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Resumen:
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The pectic material in the cell wall is responsible for textural changes in cherries. Chemical changes in the pectic material cause major changes in structure, and hence in texture, in the course of ripening and processing. The cellwall enzymes to which these changes are attributed are pectinesterase and polygalacturonase. Polygalacturonase plays a major role in altering the structure and composition of the pectic material in fruit during ripening [1-3], although in cherries A1-Detaimey et al. [4] report a low level of activity. According to Van Buren [5], changes in the pectic material in cherries appear to be due to pectinesterase action. This enzyme catalyses the hydrolytic de-esterification of the pectins, producing a higher content of lowester pectins. Its activity is enhanced in the presence of Ca 2 + and other divalent cations [6] and is induced in tissues that have suffered some damage [7]. Wu and Chang [8] have reported that the increased firmness of vegetables at temperatures where pectinesterase activity is high is caused by enzymatic de-esterification of the pectins and the formation of calcium bridges between adjacent pectin chains. Van Buren [5] found that prior incubation of canned cherries in a temperature range of 40-70 ~ C enhanced firmness and reduced the proportion of highly-esterified soluble pectins. The alterations in firmness would therefore appear to result from variations in pectinesterase activity. Prior heating at low temperatures (70 ~ C) for times ranging from 10 to 15 min induces greater resistance in the structure of the vegetable tissues to the negative effects of freezing [9]. This type of treatment prior to freezing thus considerably improves the final texture of frozen products such as green beans [10], potatoes [11], carrots and peas [12] and Brussels sprouts [13]. For the purpose of optimising the final texture of fruits of especially delicate texture such as cherries, the application of this type of treatment and a fuller understanding of the biochemical bases for such textural improvements are subjects of particular interest [14].The aim of this research was to study the effect produced on the final texture of frozen cherries by heating at 50, 60, and 70 ~ C for 3, 6, 9, and 12 min before freezing. Also examined was the effect of such treatment on pectinesterase and polygalacturonase activity, and the correlation between such activities and the firmness of the cherries before and after freezing followed by 3 and 6 months of frozen storage.
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