Three series of experiments were carried out. Each experiment consisted of six or seven tests where four or five containers were fitted with thermocouples and five were fitted with biological indicator units (BIU). The sterilization value (F0) delivered to cans of peas in brine was calculated from heat penetration data. The heat penetration data were analyzed for test-to-test reproducibility within each experimental series. Sterilization values for all tests were calculated from the BIU test results. The sterilizing values F0 (PHY) determined from physical (PHY) heat penetration data were compared with sterilizing values F0 (BIO) determined using the BIUs, both on the basis of accuracy and variability. The mean F0 (PHY) - F0 (BIO) was + 1.2 min. The mean coefficient of variation of the F0 (PHY) was 0.03 and the F (BIO) was 0.06.
Commercial biological indicator spore strips in glassine envelopes, produced by three manufacturers, were evaluated by fraction-negative procedures after being heated at 121.0 ± 0.05°C. Only one type of spore strip met the manufacturer's specifications. The strips of one manufacturer were further evaluated by fractionnegative and survivor curve-plate count procedures after being heated under several conditions (enclosed in glassine envelopes, in trypticase soy broth plus 0.0015% bromocresol purple, in Trypticase soy broth alone, in Water for Injection, directly); Trypticase soy broth plus bromocresol purple and tryptic soy agar, respectively, were used as recovery media. The heating condition affected the D-value of the spore strip. Recovery procedures also had an effect; in all cases, the D-values obtained from the survivor curve tests were larger than those obtained from fraction-negative tests carried out under the same conditions. To determine if the differences in D-values between the two evaluation procedures were caused by the recovery media, we evaluated, by both methods, one type of spore strip heated directly and in glassine envelopes, using tryptic soy agar plus bromocresol purple and Trypticase soy broth plus 1.5% agar, respectively, as the recovery media. The survivor curve results showed that for both enclosed and unenclosed spore strips, there was a marked difference between the two recovery media; however, there was no difference when fraction-negative tests were used.
The efficiency of four culture media for recovery of heat-activated and heated Clostridium sporogenes spores was studied. Yeast extract agar gave the highest spore recovery. The effect of the method of preparing the yeast extract agar on the recovery of heated spores was also evaluated. The results indicate that (a) a significantly lower spore recovery was obtained when the dextrose was omitted completely or when added to the medium before autoclaving, and (b) no significant difference in spore recovery was found between yeast extract agar freshly made or prepared and stored at 4 C up to 11 days before use.
Finally it might be mentioned that the relatively weak flavour which is usually obtained when so-called instant mashed potato products are made according to the manufacturers' instructions (e.g. by adding hot water or hot milk) is probably due to the fact that the degradative reactions leading to the volatile constituents of flavour only occur weakly, if at all. This observation and the experiments with baked potatoes support the suggestion that the desirable aroma of cooked foods is due to such degradative reactions.
ConclusionsT h e results reported are to be regarded as exploratory and only comparative within each group, and the observed differences are not necessarily true for all varieties of potato. Nevertheless, the following points have been established.
( I )There are distinct differences in the flavour volatile components between the potato varieties so far examined.
( 2 )New potatoes are notable for their lower concentrations of volatile substances rather than for the appearance of characteristic different constituents.(3) T h e effect of storage is markedly to alter the flavour 'spectrum'.(4) T h e soil types in which the potatoes are grown appear to affect the volatile substances produced but much more work will be necessary to explore this effect.Simple alterations in cooking procedures may produce marked changes in the flavours produced.( 5 )
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