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Anatomical and Biochemical Mechanism of Tissue Hardening in Heat-treated "Solo" Papaya (Carica papaya L.) Fruits
Dissertation Abstract:
The mechanism involved in the development of tissue hardening in heat-treated "solo" papaya fruits was investigated. Fruits were subjected to hot water treatment (49ºC for 120 minutes) to induce the development of hardened tissues.
Heat treatment de layed the peel color development for two days and retention of fruit firmness. At PCI6, the tirmness (in kg force) was 0.058 and 4.32 in the control and heat-treated fruits, respectively.
Heat treatment also inhibited the activity of 1-aminocyclopropane-1 carboxylate (ACC) oxidase, which consequently inhibited ethylene production. At PCI4, the ACC oxidase activity in unheated fruits was remarkably higher by 45.62 percent as compared to the heat-treated fruits.
The magnitude of this difference increased to 70.94 percent at full yellow stage (PCI6). The inhibition of ACC oxidase activity caused an accumulation of ACC in the mesocarp of heat-treated fruits.
Inhibition of ethylene production by heat treatment was associated with decreased activities of cell wall-degrading enzymes such as polygalacturonase (PG) and xylanase. Calculation based on hydrolosisviscometry (HV) method ind icated that at 50 percent viscosity loss, hydrolysis was 5.09 percent, implying that the mode of action of PG in papaya was endo or cleavage at random.
Cell walls of unheated fruits were completely dissolved at PCI6 while in heat-treated fruits with hardened tissues, cell walls were still intact. This retention of cell wall integrity was evidenced by the low leakage of ions and solutes.
Respiration pattern was not affected by heat treatment except during the initial analysis. This was evidenced by the level of internal oxygen and carbon dioxide in which values for both heated and unheated fruits were comparable.
A relatively higher percentage of microbial infection was noted in heat-treated fruits which may be attributed to the injury to surface cells during treatment, making the fruits more susceptible to microbial attack.
The mechanisms of the development of tissue hardening such as inhi bited ethylene production, decreased activities of polygalacturonase and xylanase, and red uced ion leakage were discussed.