Conditions affecting the growth of bacteria on vegetable foods

Abstract

The present study was concerned with the effect of water activity (equilibrium relative humidity) of grass juice and the release of juice from harvested leaves on the growth of bacteria.

Cultures of Aerobacter and. Bacillus licheniformis were isolated from fresh grass. The multiplication of pure cultures was examined in grass juice at different water activity (aw.) levels and was not found to be affected above 0.9754 aw. In juice of 0.9754 aw. both organisms showed an increase in the length of the l-ag phase and an increase in the mean generation time. The total populations which, developed were not less than in juices above 0.9754 aw. However, the lag time or the mean generation time of each organism at a particular aw. was different.

The liberation of juice from harvested leaves was observed microscopically and the influence on this process of pressure, temperature, exclusion of oxygen and exposure to poisonous gases and vapours was studied.

The application of weight greatly shortened the time required to liberate juice.

Juice liberation was more rapid at higher temperatures (45 C.).

Leaves kept at 45 8. under 500 g. weight released juice in about half the time required when either factor (temperature or pressure) was acting separately.

The time in which juice was liberated was halved when oxygen was excluded.

It was even more rapid when leaves were first incubated in tne absence of oxygen at 45 C. and then placed under 500 g. weight.

There was an immediate release of Juice fro® leaves exposed to pure carbon monoxide and it was somewhat slower on exposure to a partially saturated atmosphere of chloroform, benzene or toluol.

The relative rates of juice liberation from harvested leaves of grass incubated anaerobically at different temperatures were obtained by determining- the total populations and the rates of multiplication of Aerobacter in the liberated juice.

It was shown that although the total amounts of nutrient materials liberated at 20, 30 or 40 Ca were the same, the rate of release was greater at 40 C. than at lower temperatures. The juice was not available to bacteria without incubation of the leaves. Further, the amounts of nutrient materials removed from the surfaces by immersing the leaves in buffer solution supported a total population of 33 million cells/dry matter content of grass, which was shown to be small in comparison with the amounts of nutrient materials liberated as juice. The latter supported a total population of 818 million cells/dry matter content of grass.