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