Pectins in Processed Fruits and Vegetables: Part III—Texture Engineering

Authors

  • S. Van Buggenhout,

    1. Authors are with Laboratory of Food Technology and Leuven Food Science and Nutrition Research Centre (LFoRCe), Dept. of Microbial and Molecular Systems (M2S), Katholieke Univ. Leuven, Kasteelpark Arenberg 22, 3001 Leuven, Belgium. Direct inquiries to author Hendrickx (E-mail: marc.hendrickx@biw.kuleuven.be).
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  • D.N. Sila,

    1. Authors are with Laboratory of Food Technology and Leuven Food Science and Nutrition Research Centre (LFoRCe), Dept. of Microbial and Molecular Systems (M2S), Katholieke Univ. Leuven, Kasteelpark Arenberg 22, 3001 Leuven, Belgium. Direct inquiries to author Hendrickx (E-mail: marc.hendrickx@biw.kuleuven.be).
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  • T. Duvetter,

    1. Authors are with Laboratory of Food Technology and Leuven Food Science and Nutrition Research Centre (LFoRCe), Dept. of Microbial and Molecular Systems (M2S), Katholieke Univ. Leuven, Kasteelpark Arenberg 22, 3001 Leuven, Belgium. Direct inquiries to author Hendrickx (E-mail: marc.hendrickx@biw.kuleuven.be).
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  • A. Van Loey,

    1. Authors are with Laboratory of Food Technology and Leuven Food Science and Nutrition Research Centre (LFoRCe), Dept. of Microbial and Molecular Systems (M2S), Katholieke Univ. Leuven, Kasteelpark Arenberg 22, 3001 Leuven, Belgium. Direct inquiries to author Hendrickx (E-mail: marc.hendrickx@biw.kuleuven.be).
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  • M. Hendrickx

    1. Authors are with Laboratory of Food Technology and Leuven Food Science and Nutrition Research Centre (LFoRCe), Dept. of Microbial and Molecular Systems (M2S), Katholieke Univ. Leuven, Kasteelpark Arenberg 22, 3001 Leuven, Belgium. Direct inquiries to author Hendrickx (E-mail: marc.hendrickx@biw.kuleuven.be).
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Abstract

ABSTRACT:  Current interest in controlling the textural and rheological properties of processed fruit and vegetable products has stimulated research on the biochemistry of the cell wall, with particular reference to pectin and its degradation. This review covers the literature over the last decade with respect to pectin engineering in the field of fruit and vegetable processing. Several applications, illustrating that refined manipulation of chemical and/or enzymatic pectin degradation can be used as a tool to improve the texture/rheology of thermally processed and frozen fruit and vegetable products, are described. The discussion includes an evaluation of the role of novel technologies such as high-pressure processing in pectin engineering of processed fruits and vegetables. Furthermore, the possible role of pectin-related enzymes other than pectin methylesterase (PME) and polygalacturonase (PG) and of the nontraditional, ferulic acid-mediated cross-linking process is discussed. Finally, new trends, challenges, and suggestions for future pectin engineering research are covered in this review.

Introduction

Despite the general belief that food texture is a major determinant of consumer acceptance and preference for foods and beverages (Szczesniak 1971), food texture remains an elusive food quality attribute that is difficult to measure and analyze. Many food scientists (Matz 1962; Szczesniak 1963, 1979; Jowitt 1974; Bourne 1975, 1982; De Man 1976) have tried to define food texture but no single, generally accepted definition has appeared. Recently, Szczesniak (2002) defined texture as “the sensory and functional manifestation of the structural, mechanical, and surface properties of foods detected through the senses of vision, hearing, touch, and kinesthetics” whereby kinesthetics comprises the sensation of presence, movement, and position as resulting from nerve-ending stimulation. The complexity and the dynamic character of texture perception have stimulated research into analytical sensory analysis and consumer testing (Christensen 1984; Guinard and Mazzucchelli 1996; Kilcast 2004; Meullenet 2004). Although no piece of machinery, not even the most sophisticated, has been able to reproduce accurately and comprehensively the events that take place in the mouth upon food ingestion, simple, rapid, and instrumental texture evaluation is often performed as it is considered to be highly objective and appropriate in cases such as quality control (Lu and Abbott 2004). Nonetheless, whether the perturbing device is a human tooth or an engineered probe, what is being measured or analyzed in textural testing is a response to the structural organization of the material, whereby plant cell walls are the key structural components of most plant-based foods (Van Buren 1979; Waldron and others 2003). The close structure–texture relationship (Jackman and Stanley 1995; Aguilera and Stanley 1999; Wilkinson and others 2000) underscores the need to accompany textural studies with a structural examination both of the native food and samples placed under stress.

Food texture is mostly used in reference to solid foods, whereas food rheology is often confined to the behavior of liquid foodstuffs McKenna and Lyng (2003). However, very few foods are strictly solid or liquid in nature and, depending on the stress applied, will exhibit a solid- or liquid-like behavior. This is because most foods are viscoelastic, implying that they exhibit both properties of ideal liquids, which demonstrate only viscosity, and of ideal solids, which exhibit only elasticity. For different fruit and vegetable products, texture seems to be of varying importance (Beveridge 2007). In the case of, for example, celery or lettuce, texture seems a crucial characteristic, whereas products like thin soups seem to have few textural characteristics.

Processing fruits and vegetables into different product categories alters the initial mechanical properties. Research on how processes can manipulate the textural and rheological quality attributes of different products in a positive way has introduced the notion of “texture/rheology engineering.” In texture/rheology engineering, processing can be viewed as a controlled effort to preserve, transform, destroy, and/or create structure/texture changes. Hereby the term “engineered texture” connotes a controlled transformation of a native structure into any desirable structure worthy of consumption as a recognizable food (Aguilera and Stanley 1999). In some cases like, for example, frozen raspberries, the textural quality of the product should be kept as close as possible to that of the fresh produce, while in other cases like, for example, processed beans or potatoes, much more disruption of the structure and thus texture is acceptable or even desirable.

Processes aiming to alter the texture and structure of processed fruits and vegetables often focus on pectin changes (Van Buren 1979; Waldron and others 2003; Sila and others 2006a, 2006c). Pectin changes that have been studied most frequently in the context of processing-related texture changes are schematically represented in Figure 1 and are extensively discussed in Part II, the complementary publication by Sila and others. Briefly, both enzymatic and chemical pectin changes play a role in process-induced textural changes: (1) enzymatic degradation by the successive demethoxylation and depolymerization by pectin methylesterase (PME) and polygalacturonase (PG), respectively, and (2) chemical degradation via a ß-elimination reaction or acid hydrolysis. The most convenient way to direct pectin changes during processing to a certain point is to control the enzymatic pectin changes by inactivating the undesired enzymes and boosting the activity of the functionally important ones. Current knowledge of the processing stability and activity of the “traditionally“ pectin-related enzymes, PME and PG (see Part I, the complementary publication by Duvetter and others), has led to precision processing controlling pectin and, consequently, texture/rheology changes of many processed fruit and vegetable products. Recent findings on chemical pectin degradation and on pectin changes in the area of plant development, fruit and vegetable ripening and hard-to-cook plant-based foods (Waldron and others 1997a, 1997b; Kaaber and others 2007; Vicente and others 2007), indicate that pectin engineering however still has growth potential in the food processing domain. In this review, an overview of pectin engineering in the field of thermally processed and frozen fruit and vegetable products is given. Next to this recent findings are highlighted, such as the role of chemical pectin degradation, of pectin-related enzymes other than PME and PG, and of nontraditional gelation processes, that might be worthwhile evaluating in the context of processing-related pectin changes.

Figure 1—.

Schematic presentation of the most frequently studied pectin changes in the context of process-induced texture changes: PME = pectin methylesterase, PG = polygalacturonase,inline image= high temperature, OMe = methoxyl esters.

Pectin in Fruits and Vegetables

Edible portions of fruits and vegetables generally consist of parenchyma, which is an unspecialized tissue arranged into parenchyma cells and intercellular spaces. Parenchyma cells are approximately 50 to 500 μm across and polyhedral in shape (Aguilera and Stanley 1999).

Parenchyma cells are, from the inner to the outer part, composed of (1) one or more vacuoles, (2) cytoplasm, (3) the plasma membrane, (4) the cell wall, and (5) the middle lamella (Figure 2). Usually, parenchyma cells contain a single large vacuole that accounts for most of the cell volume and that is responsible for the osmotic potential of the cell. The cytoplasm contains different organelles and is connected to the cytoplasm of adjacent cells by strands of cytoplasm through the primary cell wall, called plasmodesmata. The plasma membrane has a semipermeable nature and makes osmosis possible, but it is the cell wall that provides the rigidity to allow a build-up in (turgor) pressure. The cell wall of the average parenchyma cell is thin (0.1 to 10 μm) but strong. Adjacent cells are glued together by the middle lamella, which is largely composed of pectic material.

Figure 2—.

Schematic presentation of (A) parenchyma tissue and (B) type I plant cell wall (reproduced from Davidson 2006).

Since the strength of plant tissue, and thus the texture of fruits and vegetables, is determined by the mechanical properties of the cell walls in conjunction with the internal pressure of the cells and intercellular adhesion, the cell wall-middle lamella complex is undoubtedly an important cellular structure for food scientists. The primary cell wall of parenchyma cells is composed of a mixture of cellulose, hemicellulose, and pectin, while the middle lamella can be considered as an extension of this matrix material from which the cellulose microfibrils are lacking. Pectic substances make up about 30% of the dry matter of the primary cell wall and are the primary macromolecules of the middle lamella. For details on the chemical pectin structure and the macromolecular organization of pectins we refer the reader to Part II, the complementary publication by Sila and others as well as to the recently published review of Willats and others (2006). In brief, pectin is a very complex molecule that consists of several domains, the most abundant of which are homogalacturonan (HG) and rhamnogalacturonan (RG) regions. HG is a linear chain of 1,4-linked α-D-galacturonans, which are methyl-esterified on the carboxyl groups up to 70% to 80%. Depending on the plant source, HG may be O-acetylated at C3 or C2. RGI is a family of polymers consisting of the repeating disaccharide (→4)-α-D-GalA-(1→2)-α-L-Rha-(1→). A proportion of the backbone rhamnose residues are substituted with a variety of different neutral and acidic oligosaccharides. Predominant RGI side chains are linear arabinan and galactan. The confusingly named RGII has a backbone of HG rather than RG, containing clusters of 4 different hetero-oligomeric side chains. As can be seen in Figure 1, most studies on process-related pectin changes have focused on HG. However, as is emphasized further in this review, other pectin regions might play an important role in pectin and thus textural/rheological changes of processed fruit and vegetable products.

Fruit and Vegetable Processing

Food processing critically affects the pectin structure–function relation and thus the texture of fruit and vegetable products depending on the processing technique applied and the intensity of that process (safety and/or quality). The safety and quality status of a food product is an integral part of the effect of all the reactions occurring in the system over the full history of the product until the moment of consumption. The rates at which desired and undesired safety- and quality-related reactions take place are a function of both intrinsic and extrinsic factors.

The intrinsic factors refer to the product-specific properties like, for example, pH and water activity. Other intrinsic factors that specifically influence the structural and textural properties of fruits and vegetables are: (1) the chemical composition of the cell wall or, more specific, the spatial organization and interaction of macromolecules (see previous section), (2) cell wall-related enzymes like pectinases, cellulases, and so on (see Part I, the complementary publication by Duvetter and others), (3) turgor pressure, (4) cell geometric properties like cell shape and cell size, (5) the amount and distribution of intercellular spaces, and (6) the proportion and arrangement of specialized tissues like, for example, vascular bundles. For example, plant tissue like carrot is composed of small cells (about 50 μm cross-diameter) that are closely packed with a high degree of contact and with a small volume of intercellular spaces forms a compact tissue, while plant tissue like apple, which is composed of large cells (about 300 μm cross-diameter) that have around 25% of gas-filled intercellular spaces, form a coarse or spongy texture (Khan and Vincent 1993; Zdunek and Umeda 2005). The raw material characteristics are strongly genetic- and maturity-dependent but can be affected by cultural practices like fertilizer and/or hormone applications, environmental stresses like drought, chilling stress, freezing stress, and others. The latter changes are, however, outside the scope of this review on food processing.

Extrinsic factors that affect the safety and quality of processed food products are process-specific factors like, for example, temperature, residence time, pressure, and irradiation. Many types of processing techniques are available today and more are being explored (Figure 3).

Figure 3—.

Progress in food processing technologies (adapted from Knorr 1998).

Traditionally, thermal processing (≥90  °C) and freezing (≤−18 °C) have formed the core of food preservation. Unfortunately, both high-temperature exposure during thermal processing and the formation of ice crystals during freezing often cause detrimental changes in the processed product, which may include undesirable changes in nutritional aspects and organoleptic attributes (texture). The extent of these losses depends on the nature of the process applied (for example, pasteurization compared with sterilization and blast freezing compared with cryogenic freezing) and the time–temperature combinations. For food scientists, preservation of quality in fruit and vegetable products over long periods with minimal loss of fresh-like characteristics remains a challenge.

Today, the claim for freshness in plant-based foods is among the 10 leading trends to watch and work for in the next millennium (Martin and others 2002). This has prompted extensive research for alternative and less degrading novel technologies, clearly demonstrated by the transition in food processing technologies (Figure 3) according to past, present, and future prospects (Knorr 1998). The main feature of the current and future technologies is “minimal food processing,” concept which is characterized by a combination of microbial safety with negligible food quality alterations. One of the novel technologies attracting considerable attention is high-pressure processing. Both in the moderate-temperature (high-pressure pasteurization) as well as in the low-temperature domain (high-pressure freezing), high-pressure processing of plant-based foods can result in better safety and quality of the processed products. A few texture/rheology engineering applications of high-pressure pasteurization and high-pressure freezing are shortly described in the following paragraphs.

Food processing optimization can be approached, however, from 2 different angles: next to the pure technological food processing optimization, principles of plant biochemistry can be a tool to manipulate the plant-intrinsic properties in such a way that food products become less susceptible to processing-induced quality (texture/rheology) changes. In this review, an overview is given of how basic pectin structure-function knowledge (see Part II, the complementary publication by Sila and others) can be used in food processing optimization.

Pectin Engineering of Thermally Processed Fruit and Vegetables

Thermal processes such as blanching, cooking, pasteurization, and sterilization of fruits and vegetables are known to result in softening. The initial loss of firmness is associated with loss of turgor due to membrane disruption (Greve and others 1994a). Important additional softening occurs partly as a result of solubilization and depolymerization of pectic polymers that are involved in cell-cell adhesion (Van Buren 1979; Greve and others 1994b; Cano 1996; Waldron and others 1997b, 2003). Chemical pectin depolymerization during heat treatments (see Figure 1) can be explained by ß-elimination and/or acid hydrolysis depending on the degree of methoxylation (DM) and the pH. Pectin with a high DM is more subject to ß-elimination than pectin with a low DM (Sajjaanantakul and others 1989, 1993; Krall and McFeeters 1998; Fraeye and others 2007). For example, in carrots, ß-elimination is pronounced in the highly methoxylated water-soluble pectin fraction (Sila and others 2006a). On the other hand, pectin with a low DM is subject to acid hydrolysis during thermal treatment at low pH. The lower the DM, the faster pectin is hydrolyzed (Krall and McFeeters 1998; Fraeye and others 2007). ß-Elimination and acid hydrolysis of pectin have been measured, respectively, down to pH value of 3.8 and up to pH value of 6 (Smidsrod and others 1966; Krall and McFeeters 1998); and reaction rates of ß-elimination and acid hydrolysis, respectively, increase and decrease with increasing pH (Smidsrod and others 1966; Kravtchenko and others 1992; Krall and McFeeters 1998; Sila and others 2006a; Fraeye and others 2007). Since the pH of plant cell walls is generally between 4 and 6 (Brett and Waldron 1996), it can be stated that acid hydrolysis of pectin is negligible compared to the ß-elimination reaction during thermal treatment of plant-based foods. The strong negative correlation between thermal texture degradation and the rate constant of the ß-eliminative reaction suggests that the ß-elimination reaction is the main contributing factor to thermal texture degradation during thermal processing (Sila and others 2006a; Vu and others 2006).

Concomitant with the solubilization of pectin at the intercellular connections and the accompanying depolymerization mechanisms during thermal processes of fruits and vegetables, tissue failure characteristics evolve from cell rupture to cell separation (Seow and others 1995; Waldron and others 1997b; Sila and others 2006b). Here, cell separation refers to tissue failure through the middle lamella/intercellular joints, in other words: minimal cell rupture (Figure 4). Because of the increased ease of cell separation, heat-treated fruits and vegetables lose their juicy and crispy character.

Figure 4—.

Micrographs illustrating changes in tissue failure characteristics at the cutting edge evolving from (A) cell rupture for fresh carrots to (B) cell separation for thermally treated (100 °C) carrots. (C) Micrograph illustrating cell separation and cell wall thickening in thermally (100 °C) treated carrots. Adapted from Sila and others (2006b) and Trejo Araya and others (2007).

When compared to thermal pasteurization processing, high-pressure pasteurization results in minimal pectin solubilization and depolymerization leading to products with bespoken textural attributes (Islam and others 2007; De Roeck and others 2008, Part II, the complementary publication by Sila and others). High-pressure processing initiates an instantaneous pressure-softening in fruits and vegetables (Basak and Ramaswamy 1998; Trejo Arraya and others 2007), however, in some cases, extended high-pressure processing time induces minimal texture changes (Duvetter and others 2005; Trejo Arraya and others 2007; Castro and others 2008). Generally, the effect of high-pressure processing of texture is highly dependent on the plant type, the morphological and structural features, and the pressure–temperature–time combinations. For instance, plants with a firm intact structure such as cauliflower reveal nearly fresh textures after high-pressure treatment at 400 MPa in combination with 5 °C (Prestamo and Arroyo 1998), whereas highly porous fruits such as strawberries show structure collapse when treated at 400 MPa in combination with 10 °C (Duvetter and others 2005).

In the context of food processing optimization by the implementation of plant biochemical knowledge, strengthening the middle lamella matrix, which cements the parenchyma cells together, is one of the major techniques to enhance the firmness of heat-treated fruit and vegetables. This is being achieved essentially by the salt-bridge formation between divalent cations and free carboxyl groups of the pectin chains. A reduction in pectin methylester groups provides greater opportunity for the pectic polymers to be ionically cross-linked by divalent ions such as calcium (Ca2+). Moreover, this reduction in DM reduces the rate and the extent of ß-eliminative degradation at high temperatures (Keijbets and Pilnik 1974; Sajjaanantakul and others 1993; Sila and others 2006a; Fraeye and others 2007). Therefore, a controlled demethoxylation of pectin contributes in a positive way to the texture of several products. A targeted manipulation of the DM of the system aiming to control ß-elimination can be achieved through genetic engineering, more specifically by manipulation of pectin biosynthesis or by a controlled activation of the enzyme PME. For thermally processed plant-based foods, pre-processing techniques, mostly aimed at increasing the endogenous or exogenous PME activity and thus enhancing cross-linking of pectin, are recommended (Sila and others 2008).

Exploitation of endogenous pectin methylesterase in texture engineering of thermally processed fruits and vegetables

Fruits and vegetables with negligible activity of endogenous polygalacturonase In products such as carrots (Fuchigami and others 1995; Stanley and others 1995; Ni and others 2005), green beans (Stanley and others 1995; Canet and others 2005), eggplants (Zhang and Chen 2006), cabbage, pepper, peas, and broccoli (Ni and others 2005), thermal stimulation of cell-wall-bound PME within the optimal range of PME catalytic activity (generally between 50 and 80 °C) reduces the vulnerability of the products to thermal softening. During preheating, PME, present but rather inactive in the intact tissues, becomes active when the applied heat induces loss of selective plasma membrane permeability, giving rise to diffusion of cations to the cell wall. In potatoes, preheating between 50 and 80 °C generally has a firming effect on the tissue (Walter and others 2003; Abu-Ghannam and Crowley 2006), but this effect is only partly ascribed to changes in pectin as starch retrogradation following boiling and cooling plays a role. Improved texture retention due to low-temperature preheating can be correlated with strengthened intercellular adhesion (Canet and others 2005) and with significant modification in matrix-bonding transforming the water-soluble pectin fraction into insoluble pectin (Sila and others 2006c; see Part II, the complementary publication by Sila and others).

A high-pressure pretreatment can be used as an alternative mode of preserving the textural properties of thermally processed fruits and vegetables because it significantly retards the rate of thermal softening (Sila and others 2004; Shahidul Islam and others 2007). The main reason for the increased positive effect of preheating when combined with pressures up to 400 MPa is increased catalytic activity of PME and, consequently, the extensive modification of pectins in terms of their DM (Sila and others 2004, 2007).

The availability of divalent ions is important, hence the frequent combination of preheating with Ca2+ impregnation. Ca2+ as such has 2 opposite effects on texture: (1) it firms the tissue through complexes with pectic substances and (2) it increases tissue softening by enhancing the ß-elimination reaction. The net result of Ca2+ addition, however, has been to firm the tissue (Van Buren 1979). In many cases, like, for example, with canned guava (Sato and others 2006), heat-treated carrots (Sila and others 2005; Smout and others 2005; Vu and others 2006), and sterilized onion (Kim 2006), a synergistic action of thermal pretreatment and treatment with Ca2+ to enhance PME activity and to reduce heat-induced softening is noticed. Also, when Ca2+ soaking is combined with high-pressure pretreatment, texture degradation of thermally processed carrots is further reduced. Because of the increased permeability of the carrot tissue after pressure pretreatment, Ca2+ soaking is more valuable after the pressure pretreatment than before (Sila and others 2004).

Chemical additives, such as Na+, present in the brine of fruit and vegetables during processing, negatively affect the thermal texture degradation (increasing the softening of the processed products), because these additives can compete with Ca2+ ions in the cross-linked de-esterified pectin thereby weakening the pectin-calcium-based network. For other additives, like EDTA, ascorbic acid, and citric acid, the same network-weakening effect is noticed whereby EDTA has the strongest effect (Vu and others 2006).

Fruits and vegetables with significant activity of endogenous polygalacturonase The textural properties of processed products, like tomatoes, where the endogenous PG activity level is high, are a result of chemical breakdown of the cell wall material, but also of transformations facilitated by PME and PG. The cell-wall-bound enzyme PG catalyzes the hydrolytic cleavage of the α-(1→4)-glycosidic bonds in the D-galacturonan moiety of pectic substances causing pectin depolymerization and thus texture changes. The role of PME hereby is creating the PG-substrate, demethoxylating pectin.

Loss of textural integrity in canned whole, sliced, wedged, diced, and crushed tomatoes due to heat-induced, and thus chemical, pectin depolymerization can be countered by the formation of intermolecular links between Ca2+ and pectin achieved by the addition of Ca2+ salts (Barrett and others 1998) and/or boosting the endogenous PME activity (Anthon and others 2005).

Viscosity and consistency loss of tomato puree and formulated tomato products such as ketchup on the other hand are often a direct consequence of enzymatic pectin degradation. The viscosity of different tomato products is highly dependent on pectic substances, which form an entanglement in which other particles are physically entrapped. Owing to the residual activity of the pectic enzymes, PG and PME, in tomato products processed at relatively low temperature (65.5 °C, cold break processes), the viscosity of the products is unstable during storage and the consistency is relatively low. A low-consistency tomato product will not retain its solid fraction in suspension, resulting in syneresis problems. To counter syneresis problems in tomato paste and formulated tomato products, a hot break process (77 to 93 °C) can be applied to completely inactivate both PME and PG. However, at these temperatures quality losses in terms of flavor, color, and nutritional value occur. One possible route to increase the resistance against PG attack of the pectic substances and thus to improve or preserve the viscosity of different tomato products, is to increase the formation of Ca2+-pectates (Porretta 1996). However, when PME activity is maintained, to increase the formation of intermolecular pectin bindings with Ca2+, pectin depolymerization catalyzed by PG still can cause viscosity problems. The selective inactivation of PG cannot, however, be established by thermal treatment as the thermal stability of PME is lower than that of PG. One way of dealing with this issue is to genetically transform tomato fruits into fruits with low PG activity. Tomato-based products produced from transgenic fruits, presenting less than 5% of PG activity compared to the control fruits, showed better viscosity and consistency than the control products (Porretta and others 1998).

Processing-induced selective knockout of PG is also possible. At suitable high-pressure conditions, endogenous tomato PG can be inactivated while endogenous tomato PME remains active (Rodrigo and others 2006). The level of the pressure treatment thus should be carefully chosen to obtain the desired rheological properties of tomato-based products. During treatments up to 400 MPa, the high activities of both tomato PME and tomato PG cause a drastic loss in consistency (Verlent and others 2007). At pressure conditions above 500 MPa, the rheological properties of tomato-based products seem to be dependent on the exposure temperature. At ambient temperatures, a pressure treatment between 500 and 700 MPa positively influences the viscosity of tomato-based products, limits syneresis problems, and is thus an appropriate alternative for conventional pasteurization (Krebbers and others 2003). At 60 °C, a high-pressure treatment at 500 MPa has a positive effect on the consistency of tomato homogenate (Figure 5), but other rheological quality defects appear, such as syneresis and the formation of a jelly-like translucent structure, which can be, respectively, limited and avoided by homogenization after the pressure treatment. According to the results of Verlent and others (2006), the combination of 70 °C and 500 MPa is the best condition to preserve the original rheological quality characteristics of the tomato homogenate. High pressure (700 MPa) processing at temperatures above 80 °C can limit the syneresis problems but not the high viscosity loss of conventionally sterilized tomato puree (Krebbers and others 2003). Considering the wide range of interesting applications of high-pressure for tomato-based products, it is not surprising that the use of high-pressure in the production process of tomato products has been patented (Wilding and Woolner 1997). For whole tomatoes, high-pressure treatments above 400 MPa reduce enzyme-related softening but seem not suitable to preserve the textural integrity because of the pressure-induced membrane permeabilization and the physical disruption of the tissue (Tangwongchai and others 2000).

Figure 5—.

Comparison of the index of consistency, measured with the Bostwick consistometer, of (A) untreated tomato homogenate, (B) tomato homogenate treated at 40 °C for 15 min, and (C) tomato homogenate treated at 40 °C and 500 MPa for 15 min. Adapted from Verlent and others (2006).

Use of exogenous pectin methylesterase in texture engineering of thermally processed fruits and vegetables

The use of thermal stimulation of endogenous PME in the context of texture engineering of plant-based foods is limited when PG (see the previous sub-section) or detrimental enzymes such as off-flavor and oxidative enzymes present in the tissue are also stimulated by the thermal treatment. In these cases, and also in plant tissues containing low amounts of endogenous PME, demethoxylation of pectin can be obtained by the application of exogenous PME. The simplest method for infiltration of enzymes in intact plant tissue, namely passive osmotic treatment, is limited by cell wall porosity and the macromolecular size of the infusing component(s). The movement of an infusing enzyme can be facilitated by an additional force such as a pressure gradient. Pressure- and vacuum-assisted infusion of enzyme solutions are, however, only practical for tissues containing gas voids and a permeable skin (Baker and Wicker 1996; Degraeve and others 2003). In the context of texture preservation of delicate plant-based foods like strawberries, vacuum infusion (VI) is more suitable than pressure-assisted infusion because pressurization, even at relatively low-pressure levels (100 MPa), can induce severe structural damage (Duvetter and others 2005). When compared to soaking, VI not only accelerates the infusion process but also results in a more homogeneous distribution of the infused component(s). The penetration and distribution of the infused enzyme is strongly affected by the degree of vacuum applied during infusion (Duvetter and others 2005; Guillemin and others 2006).

One of the applications of enzyme infusion is to treat fruit pieces with exogenous PME to enhance their suitability for further processing in terms of textural quality. Hereby fungal PME (food-grade) should most likely be preferred over plant PME because of its higher activity in acidic conditions (Duvetter and others 2005) and because of its random de-esterification pattern, which has a positive effect on the strength of the formed gel (Löfgren and others 2005). Also, fruit enrichment with Ca2+ in the presence of PME appears to play a key role (Degraeve and others 2003). For example, no significant differences between the hardness of PME-treated (without Ca2+) and untreated strawberry halves were observed by Banjongsinsiri and others (2004), whereas vacuum treatment of PME-solution containing Ca2+ significantly increased the firmness of fruits like canned half peaches (Javeri and others 1991) and pasteurized apple cubes, strawberry halves, and raspberries (Degraeve and others 2003). Also, the firmness of pressure-treated (400 to 550 MPa) strawberry halves could be significantly improved by infusing the fruit halves with PME and Ca2+ prior to the pressure treatment (Duvetter and others 2005).

The lower DM of pectin in fruits infused with PME and Ca2+ compared to the DM of pectin in fresh or nontreated fruits is a strong argument to indicate that the increased resistance of PME/Ca2+-infused fruits towards thermal texture losses is (partly) caused by the action of PME (Duvetter and others 2005). This is also illustrated by the microphotographs of PME/Ca2+-infused strawberry fruits (Figure 6) showing densely stained cell wall suggesting the presence of a strong pectin-Ca2+-network in the middle lamella of the cell wall of infused fruits.

Figure 6—.

Detailed micrographs of (A) untreated and (B) PME/Ca2+-infused (infusion with 0.12%[v/v]Aspergillus aculeatus PME and 0.5%[w/w] CaCl2 during 5 min at 10 hPa and room temperature) strawberry tissues indicating the dense and strong staining of cell walls of cortical tissues treated with PME and Ca2+. Adapted from Van Buggenhout and others (2007).

Role of other pectin-related enzymes in the texture engineering of thermally processed fruits and vegetables

Hard-to-cook problem In contrast to most fruits and vegetables, a number of plant-based foods such as common beans (Phaseolus vulgaris), lentils (Lens culinaris), Chinese water chestnut (Eleocharis dulcis), chufa (Cyperus esculentus), sugar beet (Beta vulgaris), asparagus (Asparagus officinalis), and cassava roots (Manihot esculenta) fail to soften during (prolonged) cooking. The hard-to-cook phenomenon is related to the lack of cell separation during cooking and involves changes in cell wall and middle lamella polysaccharides. The mechanism of the hard-to-cook phenomenon and the underlying cell-wall chemistry, however, is very complex and difficult to understand.

In monocotyledons like Chinese water chestnut (Parker and other 2003), chufa (Parker and other 2000), and asparagus (Rodriguez and others 2005; Jaramillo and others 2007), the hard-to-cook phenomenon is related to a storage-induced increase in ferulic acid cross-linking in xylans in the parenchyma tissue. The ferulic acid dimers that play a dominant role in this toughening process are: 8,8′-diferulic acid aryltetralin form in Chinese water chestnut, 8-O-4′-diferulic acid in chufa, 8-O-4′-diferulic acid in green asparagus, and 8-O-4′-diferulic acid and 8,5′-diferulic acid benzofuran form in white asparagus. In sugar beet, being a dicotyledon, resistance to thermal softening is related to pectin cross-linking. However, in contrast to most dicotyledons in which cross-linking of the pectic polymers via Ca2+ plays a cardinal role in cell-cell adhesion, pectin cross-linking via ferulic acid dimers seems to be the key factor in cell–cell adhesion of sugar beet (Waldron and others, 1997a). Most likely, pectic polymers cross-link via the arabinan-rich and galactan-containing neutral RGI side chains to which most of the monomeric ferulic acid is esterified (Colquhoun and others 1994). Over 20% of the ferulic acid in sugar beet is in dimer form and the main dimers involved in pectin cross-linking are 8-O-4′-diferulic acid and 8,5′-diferulic acid benzofuran form. Because pectic substances are less heat-stable than the arabinoxylan hemicelluloses and only 20% instead of 40% of the ferulic acid is in dimer form, cell–cell adhesion in sugar beet can be, in contrast to cell–cell adhesion in, for example, Chinese water chestnut, weakened after prolonged (several hours) heating at 100 °C. Beetroot, which is, like sugar beet, in the family of Beta vulgaris var. vulgaris, contains the same types of phenolics present in sugar beet. However, only 10% of ferulic acid in beetroot is found in dehydrodimer form, which can explain the relatively rapid increase in the ability of the beetroot cells to separate and the softening observed after 25 to 30 min of cooking (Waldron and others 1997a). The formation of diferulic acid moieties and thus pectin cross-links in the cell wall of beetroot can be enhanced by incubating the tissue in hydrogen peroxidase (Ng and others 1998). Also, texture improvement of other processed products might be facilitated by controlling or increasing the ferulic acid cross-linking. In most dicotyledons, increased phenolic cross-linking will necessitate an increase of suitable phenolic components and an enzyme catalyzing the oxidative coupling reaction. The cell walls of most edible fruits and vegetables already contain suitable peroxidase (POD) enzymes for cross-linking ferulate moieties. However, when regulating the activity of endogenous plant POD to increase pectin cross-linking, the production of hydrogen peroxidase also needs to be controlled as POD uses hydrogen peroxide during catalysis and it is prohibited to add hydrogen peroxide to foods. Other oxidizing enzymes, like laccases, which are capable of using oxygen instead of hydrogen peroxide, might be of interest for future research investigations in the texture engineering area.

The main mechanism of the hard-to-cook phenomenon in beans, lentils, and cassava roots involves the interaction of phytate and divalent cations with pectin. Phytic acid is known to influence the rate of softening of plant tissues during thermal processing by acting as a chelator of divalent cations, thereby preventing them from combining with pectic polysaccharides to form cross-linked insoluble pectates. Legumes and roots with high levels of intracellular phytate thus appear as easy-to-cook. When phytate, however, is hydrolyzed by the enzyme phytase, the chelation potential vanishes, allowing cross-linking of pectic substances via Ca2+-bridges. To maintain the easy-to-cook property of beans and lentils, unsuitable storage conditions such as high temperature and relative humidity, allowing phytase activity, should be avoided (Galiotou-Panayotou and others 2008). The difference in thermal softening of cassava roots from different varieties is related to differences in the level of intracellular phytate and the level of divalent ions. However, as these tissue characteristics can only partly explain the observed differences in texture after cooking, some other, yet unknown, mechanism must be involved in the thermal stability of the texture of cassava roots (Favaro and others 2007).

Ripening-related enzymes other than pectin methylesterase and polygalacturonase Most studies on pectin depolymerization during processing have focused on ß-elimination and on PME and PG. Evidence from ripening-related texture changes in fruits and vegetables now suggest that a number of other classes of enzymes, such as pectate lyase and rhamnogalacturonan hydrolases and lyases, are likely to be involved in pectin degradation.

Pectate lyase (PL) It catalyzes the eliminative cleavage of de-esterified pectin, generating 4,5-unsaturated oligogalacturonates. PLs secreted by plant pathogenic bacteria are, in the presence of Ca2+, able to macerate the parenchymatous tissue of various dicotyledon plants. As PL genes have now also been detected in many plant species, it is clear that PL is not a strictly microbial enzyme and can play an important role during plant developmental processes (Marin-Rodriguez and others 2002; Vicente and others 2007). Suppression of PL gene expression in transgenic strawberry fruits for example results in substantially firmer fruits and reduced cell wall swelling (Jimenez-Bermudez and others 2002; Sesmero and others 2007).

The net loss of galactose from cell walls during fruit ripening suggests that degradation of galactosyl residues is involved in pectin depolymerization and solubilization and thus in fruit softening.

ß-Galactosidase It is one of the main enzymes responsible for the removal of galactose from galactan, arabinagalactan, and galactose-containing side-chains of cell wall polysaccharides, may be one of the key enzymes in fruit softening during ripening (Brummell and Harpster 2001; Ogasawara and others 2007). Transgenic tomato fruits with reduced ß-galactosidase activity are significantly firmer than control fruits (Smith and others 2002). Also, the biochemical background of undesirable hardness in stored prepeeled potatoes after cooking has been partly associated with the removal of galactose and arabinose in the side chains of pectin. Possibly, the reduction of steric hindrance by debranching the pectin molecules can ease the formation of Ca2+ bridges. In this way, arabinan- and galactan-degrading enzymes can favor hardening of the potato tissue (Kaaber and others 2007).

From the above, it is clear that enzymes other than PME and PG are also involved in fruit softening during ripening. Two examples of these enzymes are described here but the role of other enzymes might be elucidated with future studies. It might be worthwhile to screen for the presence of these enzymes in different fruit and vegetables and to study their processing activity and stability to clear up their role in processing-related softening.

Pectin Engineering of Frozen Fruits and Vegetables

Owing to ice crystal formation, freezing of fruits and vegetables is inevitably accompanied by irreversible structural damage of cell wall, middle lamella, and protoplast and thus by serious textural quality deterioration. Ice crystallization in plant tissue starts in the spaces between the cells because the freezing point of the intercellular fluid is higher than that of the cytoplasm (Brown 1976). Initially, the intracellular solution will remain unfrozen, presumably because the plasma membrane acts as a physical barrier, preventing ice nucleation of the cytoplasm from extracellular sites (Stepkonus 1999). This results in disequilibrium between the chemical potential of water in the intracellular solution and that of the partially frozen extracellular solution that is manifested by supercooling of the intracellular solution. Since different cell barriers have different permeabilities to water and exhibit different abilities to sustain supercooling, the patterns of ice formation and the internal time-concentration profiles for plant cells will have a complex relationship and reaction to freezing conditions. However, in most cases, slow freezing will lead to significant cell dehydration and further extracellular ice formation. Formation of large extracellular ice crystals can cause cell separation and eventually cell rupture (Munoz-Delgado 1977); and very extensive cell dehydration may lead to destruction of membrane compartmentalization (Stepkonus 1999). On the other hand, rapid freezing will lead to a small amount of cellular dehydration and, as a consequence, excessive supercooling of the intracellular solution will increase the probability of intracellular freezing (Mazur 1963; Reid 1999). Although intracellular freezing is an inherently lethal event (Acker and McGann 2003), intracellular freezing is advantageous for the overall eating quality of frozen products as the gross tissue structure is maintained (Reid 1999). If the use of adequate quality of raw material is assumed, the most important factor affecting the final quality of frozen fruits and vegetables is thus the control of temperature during and subsequent to the freezing process (Brown 1976). In this context, new freezing techniques like high-pressure-shift freezing (HPSF), which promotes the formation of many small ice crystals throughout the whole sample depth, offers some possibilities to improve texture and structure preservation of fruit and vegetable products. For some particular products, the results of several recent studies examining the effect of freezing rate and HPSF on the textural and structural quality are summarized in Table 1. This summary makes clear that, although it is generally accepted that higher freezing rates produce smaller ice crystals, less migration of water, less breakage of cell walls, and consequently less texture damage, no unique set of freezing conditions can be considered as optimal. HPSF followed by suitable freezing conditions after pressure release seems to be able to preserve the texture of plant tissues that depend mainly on their skeletal frame. However, it is not effective in improving the texture of all products, especially not when the product's texture depends chiefly on turgor pressure.

Table 1—.  Summary of several recent studies investigating the effect of freezing rate (FR) and high-pressure-shift freezing (HPSF) on the textural and structural quality of frozen fruits and vegetables.
ProductFreezing processEffect of freezing rate (FR)Authors
Raspberry blackberryForced convection freezing and freezing by immersion both using liquid nitrogenIncreased firmness by using a high FR (>−2.2 °C/min) but unfavorable effects of ultrarapid freezing (>−32 °C/min)Sousa and others 2005
StrawberryAir-blast freezingHardness loss not limited by using a FR higher than −1 °C/minSaray and others 1999
StrawberryAir-blast freezingTissue damage limited by using a high FR (−2.43 °C/min)Delgado and Rubiolo 2005
StrawberryForced convection freezing using liquid nitrogen, freezing by immersion in ethylene glycol, and freezing in a conventional freezerHardness loss and tissue damage not limited by increasing the FRVan Buggenhout and others 2006a
CarrotForced convection freezing using liquid nitrogen compared with freezing in a conventional freezerIncreased firmness by using a high FR (−5 °C/min)Fuchigami and others 1995
CarrotAir-blast and cryogenic freezingIncreased firmness by using a high FR (temperature of cryogenic freezer < −70 °C)Kidmose and Martens 1999
CarrotForced and natural convection freezing using liquid nitrogen compared with freezing in a conventional freezerIncreased firmness by using a high FR (>−2.4 °C/min)Roy and others 2001
CarrotForced convection freezing using liquid nitrogen, freezing by immersion in ethylene glycol, and freezing in a conventional freezerIncreased firmness and limited tissue damage by using higher FRVan Buggenhout and others 2006b
PotatoForced convection freezing using liquid nitrogenIncreased firmness by using a high FR (−2 °C/min)Alvarez and others 1997
PotatoAir-blast freezing, freezing by immersion in ethylene glycol and liquid nitrogenIncreased firmness by increasing the FR but unfavorable effect of ultrarapid freezingCarbonell and others 2005
StrawberryHPSF (−15 °C/200 MPa)Tissue damage and softening not limitedVan Buggenhout and others 2006a
Peach mangoHPSF (−20 °C/200 MPa)Improved structure preservation in large productsOtero and others 2000
CarrotHPSF (−15 °C/200 MPa)Tissue damage and softening not limitedVan Buggenhout and others 2006b
CarrotDifferent high pressure freezing processes (−20 °C/0.1 to 700 MPa)Improved texture and structure by HPSFFuchigami and others 1996, 1997a, 1997b
CabbageDifferent high pressure freezing processes (−20 °C/0.1 to 700 MPa)Improved structure but loss of crispness by HPSFFuchigami and others 1998
BroccoliHPSF (−20 °C/210 MPa and −16 °C/180 MPa)Cell disorganization and destruction of vacuole membranePrestamo and others 2004
PotatoHPSF (−27 °C/250 MPa)Improved texture but high membrane damageLusher and others 2005
EggplantHPSF (−20 °C/200 MPa)Improved structure preservation in large productsOtero and others 1998

To inactivate enzyme systems present in (fruit and) vegetable products that could otherwise induce deterioration during frozen storage, blanching is often performed prior to the freezing step. Because blanching time is, in general, relatively short (1 to 10 min), the extent of heat-induced textural changes during blanching is usually smaller during blanching than, for example, during cooking or retorting. However, softening and structural damage of blanched-then-frozen products is often worse than the damage of frozen unblanched products (Prestamo and others 1998; Roy and others 2001).

From the above, it is clear that the role of pectin changes is minor in freezing-induced texture degradation when compared to thermal texture degradation. Nevertheless, pectin alterations that strengthen the middle lamella complex result in reduced freezing-induced structural and thus reduced textural damage of several products.

Exploitation of endogenous pectin methylesterase in texture engineering of frozen fruits and vegetables

Like for thermally processed fruits and vegetables, preheating and/or addition of Ca2+ can be used as a tool to reduce texture and structure degradation of frozen and blanched-then-frozen fruit and vegetable products. Freezing-induced damage in, for example, sweet cherries can be attenuated by immersion in 100 mM CaCl2 followed by a low-temperature pretreatment at 50 °C stimulating endogenous PME activity (Alonso and others 1995, 1997, 2005). Replacing a conventional blanching treatment by a stepwise blanching treatment, that is, a treatment at low-temperature followed by conventional blanching, results in a significantly better texture for frozen-then-thawed radish, mustard, and green beans (Chen and others 1989). A maximal firmness increase with respect to the control, blanched, frozen jalapeño pepper (Pérez-Aleman and others 2005) and potato (Carbonell and others 2005) is also obtained by a combination of Ca2+ addition and stepwise blanching. A detailed study on the impact of Ca2+ soaking followed by a thermal (60 °C) or high pressure (60 °C/300 MPa) treatment on the texture and structure of frozen carrots was performed by Van Buggenhout and others (2006b). When applied prior to slow freezing in a conventional freezer, these pectin-modification treatments had no significant effect on the resulting textural and structural quality. When combined, on the other hand, with rapid freezing by immersion in ethylene glycol or cryogenic freezing, a negative effect was observed. And when pretreatments were applied before HPSF, a desired hardness improvement was observed (Figure 7). These results clearly indicate that the effectiveness of pectin modifying treatments to enhance the suitability of the product to be frozen is dependent on the freezing conditions and that a controlled freezing operation that assures a high degree of supercooling is a necessary factor in controlling the textural quality of frozen fruits and vegetables.

Figure 7—.

Microphotographs and tissue particle classes of high pressure frozen carrots illustrating the positive effect of soaking in Ca2+ followed by a thermal pretreatment at 60 °C on the microstructural damage. The pectin modifying treatment reduced the tissue damage from 18%± 6% (A and B) to 3%± 3% (C and D). Colors in microphotographs B and D indicate the particle area classes: red (100 to 500 μm2), green (500 to 1000 μm2), dark blue (1000 to 5000 μm2), yellow (5000 to 10000 μm2), and light blue (10000 to 200000 μm2). Light blue particles are considered to be tissue damage as particles larger than 10000 μm2 are absent in the fresh carrot tissue used during the experiments. Adapted from Van Buggenhout and others (2006b)

Use of exogenous pectin methylesterase in texture engineering of frozen fruits and vegetables

As it has been known that infusion of PME is a tool to improve the texture of thermally treated fruits and that treatments targeting the endogenous pectin can be used to ameliorate the textural quality of both thermally treated and frozen fruits and vegetables, one might expect that exogenous PME could also be used as a processing aid in the frozen fruit and vegetable domain. The few studies that have been performed on the latter issue show promising perspectives of PME infusion to create frozen plant-based products with novel properties meeting consumer quality demands. Strawberry fruits pretreated with fungal PME and Ca2+ and subsequently frozen at −20 °C (Suutarinen and others 2000) or frozen by immersion in ethylene glycol, by cryogenic freezing, and by HPSF (Van Buggenhout and others 2006a) are significantly harder after thawing when compared to the untreated, frozen-then-thawed strawberries. Also, the sensory quality of jam made from these pretreated frozen strawberries is higher than the control jam quality prepared with untreated frozen strawberries. The lower DM, the strengthened cell-cell adhesion and, consequently, the stabilized structure of strawberry vascular tissue, cortex, and pith of pretreated fruits emphasize the role of PME during prefreezing treatments. Also, liquid retention upon thawing the strawberries is maximized by the action of PME. The extent of textural and structural quality improvement resulting from infusing PME in fruits is, however, dependent on the degree of supercooling obtained during freezing and can thus be maximized by combining VI of PME and Ca2+ with cryogenic freezing or HPSF.

Conclusion and Future Trends

Increased interest in controlling the textural quality of processed fruit and vegetable products has stimulated research on (1) novel food processing techniques and on (2) the biochemistry of the cell wall, with particular reference to pectin and its degradation.

For particular fruit and vegetable products, high-pressure technology in the low- and high-temperature domains seems to be a good, less texture-degrading alternative for conventional thermal and freezing processes. Next to the currently studied high-pressure freezing and pasteurization processes, however, high-pressure sterilization could offer some possibilities in the pectin and texture engineering domain. The impact of high-pressure sterilization and other, possibly less degrading technologies on pectin and texture changes in processed fruits and vegetables should be studied in future pectin engineering work.

The current understanding of chemical and enzymatic pectin modifications has been used to control process-induced pectin changes with the eventual aim of improving the textural quality of thermally treated and frozen fruit and vegetable products. The technological importance of pectin methylesterase (PME) and polygalacturonase (PG) in pectin engineering is well known. Several applications have shown that refined manipulation of the PME and/or PG activity results in fruit and vegetable products with satisfactory textural properties. However, most of this study has concentrated on the thermal processing domain, while pectin-engineering studies in the freezing technology domain are rather limited up to the present.

Other pectin-related enzymes, such as pectate lyase (PL), have proven their role in fruit and vegetable textural changes during plant development and ripening, but their significance in processing-related textural changes remains unknown up to the present. The same holds true for enzymes catalyzing changes in pectin side chains, like ß-galactosidase. Screening for the presence of these enzymes in different fruits and vegetables and studying their processing activity and stability and their effect on pectin structure might enlarge the enzymatic toolbox for precise pectin engineering of processed fruit and vegetable products.

Present knowledge on pectin gel-forming characteristics indicates that pectin cross-linking is not limited to the interaction between homogalacturonan chains and Ca2+ bridges. Pectin gelation involving the feruloyl groups on the pectin side chains can be induced by oxidative enzymes like peroxidase and laccase. Prospects for the use of this nontraditional gelling mechanism in processed food applications in the future look promising.

Elucidation of pectin conversions is essential in pectin engineering. However, the identification and quantification of pectin changes are hindered to some extent because it is extremely difficult to extract native cell wall pectin without modifying its structure, and it is even more challenging to isolate pectin complexes for in vitro analysis without disrupting molecular associations and generating artefacts. Recent advances in antisense RNA technology and antibodies against specific pectin epitopes have been developed and used to study pectin changes during plant development and ripening (see Part II, the complementary publication by Sila and others). Applying these in situ analysis tools in the fruit and vegetable processing domain could provide a broader understanding of pectin-related texture changes.

In summary, future pectin engineering work in the field of fruit and vegetable processing should include: (i) research on the role of novel technologies in minimizing pectin and texture changes, (ii) more research in the frozen fruit and vegetable domain, (iii) research on the role of pectin-related enzymes other than PME and PG, (iv) research on the role of nontraditional pectin cross-linking mechanisms, and (v) in situ analysis of pectin-related enzymes and of processing-induced pectin structural changes, focusing both on homogalacturonan and on pectin side chain regions (Figure 8). These advances should be carefully managed to maintain the deserved reputation of pectin as a natural product and to counter possible unwanted cell wall-dependent changes, in addition to those responsible for texture, that are regulated by modulating the pectin changes. One important issue in this context will be the bioavailability of nutrients. It is well known that release of nutrients like ß-carotene and lycopene from plant foods only occurs when the cells in the food matrix are disrupted. And although food processes like cooking and heating can reduce the total amount of food matrix nutrients, the extractability and, thus, the bioavailability of nutrients may increase by the above-mentioned processes at the same time (Parada and Aguilera 2007). One of the main reasons for the increased bioavailability of several nutrients by food processing is the breaking-down of plant cell walls. Pectin-like fibers are also proposed to be among the key factors in reduced bioavailability of carotenoids in carrot juice (Zhou and others 1996). Therefore, it might be that optimizing fruit and vegetable quality in terms of health (nutrient bioavailability) is counteracted by optimizing the texture/structure of fruits and vegetables (pectin engineering). Finding the balance between the various quality parameters of processed fruit and vegetable products will be the major challenge for future food technologists.

Figure 8—.

Schematic presentation of current and future pectin engineering work in the field of fruit and vegetable processing. PME = pectin methylesterase; PG = polygalacturonase; PL = pectate lyase;inline image= galacturonic acid; ▾= methoxyl group; POD = peroxidase;inline image= ferulic acid;inline image= Ca2+.

Acknowledgment

The authors thank the Research Council and the Interfaculty Board for Development Cooperation of KU-Leuven for their support. Sandy Van Buggenhout is a Postdoctoral Research Fellow of the Research Foundation Flanders (FWO).

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