Minimizing texture and structure degradation of frozen fruits and vegetables : use of enzyme technology and high pressure-low temperature processes

Although freezing is one of the most important preservation methods for fruits and vegetables, the evolution of texture in frozen fruits and vegetables is very complex and difficult to understand. As freezing damage is the consequence of many separated processes, it is not always clear whether the damaging step occurs during pretreatment, freezing, storage or thawing. For each of these steps, but especially for the pretreatment and the conventional freezing step, a lot of research has been carried out in order to optimize the final quality of the frozen products. At the level of pretreatment, enzyme technology offers, among other treatments, some possibilities to improve the suitability of fruits and vegetables to be frozen. In this context, a controlled activity of pectinmethylesterase is sometimes desired because the enzyme catalyzed de-esterification of pectin creates the possibility of intermolecular interactions between Ca++ ions and low esterified pectin and can consequently reinforce cell-cell adhesion. At the level of optimization of the freezing and thawing step, high pressure-low temperature processes are relevant to mention. Of the latter emerging processes, the most promising processes in the food domain are high pressure shift freezing and high pressure thawing as a tool to increase the freezing and thawing rate and high pressure induced crystallization processes as a tool to reduce the microbial load of frozen foods. The experimental part of the work is hypothesis driven: throughout the work a number of hypotheses concerning the optimization of textural quality of frozen fruits and vegetables by the use of different high pressure-low temperature processes as well as pectin modifying treatments were evaluated. In the context of these pectin modifying treatments, two case studies were selected: carrots, a system where mainly endogenous pectinmethylesterase is responsible for pectin conversions and strawberries, a system suitable for enzyme infusion. Next to this, the use of a microscopy based approach including image analysis and detailed tissue particle parameter analysis to study freezing damage in fruits and vegetable was examined. In this way, evaluating the hypothesis that textural quality of frozen fruits and vegetables is closely related to their tissue integrity is included in the work. First it was investigated whether the freezing damage of frozen fruits and vegetables could be reduced (compared to slow, rapid, and cryogenic freezing at atmospheric pressure) by the use of high pressure shift freezing. This question was answered negatively: although the freezing time was remarkable reduced by high pressure shift freezing, the extensive hardness loss of slowly frozen carrots and strawberries could not significantly be reduced by the application of high pressure shift freezing. This observation was correlated with serious microstructural damage that arose mainly during the completion of the freezing process (at atmospheric pressure). Rapid and cryogenic freezing on the other hand could reduce the structural damage of frozen carrots and thus could be used to improve the textural quality of frozen carrots. This was however not the case for frozen strawberries. Secondly, it was studied to what extent textural and structural changes during different freezing processes were related to pectin conversions and thus whether pectin modifying pretreatments could enhance the suitability of fruits and vegetables to be frozen. Calcium soaking followed by a thermal (60°C) or high pressure (300 MPa/60°C) treatment and vacuum infusion with pectinmethylesterase and Ca++ reduced the degree of esterification of respectively carrot and strawberry pectin. These respective pretreatments were effective in minimizing the structural damage and thus in obtaining an improved textural quality of high pressure frozen carrots and of rapidly, cryogenically and high pressure frozen strawberries. Adding pectinmethylesterase and Ca++ by the use of vacuum in osmodehydrated and osmodehydrofrozen strawberry fruits resulted in a minor positive effect in terms of hardness values, but the positive effects on drip loss and visual appearance after thawing were obvious. Next, the use of another high pressure-low temperature process, namely high pressure thawing, for texture preservation was studied. High pressure thawing noticeably reduced the thawing time of both carrots and strawberries, but remarkable textural quality improvements were not observed when compared to thawing at atmospheric pressure. This observation, together with the finding that textural and structural quality of both carrots and strawberries were not altered considerably during frozen storage, led to the statement that the combination of pretreatment and freezing conditions is the most important factor for texture preservation/optimization of frozen fruits and vegetables. The last high pressure-low temperature process, whose possibilities in the food domain were studied, is high pressure induced crystallization. Although post-freezing high pressure induced crystallization processes have recently been proven to reduce the microbial load of frozen products, the possibility of these processes to replace a pre-freezing blanching step for fruits and vegetables seemed rather limited. Post-freezing high pressure induced crystallization processes above 200 MPa (up to 500 MPa) in combination with temperatures of -10°C down to -26°C were able to sufficiently inactivate lipoxygenase in frozen model and real systems but were not able to sufficiently inactivate other quality related enzymes such as pectinmethylesterase, polygalacturonase, peroxidase, and polyphenoloxidase. Moreover, volume changes during high pressure induced crystallization due to the transformation of ice I to ice III crystals negatively affected the textural and structural quality of pectinmethylesterase/Ca++-infused, frozen strawberries and pretreated, frozen carrots. Throughout the work, a high correlation between textural quality, measured as instrumental hardness, and structural integrity, analyzed by light microscopy followed by image analysis, was observed. Moreover, the detailed image analysis step allowed detecting changes in tissue particle size and shape and thus quantifying the tissue damage due to cell wall breakage. Therefore it could be concluded that the the microscopy based approach evolved in this work is, in addition to conventional analytical tools, useful to study freezing damage in frozen fruits and vegetables.status: publishe

Modeling lycopene degradation and isomerization in the presence of lipids

The kinetics of thermally induced degradation and isomerization of lycopene in olive oil, fish oil, and an olive oil/tomato emulsion were studied in detail. Special attention was paid to the isomerization reactions using a multi-response modeling approach. The type of oil had a significant impact on lycopene degradation kinetics, being faster in fish oil compared with olive oil. The estimated kinetic parameters to describe lycopene degradation in olive oil were not significantly different from those in an olive oil/tomato emulsion. The overall Z-isomer formation and elimination in olive oil, fish oil, and olive oil/tomato emulsion was similar. Only the kinetic parameters for 13-Z-lycopene formation differed significantly in the two oils. Although the isomerization rate constants for the emulsion were lower than for olive oil, the isomerization reactions showed similar temperature dependency. This study shows that the kinetics of thermally induced degradation and isomerization of lycopene in oil and in an olive oil/tomato emulsion can be described using the same model. The food system, however, has an influence on the model parameters, especially on the rate constants. © 2011 Springer Science+Business Media, LLC.status: publishe

Investigating the role of pectin in carrot cell wall changes during thermal processing: A microscopic approach

Changes in cell wall integrity upon thermal treatment were assessed in carrot cells using novel microscopic approaches using Congo red and different cell wall polysaccharide specific probes (JIM7, LM10, LM11, LM15, LM21, LM22 and CBM3a). Strong thermal processing induced an increased accessibility of cellulose and hemicelluloses by Congo red and the specific probes, except galactomannan, which detection was not affected by the thermal processing. Detection of pectin by JIM7 disappeared upon thermal processing, pointing at the leaching out effect of pectin from cell wall due to beta-elimination. Changes observed after thermal processing were moreover similar to changes observed after enzymatic degradation of pectin, and a combination of thermal and pectinases treatments did not cause additional effects. These observations indicated that the presence of native pectin is the main factor governing cell wall polysaccharides accessibility and overall cell wall integrity in carrot, which can be modulated through thermal processing.[i]Industrial relevance[/i]: This work provides new evidences on the specific role of pectin in carrot cell wall integrity, more specifically on how it can be modulated by thermal processing. New light microscopy approaches to assess changes in cell wall integrity are presented. This information is important for food industry since plant cell wall acts as a structural barrier for the release of carotenoids and other micronutrients in plant-based food products

Experimental and numerical analysis of an apparatus to apply controlled shear/elongation in fluid flows

This paper reports on the detailed flow physics of a device to control shear/elongation in a fluid flow. The device consists of a sphere which periodically moves up and down inside an axisymmetric confinement. Detailed analysis of the flow field inside the setup is reported in this paper and an analysis of the flow structures and the shear rates inside the confinement is made. The flow field is experimentally measured using Time-Resolved Particle Image Velocimetry (TR-PIV) and numerically obtained by a Direct Numerical Simulation. The numerical simulations are validated with experimental data and the numerics were able to capture all the flow features found in the experiments. Shear rate profiles along pathlines are calculated and it was found that the shear rate profiles are very similar for different initial positions in the flow field. Due to the up and down movement of the probe, both positive and negative shear is created, which is not possible with other shear controlled devices such as rotating cylinders, moving plates, and contractions. Therefore, this apparatus is able to simulate more realistic flow conditions as can be found in the processing industry. © 2014 Elsevier Ltd.publisher: Elsevier articletitle: Experimental and numerical analysis of an apparatus to apply controlled shear/elongation in fluid flows journaltitle: Chemical Engineering Science articlelink: http://dx.doi.org/10.1016/j.ces.2014.04.006 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved.status: publishe

Processing tomato pulp in the presence of lipids: the impact on lycopene bioaccessibility

Industrial processing of tomatoes into different end-products includes mechanical treatments, several thermal treatment steps, and the addition of ingredients which might induce changes in lycopene bioaccessibility. Here the influence of high pressure homogenisation (HPH) (100bar) and microwave heating (20min at 70, 90, and 120°C) of tomato pulp was evaluated in the absence and in the presence of three different oils (5%) with distinct fatty acid composition (coconut oil, olive oil, and fish oil). Lycopene bioaccessibility in the processed samples was studied by quantifying the fraction of lycopene that was transferred from the food matrix to the aqueous micellar phase during in vitro digestion.Adding lipids prior to processing clearly enhanced the lycopene bioaccessibility. However, the type of lipid added was of minor importance compared to the process conditions applied. HPH or microwave heating of tomato pulp in the presence of lipids during 20min at 70 and 90°C did not improve the lycopene bioaccessibility significantly. When HPH was applied prior to the heat treatment, microwave heating at 90°C could improve the lycopene bioaccessibility. It is hypothesised that HPH damages the cellular barriers for lycopene bioaccessibility, which can be further disrupted by thermal processing improving lycopene release during digestion. Finally, applying conditions of 20min at 120°C as such facilitated the lycopene bioaccessibility remarkably. HPH preceding this thermal treatment was of no extra value in terms of lycopene bioaccessibility.status: publishe

A review on the relationships between processing, food structure, and rheological properties of plant-tissue-based food suspensions

 Nowadays, there is much interest in controlling the functional properties of processed fruit- and vegetable-derived products, which has stimulated renewed research interest in process-structure-function relations. In this review, we focus on rheology as a functional property because of its importance during the entire production chain up to the moment of consumption and digestion. This review covers the literature of the past decade with respect to process-structure-rheology relations in plant-tissue-based food suspensions. It became clear that the structure of plant-tissue-based food suspensions, consisting of plant-tissue-based particles in an aqueous serum phase, is affected by many unit operations (for example, heat treatment) and that also the sequence of unit operations can have an effect on the final structural properties. Furthermore, particle concentration, particle size, and particle morphology were found to be key structural elements determining the rheological properties of these suspensions comprising low amounts of starch and serum pectin. Since the structure of plant-tissue-based products was shown to be changed during processing, rheological parameters of these products were simultaneously altered. Therefore, this review also comprises a discussion of the effect on rheological properties of the most relevant processing steps in the production of plant-tissue-based products. Linking changes in rheology due to processing with process-induced alterations in structural characteristics turned out to be quite intricate. The current knowledge on process-structure-function relations can form the basis for future improved and novel food process and product design.status: publishe

Comparative study of cell wall polysaccharides from pulp and peel of dragon fruits (Hylocereus spp.)

Cell wall polysaccharides from the pulp and the peel of white-flesh and red-flesh dragon fruits (Hylocereus spp.) were isolated using various solvents in order to obtain different pectin fractions. The galacturonic acid (GalA) content, the degree of methoxylation (DM), the neutral sugar composition and the molar mass distribution of these pectin fractions were determined, as well as their affinity towards some specific anti-pectin antibodies. The results showed that cell wall polysaccharides of dragon fruit samples contained significant amounts of pectic substances with a low average DM value. No remarkable differences between the pectic substances of the different dragon fruit varieties were observed while there were large differences between the pectic fractions of the peel and the pulp samples. The pectic substances in the peel are fairly linear and highly water-soluble whereas in the pulp, on the contrary, pectin is more branched and contains comparable amounts of loosely-bound, ionically-bound and strongly-bound pectic fractions. Using anti-pectin antibodies with different specificities (towards different pectin structures), it was revealed that a wide range of epitopes, including long blocks of unesterified GalA residues as well as blocks with consecutive esterified GalA residues, are present in the pectin fractions of peel/pulp dragon fruits

The effect of pectin concentration and degree of methyl-esterification on the in vitro bioaccessibility of carotenoid-enriched emulsions

Soluble fibers, like pectin, are known to influence the physicochemical processes during the digestion of dietary fat and may therefore affect the absorption of lipophilic micronutrients such as carotenoids. The objective of the current work was to investigate whether the pectin concentration and degree of methyl-esterification (DM) influence the bioaccessibility of carotenoids loaded in the oil phase of oil-in-water emulsions. The in vitro β-carotene bioaccessibility was determined for different oil-in-water emulsions in which 1 or 2% citrus pectin with a DM of 99%, 66% and 14% was present. Results show that pectin concentration and DM influence the initial emulsion properties. The most stable emulsions with the smallest oil droplets (D(v,0.9) of 15–16μm) were obtained when medium or high methyl-esterified pectin was present in a 2% concentration while gel-like pectin structures (D(v,0.9) of 114μm), entrapping oil droplets, were observed in the case where low methyl-esterified pectin was present in the aqueous emulsion phase. During in vitro stomach digestion, these gel-like structures, entrapping β-carotene loaded oil droplets, significantly enlarged (D(v,0.9) of 738μm), whereas the emulsion structure could be preserved when the medium or high methyl-esterified pectin was present. Initial emulsion viscosity differences, due to pectin concentration and especially due to pectin DM, largely disappeared during in vitro digestion, but were still significant after the stomach digestion phase. The observed differences in emulsion structure before and during in vitro digestion only resulted in a significant difference between emulsions containing low methyl-esterified pectin (β-carotene bioaccessibility of 33–37%) and medium/high methyl-esterified pectin (β-carotene bioaccessibility of 56–62%).publisher: Elsevier articletitle: The effect of pectin concentration and degree of methyl-esterification on the in vitro bioaccessibility of β-carotene-enriched emulsions journaltitle: Food Research International articlelink: http://dx.doi.org/10.1016/j.foodres.2014.01.031 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved.status: publishe