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EXOTHERMIC EYENTS IN COCOA (T. CACAO L) SEEDS ASSOCIATED WICI IIX) W TEMPERA Tl RE CI IANGE

Seed mointurc contont <%)

Fig. 2: Chan ges in exothenn temperaturę of embryonic axes in response to aehydration. When freezable watrr was present (unshaded) the exotherm temperaturę is linearly reUiled to the tissue moisture content which was significant at the 0.01 leuel of probability.

subfreezing temperaturesbetween-14°Cand-38°C (Figs. 1 and 2). The exotherms decreased in height as seed moisture was reduccd. Exotherms were not observed when ihe embryonic axes were dried below 27% moisture. Decrease in mag-nitude of the exotherm with dehydration and its eventual absence at Iow moisture levels indicates that the exotherms are caused by freezing of water within tłu* embryonic axis. The exotherms were also within the rangę of exothermic temperatures reported for freezable water for other seed tissues (Becwar at ai 1983). As only a single exotherm was observed for each embryo axis, no other celi eon-stituents exhibiled a detectable phase transition within the temperaturę rangę studied (+20°C to

-70°C).

Individual exotherm was narrowly spiked and peaked vertically before falling morę gradually to give a slightlyskewed profile (Fig. 1). Therefore.all freezable water in the embryonic axis was frozen instantaneously after supercooling to -14°C or lower. Embryonic axes dried below 27% did not exhibitexotherms in their DTA profiles indicating that this contained no freezable water.

When freezable water was present, decrease in tissue moisture caused a reduction in theexotherm peak and also a reduction of the exotherm temperaturę (Fig. 2). Regression between exotherm temperatures and embryonic axes moistures was

linear (r² = 0.86, p = 0.01), with a regression func-tion Y = - 49.14 + 0.5 IX. This reduction in exo-therm temperaturę is likely to be related to in-creased freezing point depression brought about by solution concentration as a result of moisture loss from the tissue.

Exothennie Events in Excised Cotyledons

w

Unlike the embryonic axes, excised cotyledons showed two exotherms, one occurring at sub-zero temperatures and the other at temperatures above 0°C (Figs. 3 and 4). The subfreezing exotherm was similar to that detected in the embryo axes. Ii was narrowly spiked and observed between -19°C to -25°C at moisture contents of 15% to 27% (Figs. 3 and 43). Exotherms were not detected in cotyledons at moisture contents below 15%. This sug-gests that theexotherm is similar to that detected in the embryonic axes, and is due to freezing of water

in the cotyledon tissues.

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The second exolherm was broader, and oc-curred at temperatures above 0°C (Figs. 3 and 4A). Itcommencedatcool temperatures between +15°C to+ll° C and terminated at + 10*('. to + 5°C. This cool temperaturę exotherm was present in all

Rcfcrence temperaturę ( °C)

Fig. 3: DTA cooling (-1"C/min) profiles of cotyledon tissues at 12%_% 75%, 16%, and 22% moisture contents. Theexotherms occurring at temperatures above freezing are non-aqueous in on gin. I he subfreezing exotherms occurred when freezable water was frozen. SeeFig. 1 forfurther explanation.

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PERTANIKA VOE. 14 NO.l. 1991