009; Wright and others 2009; Campbell and others 2011a, 2011b; White and others 2013).
The physical properties of dairy powders impact various functional characteristics and may also impact the sensory properties. Because lipid oxidation is the source of many off-flavors in dried dairy ingredients, an understanding of the effect that distribution and physical characteristics of lipids in dried dairy ingredients have on flavor and flavor stability is of great importance to the dairy industry. The goal of this manuscript is to investigate the influence of the distribution of fat on the flavor of dried dairy ingredients. A link between fat distribution in dried dairy ingredients and flavor and flavor stability has yet to be investigated.
Flavor Deterioration in Dairy Products
It is generally recognized that the main reactions that deteriorate the flavor of dried dairy ingredients are lipid oxidation and Maillard browning (Karagul-Yuceer and others 2001; Farkye 2006). Generally, lipids alone do not contribute to the flavor of foods due to their low volatility but products formed during the decomposition of lipids can impact the flavor significantly (McClements and Decker 2008). Hydrolytic rancidity and autoxidation are the 2 main types of decomposition of lipids. Hydrolytic rancidity refers to the liberation of free fatty acids from the glycerol backbone whereas autoxidation involves a complex sequence of chemical changes due to the interaction of unsaturated lipids with oxygen (Frankel 1998a; McClements and Decker 2008). Hydrolytic rancidity in milk is mostly attributed to endogenous lipoprotein lipase enzymes (Deeth 2006). Lipid autoxidation decomposes fatty acids into volatile compounds that are generally aldehydes, ketones, carbonyls, alcohols, and acids (Frankel 1998b) and these are the primary source of off-flavors in dried dairy ingredients.
Lipid oxidation in milk powders has been studied extensively. Lloyd and others (2009a) demonstrated that common off-flavors in WMP produced in the United States were grassy and painty. These off-flavors increased with storage time and were correlated to an increase in various lipid oxidation products. Aldehydes and ketones are among the main volatile compounds responsible for off-flavors in WMP and SMP (Shiratsuchi and others 1994; Karagul-Yuceer and others 2001; Karagul-Yuceer and others 2002; Carunchia Whetstine and others 2007; Lloyd and others 2009a, 2009b). Lipid oxidation and flavor of both SMP and WMP can be influenced by many factors including light exposure, anti-oxidant addition, preheating treatment, storage temperature, nitrogen flushing, moisture content, and relative humidity (Hall and Lignert 1984; McCluskey and others 1997; Stapelfeldt and others 1997; Hardas and others 2002; Lloyd and others 2009b). The concentration of unsaturated fatty acids plays a role in the oxidation stability of milk powders (Romeu-Nadal and others 2007). The physical distribution of fat in the powders also affects lipid oxidation in milk powders and will be discussed in subsequent sections of this manuscript.
Lipid oxidation is the primary contributor to a decrease in shelf life and an increase in off-flavors in WPC34, WPC80, and WPI (Carunchia Whetstine and others 2005; Wright and others 2009; Evans and others 2009, 2010). In sweet whey powder, a combination of lipid oxidation and Maillard reactions contribute to off-flavors (Mahajan and others 2004; Sithole and others 2005). Off-flavors in dried whey proteins are often associated with different processing steps. The use of starter culture increases lipid oxidation in liquid whey due to their hydrolytic enzymatic activity, which can then carry through into WPC or WPI (Campbell and others 2011a). Mesophilic starter cultures impact the oxidative stability of WPC more than thermophilic starter cultures (Liaw and others 2011). Because the orange colorant annatto used in Cheddar cheese manufacture is also found in liquid Cheddar whey, it must be bleached to obtain a colorless powder. Bleaching of Cheddar whey increases off-flavors and lipid oxidation (Croissant and others 2009; Listiyani and others 2011; Jervis and others 2012; Kang and others 2012). Other unit operations that increase lipid oxidation include storage of liquid whey or retentate, agglomeration, and instantization (Wright and others 2009; Whitson and others 2011; Campbell and others 2011b; White and others 2013). Because lipid oxidation is responsible for off-flavors and loss of shelf life in both milk powders and dried protein powders, increased lipid oxidation due to increased surface free fat in the powders could be detrimental to the flavor and flavor stability.
In order for flavor perception to occur, flavor compounds must be volatile in the food system as well as in the mouth. Interactions between flavor compounds and constituents in dried dairy ingredients can impact flavor release. Thus, flavor quality of dried dairy ingredients could be improved if volatile compounds responsible for off-flavors were less volatile, therefore increasing the sensory detection threshold of the volatile compounds. Milk proteins that contain nonpolar amino acids are of interest in flavor binding due to the nonpolar nature of flavor compounds. The measurement of flavor binding of proteins is generally done either by headspace analysis or by equilibrium dialysis (O'Neill 1996).
Roberts and Pollien (2000) investigated the influence of milk components on the volatility of different flavor compounds. The main factor in flavor retention in milk was the milk fat content with milk fat concentrations up to 1.5% in their experimental design. This was due to the lipophilic nature of many flavor compounds. The volatility of some compounds (diacetyl, 2,3-pentanedione, guiacol) was not affected by the concentrations of the different milk components and some (3-methyl butanal, 2-methylpropanal, 4-ethylguiacol) decreased with decreased milk fat content. Volatility of other compounds (β-damascenone and 1-octen-3-one) decreased with decreasing milk solids-not-fat suggesting that there was also binding with protein. In another study, the effects of lipid type and solid fat content on volatile compound release were investigated. Lipid type did not have a significant effect but an increase in solid fat content increased the volatile compound release in milk-based emulsions (Roberts and others 2003). These studies demonstrate that fat in milk products influences volatile compound release whether it is due to concentration or physical state. Given the large impact that fat has on volatile compound release, it is probable that the distribution and emulsification of fat in dried dairy powders has a strong influence on the volatile compound release and overall flavor.
Hansen and Booker (1996) investigated the influence of casein and whey protein on the binding of flavor compounds used in ice cream mixes. Their work demonstrated that whey proteins reduced flavor compound intensities more than casein. It was hypothesized that the high thermal treatment denatured more of the whey proteins than caseins, which exposed more nonpolar regions to bind the flavor compounds (Hansen and Booker 1996; O'Neill 1996). However, other studies have observed a decrease in flavor binding by β-lactoglobulin and WPI upon heating above denaturation temperatures (O'Neill and Kinsella 1988; McNeill and Schmidt 1993). This has been suggested to be due to structural changes and aggregation that occurred during the heat treatment (Kuhn and others 2006). Sodium caseinate also decreased vanillin concentrations in dairy protein beverages (McNeill and Schmidt 1993; Li and others 2000) but WPC decreased vanillin flavor intensity more than sodium caseinate (Hansen and Heinis 1991). Extensive research has been done on the flavor binding properties of various milk proteins and has been reviewed by Kuhn and others (2006). In WPI, the major protein responsible for binding flavor compounds was β-lactoglobulin (Kuhn and others 2007). Collectively, these studies provide strong evidence for the binding of flavors by milk-derived proteins. The encapsulation of fat in dried dairy powders by proteins capable of binding flavors could increase the sensory quality by reducing the volatility of the compounds responsible for off-flavors.
Surface Free Fat
The term free fat is defined as fat that is no longer emulsified (Palanuwech and others 2003). In dried dairy ingredients, proteins and phospholipids are the most common emulsifiers. Free fat can be an indicator of damage to the milk fat globule membrane (MFGM) (Kim and others 2002). A more complete definition of free fat in dairy powders is fat that is not entirely coated by amphiphilic molecules or protected by a matrix of carbohydrates and proteins during drying (Vignolles and others 2007). When free fat is on the surface of the powder particles, it is referred to as surface free fat. The surface free fat in milk powders can alter important properties of the dried milk powder such as: oxidative stability, wettability, dispersability, solubility, flowability, and ability to be used in chocolate-processing applications (Vignolles and others 2007).
Free fat is most commonly extracted using an organic solvent such as hexane or petroleum ether (Vignolles and others 2007). The use of polar solvents is avoided because they can lead to the extraction of total fat (Buma 1971). During a free fat extraction, a fixed amount of organic solvent is added to a fixed amount of powder and swirled gently for a given amount of time. The solvent is then filtered and the fat is measured gravimetrically after evaporation of the solvent. Because of this, free fat can also be referred to extractable fat. Increasing extraction time and temperature increased the amount of free fat that was extracted (Buma 1971; Kim and others 2002).
Because it can take more time to extract the free fat from the interior of the particle, there is the ability to extract different fractions of free fat, whether from the surface or the interior (Kim and others 2005b). Kim and others (2005b) were able to separate 3 different fractions of fat: surface free fat, inner free fat, and encapsulated fat. Only minor changes in fatty acid composition were found in the fat extracted from different fat fractions with the high melting saturated fatty acids being slightly more represented in the surface free fat than the interior free fat or the encapsulated fat. In both the surface free fat and the inner free fat, the oleic, linoleic, and linolenic acid composition accounted for approximately 22% of the total fatty acid profile. These unsaturated fatty acids are highly susceptible to lipid oxidation (Frankel 1998a). These results demonstrate that the surface free fat is rich in unsaturated fatty acids and suggest that surface free fat could be susceptible to lipid oxidation. Truyen and Orsi (1977) observed greater concentrations of unsaturated fatty acids in surface free fat in milk powders and increased concentrations of polar lipids in emulsified fat, fat not solvent extractable. In the manuscript, it was not specified whether the free fat was surface or inner free fat but it is assumed to be surface free fat due to the extraction being very similar to the surface free fat procedure described by Kim and others (2005b). The extraction time was 10 min whereas the extraction time for inner free fat by Kim and others (2005b) was 48 h.
High levels of unsaturated fatty acids such as oleic, linoleic, and linolenic acid make lipid oxidation a concern in milk and milk-derived products because they are among the most common unsaturated fatty acids to undergo lipid oxidation (Frankel 1998a). Surface free fat may be more susceptible to oxidation than emulsified fat because emulsified fat is encapsulated with proteins and phospholipids, which have anti-oxidant properties and will be discussed further.
Surface free fat in dairy powders can be highly influenced by processing and storage. In general, parameters that can be manipulated include inlet air temperature, outlet air temperature, feed solids concentration, and atomizing conditions. Elevated inlet air temperatures increase the particle size because the crust on the particle surface is formed more quickly, leaving less time for the particle to shrink (Birchal and others 2005; Nijdam and Langrish 2006). Larger particles can encapsulate more fat, thus decreasing the surface free fat content (Buma 1971; Beristain and others 2001). Increased inlet air temperatures decreased surface free fat in WMP whereas increasing outlet air temperatures increased free fat (De Vilder and others 1976; Kelly and others 2002). Increased inlet temperatures and increased feed solids concentration increased the surface free fat in spray-dried WPC80 (Park and others 2014). Lactose crystallization increased general free fat by damaging the MFGM and proteins that encapsulate the fat droplets (Aguilar and Ziegler 1994).
Surface Composition of Dairy Powders
The way that dairy powders are produced can greatly influence the composition on the surface of the dried particles. During spray drying, the concentrate feed is sprayed into small droplets, which are mixed with hot air to evaporate the water, leaving a dry particle with a low moisture content (<0.5%). The drying of particles in the spray dryer can be classified into 2 different periods. The 1st period is when the bulk of the water is evaporated. During this period, the water can move freely to the surface of the droplet and thus keeps the surface saturated with water. The temperature of the droplet during this time is prevented by rising above the wet bulb temperature due to the cooling that occurs when water evaporates (Fellows 2009). The wet bulb temperature during drying is generally no greater than 60 °C (Schuck 2013). During the 2nd period, enough moisture has been removed from the droplet that the surface is no longer saturated with water. At this point, a crust made of solid particles forms and the amount of water that is evaporated decreases. Because the water evaporation rate decreases, the temperature of the dried particles increases (Birchal and others 2006; Kim and others 2009b).
The solids composition of dried dairy ingredients is primarily made up of fat, protein, and carbohydrate (lactose). The distribution of these components on the surface of dried powder particles can affect different functional properties. Kim and others (2009a) demonstrated that the surface composition of SMP, WMP, and instantized WMP was determined solely by the spray drying process and not by subsequent fluidized bed drying. During spray drying, the fat, protein, and lactose reorient themselves where the fat and protein migrate to the surface due to their hydrophobicity and the lactose migrates to the center due to its hydrophilic nature. This makes the surface composition of the powder different from the composition of the entire powder including the interior, or bulk composition. In order to analyze the surface composition of dairy powders, scanning electron microscopy and a technique called electron spectroscopy for chemical analysis (ESCA) are utilized (Kim and others 2009a). In ESCA, the milk powder is assumed to be made of protein, fat, and lactose. By analyzing the elemental composition of the surface, mainly carbon, oxygen, and nitrogen, the relative percentages of fat, protein, and lactose on the surface can be calculated (Kim and others 2009a, 2009b). A more in depth explanation of this technique has been described by Faldt and others (1993).
Kim and others (2002) investigated the bulk and surface compositions of commercially produced SMP, WMP, cream powder, and WPC. In SMP, the bulk composition of lactose, protein, and fat was 58%, 41%, and 1% respectively and the surface composition was 36%, 46%, and 18%, respectively. In WPC, the bulk composition of lactose, protein, and fat was 8%, 86%, and 6%, respectively, with the surface composition much different, 6%, 41%, and 53%, respectively. In WMP and cream powder, fat represented 98% and 99% of the surface composition, respectively. Gaiani and others (2007) observed that a native caseinate powder with 0.4% lipid had a surface lipid content of 6%. It was hypothesized that the high spray drying outlet temperature (90 °C) was above the melting temperature of the powder lipids. As a consequence, the lipids were in the melted state and had increased mobility throughout the particle (Kim and others 2005a, 2005b; Nijdam and Langrish 2006). The fact that fat is overrepresented at the surface of dairy powders can have implications for different functional properties such as flowability, particle stickiness and solubility (Kim and others 2005a; Nijdam and Langrish 2006). Free fat on the surface of dairy powders could be more susceptible to lipid oxidation due to greater access to oxygen. Thus, decreased surface free fat could reduce off-flavors and increase flavor stability.
The presence of fat on the surface decreased the wettability of spray-dried emulsions stabilized by both whey and milk proteins due to the hydrophobic nature of the fat (Faldt and Bergenstahl 1996; Millqvist and others 2001). Increasing concentrations of lactose on the particle surface increased the wettability. When stored in a humid environment, the fat was redistributed to the surface at the expense of lactose (Faldt and Bergenstahl 1995; Faldt and Bergenstahl 1996). This observation was confirmed by Shrestha and others (2007) who observed in SMP that fat and protein were more likely to migrate to the powder particle surface than lactose. Kim and others (2005a) reported that surface fat inhibited the flowability of dairy powders. SMP with low surface fat was observed to flow better than powders with high surface fat coverage (WMP, cream powder, WPC). Higher surface fat has been correlated to increased oxidation in dairy powders (Granelli and others 1996). Lloyd and others (2009a, 2009b) did not observe a correlation with free fat and flavor stability in WMP produced in the United States. The range of surface free fat in the U.S. WMP was 1.1% to 7.7% and international WMP ranged from 2.8% to 6.7%. A possible reason for the lack of correlation was that the WMP were made at 4 different manufacturing facilities, confounding the effect that surface free fat alone would have on flavor stability in WMP.
Kim and others (2009a) reported that spray drying was the most important manufacturing process in determining the surface composition of spray-dried milk powders. Fluidized bed drying had no significant effect on the surface composition of the milk powders. As the particle dries, the Peclet number and the initial saturation of the concentrate to be dried influence the particle formation (Vehring and others 2007). The Peclet number is defined as the ratio between the diffusion coefficient of the solute and the evaporation rate. As the particle dries, the shape, size, and surface composition are determined by the ability of the components to reposition themselves due to the droplet viscosity or the presence of precipitates (Vehring and others 2007). This was observed by Nijdam and Langrish (2006) with the drying of milk powder. Increasing fat content in milk powders increased the surface fat coverage with the most dramatic increase seen in powders ranging from 0% to 5% fat. Spray drying at increased inlet temperatures favored the accumulation of lactose on the surface rather than protein. The theory proposed was that higher temperatures lead to accelerated formation of the surface crust, leaving less time for larger molecules such as proteins to reach the surface. The increased viscosity of the droplets would also reduce the amount of fat able to migrate to the surface. An increase in concentrate viscosity during spray drying was observed to decrease solubility of SMP (Baldwin and others 1980). Kim and others (2002) observed that of the milk components, fat migrated to the surface more than lactose or protein. Their results also showed a dramatic increase in surface free fat in dairy powders with less than 6% fat, a WPC with 6% fat bulk composition had 53% of the particle surface covered in free fat. These results along with those of Nijdam and Langrish (2006) suggest that the bulk fat content of the powder has a significant effect on surface free fat when the bulk fat content is low. As the percentage of bulk fat in the powder increases, its effect on surface free fat diminishes significantly.
Particle size distribution of dairy powders is also of importance. Dairy powders are often agglomerated to increase the average particle size and porosity. Larger sized particles are more soluble because they are more porous and thus allow for an increase in wetting ability. Nijdam and Langrish (2006) observed that regardless of the fat content, milk powders spray dried at elevated inlet temperatures resulted in an increased average particle size. Increased fat content in the milk powders decreased the particle size and increased surface free fat. Elevated inlet temperatures also increased the particle size of spray-dried WPC80 but increased feed solids concentration increased the particle size to a greater extent (Park and others 2014). In the spray-dried WPC80, a decrease in surface free fat was observed in WPC80 that had a larger particle size. Fitzpatrick and others (2004) documented the particle sizes of 26% fat milk powders with varying free fat contents. Although not a major focus of the study, it is of interest to note that for the powders with the lowest free fat content, increased particle size reduced the free fat content. These results suggest that particle size also plays a large role in the surface free fat content in dried dairy ingredients. As the particle size decreases, the surface area per unit mass increases, leaving more particle surface to be covered by free fat and less to be encapsulated (Buma 1971).
Free Fat and Lipid Oxidation
The 2 main classes of compounds responsible for encapsulating fat in dried dairy ingredients are phospholipids and proteins. Due to their close proximity to fat during emulsification, both are of interest in regards to their ability to promote or inhibit lipid oxidation. Also, due to their hydrophilic and hydrophobic properties, they may be able to slow the migration of fat to the surface by interacting with both the fat and the hydrophilic lactose interior. Phospholipids are amphiphilic and contain 2 hydrophobic acyl chains and a hydrophilic portion (Rombaut and others 2006). Phospholipids contain 2 fatty acids esterified on the glycerol backbone at the sn-1 and sn-2 positions with a phosphoric acid on the sn-3 position through a phosphate ester bond (Rombaut and Dewettinck 2006).
Phospholipids account for about 1% of the total bovine milk lipids and about 60% of these come from the MFGM (Gallier and others 2010). The phospholipid content of various dairy products derived from milk are shown in Table 1. During processing steps (heating, agitation, homogenization, and aeration) the MFGM is ruptured and the phospholipids enter into the aqueous phase (Rombaut and Dewettink 2006). This is why phospholipids can be found in products that are not fat rich. In many cases, the proportion of phospholipids to total fat is higher in products with little fat.
Table 1. Polar lipid content of various dairy products
|Whey powdera||21.1 to 41.7|
|Creamb||0.35 to 0.86|
|Butterb||0.20 to 0.27|
|Buttermilkb||21.7 to 33.1|
|WPCe||10.8 to 45|
It is unclear whether phospholipids native to dried dairy products are pro- or anti-oxidants. It appears that their effect on lipid oxidation is product specific. Phospholipids can negatively affect the flavor of dairy products through oxidation of lipids due to their high levels of unsaturated fatty acids (Sattar and deMan 1975; Sessa 1985). Phospholipids also act as pro-oxidants by lowering the surface tension of the lipid, allowing oxygen from the headspace to diffuse to the oil and thus, increase lipid oxidation (Choe and Min 2006). In WPC, phospholipids are concentrated along with the protein. In WPC75, phospholipids were as much as 23% of the total lipid content (Vaghela and Kilara 1995). This renders proteins as an important part in the encapsulation of fat in dried dairy ingredients. The concentrated phospholipids associated with the MFGM increase the potential for lipid oxidation to occur in dried whey ingredients. Wright and others (2009) observed that WPI that was instantized with lecithin, a phospholipid, had decreased shelf life due to increases in lipid oxidation products and off-flavors.
Phospholipids have also been reported to have anti-oxidant activity, which can impact the flavor of foods (Chen and Nawar 1991). The mechanism for the anti-oxidative effects of phospholipids is still not clear. The phospholipids with polar groups containing nitrogen are effective anti-oxidants under most conditions (Choe and Min 2006). Phospholipids also chelate metals, which decrease lipid oxidation. If the concentration of the phospholipids is too high then the phospholipids act as pro-oxidants. Yoon and Min (1987) reported that phospholipids were only anti-oxidants when Fe2+ was present and chelated. Sources of Fe2+ in dried dairy ingredients include metalloproteins such as lactoferrin, serum transferrin, and ovotransferrin and are found in many dried dairy ingredients (Jervis and Drake 2013). It is possible that in dried dairy ingredients with increased fat emulsified by native phospholipids that decreased off-flavors would result due to anti-oxidant properties of the native phospholipids. Phospholipids also could reduce lipid oxidation by encapsulating the fat and preventing it from migrating to the surface due to their hydrophilic properties and the hydrophilic nature of the lactose rich interior of the spray-dried particles.
Milk proteins are the other main class of compounds involved in the encapsulation of fat in dried dairy ingredients. Anti-oxidant properties of native milk proteins that encapsulate and reduce surface free fat may also in turn reduce lipid oxidation and off-flavors in dried dairy products. The ability to unfold at the oil/water interface and expose the hydrophilic and hydrophobic amino acids allows proteins to encapsulate fat globules. Casein has been demonstrated to have greater anti-oxidant effects than whey proteins (Allen and Wrieden 1982; Hu and others 2003). It was hypothesized that differences in anti-oxidant properties among the proteins was due to interfacial thickness, chelating properties, and free-radical scavenging amino acids. The anti-oxidant activity can come from the electrical charge proteins impart on the fat droplets, repelling pro-oxidant metals away from fat droplets (Donnelly and others 1998). Others have stated that the effectiveness of casein to prevent lipid oxidation is due to the ability to bind copper and other pro-oxidant metals and to unfold and surround the fat globule membrane (Frankel 1998b). β-lactoglobulin was observed to be a mild anti-oxidant and a loss in anti-oxidant activity was attributed to structural changes during heating (Liu and others 2007). The anti-oxidant or pro-oxidant properties of proteins are highly dependent on the food system and unexpected activity can arise from different interactions between food components.
Milk proteins may also decrease lipid oxidation in dried dairy ingredients by encapsulating milk fat and preventing it to reach the surface of the powders. Kim and others (2002) observed that fat that was encapsulated by protein was preferentially located beneath the layer of surface free fat. They also demonstrated that oxygen uptake for powders with higher levels of surface free fat was greater than in powders with lower levels of surface free fat. Because oxygen is key to lipid oxidation and an increase in oxygen content correlates to greater lipid oxidation, this demonstrated that powders with higher surface free fat were more susceptible to lipid oxidation and potentially off-flavor development. De Vilder and others (1976) observed a positive correlation between surface free fat and particle porosity. Nitrogen was able to penetrate the milk powder particles with increased surface free fat. This suggests that decreased surface free fat could also decrease lipid oxidation by limiting oxygen exposure to the interior of the powder particles. Hardas and others (2000) observed increased oxidation in surface free fat compared to encapsulated under the surface in emulsions made with milk fat. In their study, surface free fat had a greater increase in peroxide value and hexanal over time and a greater decrease in linoleic and linolenic acid contents compared to the encapsulated fat. These studies suggest that efforts should be made to reduce the amount of free fat on the surface of dried dairy ingredients to improve flavor and flavor stability.
Implications for Flavor and Flavor Stability
The protein-binding properties, surface composition, and surface free fat are all important characteristics that influence the flavor of dried dairy ingredients. Because spray drying and storage of dried dairy ingredients influence these properties, an emphasis should be placed on parameters during these unit operations to improve the flavor and flavor stability. A reduction in surface free fat could reduce off-flavors in dried dairy ingredients due to the properties of the emulsifying native proteins and phospholipids. Emulsifying proteins in dried dairy ingredients can bind more off-flavors, decrease oxygen permeability, and decrease lipid oxidation and native phospholipids potentially have anti-oxidant effects. Park and others (2014) observed this effect directly in WPC80 where decreased surface free fat corresponded to decreased off-flavor intensity and associated lipid oxidation compounds. Lower levels of surface free fat were observed in WPC80 spray dried at increased feed solids concentration. Higher feed solids concentration during spray drying in WPI was demonstrated to increase whey protein denaturation (Anandharamakrishnan and others 2007). The denatured whey proteins would have a greater ability to encapsulate the fat due to their exposed hydrophobic regions. This is of importance because preheat treatment is an important unit operation in the manufacture of dried dairy ingredients. While it is possible that preheat treatment could in theory reduce surface free fat due to greater milk fat encapsulation, its effect on flavor would be more difficult to predict since many thermally induced flavor compounds would result and confound any benefits of reduced surface free fat. Keogh and O'Kennedy (1999) observed higher levels of fat oxidation on the surface of spray-dried whey protein/milk fat emulsions. A summary of research related to the influence of process parameters on characteristics of dairy powders related to flavor and surface free fat is shown in Table 2.
Table 2. The role of various process parameters on surface free fat and/or flavor of dairy powders
|WMP||Spray dryer nozzle size, outlet air temperature, feed solids concentration||Increased nozzle size and outlet air temperature increased surface free fat and increased feed solids concentration decreased surface free fat||Kelly and others (2002)|
|WMP||One-stage and 2-stage drying and homogenization||The use of a 2-stage drying process involving spray drying and fluidized bed drying decreased surface free fat. Homogenization also decreased surface free fat||De Vilder (1980)|
|WMP||Lactose crystallization by high shear and elevated temperature in a mixer||Increasing lactose crystallization with high shear and elevated temperature increased the amount of free fat in WMP to almost 80% compared to low shear and decreased temperature.||Koc and others (2003)|
|WMP||Preheat treatment prior to spray drying||WMP that was classified as low-heat WMP was consistently higher in lipid oxidation throughout extended storage than medium or high-heat WMP.||Stapelfeldt and others (1997)|
|WPC80||Spray dryer inlet temperature, feed solids concentration||Increased inlet temperature and feed solids concentration decreased off-flavors along with surface free fat while increasing particle size.||Park and others (2014)|
|WPC34 and SPC34||Freeze drying and spray drying||The heat used during the spray drying process had little effect on the flavor of WPC34 and SPC34 because no consistent differences in flavor were observed between freeze drying and spray drying.||Evans and others (2009)|
|WPC/WPI Emulsion||Homogenization conditions and composition of the feed||Increasing the lactose: WPC concentration reduced free fat but not surface fat. The higher levels of fat on the surface of powder particles increased the level of oxidation during storage.||Keogh and O'Kennedy (1999)|
|WPI||Spray dryer outlet air temperature, feed solids concentration||Increased outlet air temperature and feed solids concentration increased the amount of whey protein denaturation.||Anandharamakrishnan and others (2007)|
Future experiments should be conducted to investigate the effect of decreased free fat content on the flavor and oxidative stability of various dried dairy ingredients, both milk powders and protein ingredients of differing fat contents. Although surface free fat has been studied extensively in WMP, a link between surface free fat, flavor, and flavor stability has only been recently proposed in WPC80. In particular, spray drying parameters and processing steps prior to spray drying should be optimized for the sensory properties of the dried powders. These parameters should include homogenization pressures, concentrate solids concentration, inlet and outlet air temperatures, and atomization conditions. The effect of surface free fat in various dried dairy ingredients on flavor stability over time would be very useful to the dairy industry as a whole.
Dried dairy ingredients continue to be a focus for the dairy industry due to their reduction in shipping costs, versatility, and extended shelf life. Because flavor is the limiting factor in consumer liking of dried dairy ingredient applications, improved flavor and flavor stability by altering powder characteristics is of great importance. Advances in spray drying technology and research have resulted in discoveries regarding the influence that spray-drying parameters have on protein binding, particle surface composition, and free fat. Understanding how these powder characteristics relate to the flavor and flavor stability of dried dairy ingredients will help the dairy industry produce products with improved flavor and increase their implementation in the food industry.
Funding provided in part by the Dairy Research Inst. (Rosemont, Ill., U.S.A.). The use of trade names does not imply endorsement nor lack of endorsement by those not mentioned.