Effects of temperature and holding time during pre-blanch on pH, pectic substances and quality characteristics of canned green beans Public Deposited

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  • Physical, chemical and sensory attributes were examined on canned green beans (Oregon 1604) of sieve sizes 3 & 4 and 5 & 6 after one and three months of storage at room temperature (RT). The green beans subjected to different pre-blanch conditions in water (T1t1: 160°F - 30 min.; T1t2: 160°F - 90 min.; T2t1: 200°F - 30 min.; T2t2: 200°F - 90 min.) were blanched in a steam exhaust line for 2 min. and 50 sec. at 208-210°F to inactivate the enzymes still active. The control (T3: direct blanch) was significantly different from the others. Heat treatment prior to blanching and the other variables (sieve size and storage) had highly significant effects on all quality attributes of canned green beans. The pH was significantly lowered by pre-blanch and by canned storage. This decrease was considerably faster for sieve 5 & 6 than for sieve 3 & 4. The lowest pH value was obtained for T1t2 and corresponded to the most firm and most green canned green beans. The effects of increasing sieve size, temperature and time of pre-blanch were significant (P ≤ 0.01) on slough. The decrease of sloughing during storage was not found to be significant. The resistance to compression and shear was significantly increased by the lower temperature of pre-blanch (160°F), which activated pectin methylesterase (PME), and also by a longer canned storage time (3 months). Larger green bean sieve size and increasing temperatures, lowered the resistance to compression and shear which was negatively correlated with pH and slough. EDTA-soluble pectins (EDTA-SP) which were significantly higher with lower temperature of pre-blanch and in smaller sieve size were not affected by storage at RT. EDTA-SP, highly positively correlated to resistance to compression and shear, showed a negative relationship with pH and slough. However, water-soluble pectin (WSP) increased with larger sieve size, longer storage and higher temperature, and declined when time of pre-blanch increased. Sensory attributes for texture, appearance, color and flavor had a high degree of correlation with chemical and physical parameters. The judges were more able to distinguish changes introduced by pre-blanch treatments than those related to different stages of storage or different sieve sizes. There was a loss of green color and an increase of yellow and brown color associated with heat treatments. While treatment T1t2 (160°F-90 min.) had the highest retention of green color, the control (T3) displayed the highest loss. Also, firmness and brine clarity of the canned green beans were improved with pre-blanch at the lower temperature (160°F).
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