| Summary: | 碩士 === 國立臺灣大學 === 園藝學研究所 === 89 === This study has investigated the physical changes of avocado during its ripening and influences caused by different pre-treatments before frozen processing of avocado. Changes in the texture, color, and flavor after -18℃ frozen were also investigated in order to evaluate the practicability of avocado frozen processing.
Local purple-skin and green skin avocado variety were taken as material in this study. Avocados were harvested and storage under 28±2℃ for ripening. The changes of color, Transmission force Test of displacement and resistance, water content, crude lipid were investigated until the softening of avocado. Relative change was observed in the water content and crude lipid change in two avocado varieties. Sum total of water content and crude lipid was increased in two avocado, namely in purple-skin avocado from 91.99% to 92.24%, 90.56% to 91.56% in green-skin avocado. L value and b value of the avocados decreased during storage but b value increased. L, a, b value of avocado was found have significant correlation with chroma. Young’s modulus value can be used as ripening index of avocado.
Two varieties of avocado were homogenized by one part of pulp with three part of water. pH adjustment, different adding amount of ascorbic acid, sodium chloride, EDTA were treated to avocado puree. Relationship of above treatment with puree’colority, stickiness, content of peroxidase, polyphenol, and pectin methylesterase were investigated. The finding showed at low pH, adding of high dose ascorbic acid, sodium chloride and EDTA resulted better colority and lower enzyme activity, whereas in green-skin avocado, the existence of sodium chloride caused poorer color. Polyphenol activity was higher in green-skin avocado than that of purple-skin avocado, and it also found to have greater color change, but do not inhibit the occurrence of browning. pH adjustment significantly influenced to stickiness of juice but others treatment shown only limited influence.
Avocado was sliced and given with frozen storage. Changes of displacement and resistance force curve for Transmission force Test determined with Texture Analyzer. Cell structure was observed using microscope. The result showed that cell structure was seriously damaged, but purple-skin avocado encountered lesser cell damage, the immersion and individual quick frozen method in texture for transmission force test had larger stress.
Avocado puree was packed after adding of 1000 ppm ascorbic acid and 500 ppm of EDTA, packed with 1000 ppm ascorbic acid and 500 ppm EDTA treated ice coat, and packed with multi-layer package and storage under -18℃. Change in color, lipid oxidation and chloroplast have significant difference in the first two months storage. Storage of avocado in puree style was higher than in flesh style in their TBARS value of color change and lipid oxidation. Stickiness increased significantly during storage time where purple-skin avocado was higher. Avocado was found low in flavor and texture evaluation, but generally acceptable. Bitterness of avocado was found lower than 10%.
In summary, the formation of bitterness after heated in avocado can be reduced by frozen storage treatment. Different pre-treatment in frozen storage also reduced its color browning, lipid oxidation and odor formation. As a result, storage life can be extended.
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