Effect of Freezing on Microorganisms in Various Menstra

We showed 1 that prolonged action of freezing temperatures destroyed many yeasts and bacteria although with some species absolute sterility of the suspensions was not attained even after freezing at —15°C for 160 weeks. These investigations have been continued to determine the behavior of microorganisms in a large number of commercially frozen fruits and vegetables. Although the numbers of viable microorganisms have shown considerable reduction, there was no sterility even after storage at —16°C for about 2 years. Some 18 pure cultures of various microorganisms have been maintained in the frozen condition in 16 different menstra of varying hydrogen ion, salt and sugar concentrations as well as several fruit juices. To date the suspensions have been frozen for 19 months. In each case there has been a distinct decrease in the numbers of viable cells. Escherichia coli, a short non-spore-forming rod from cherries, a similar organism from strawberries and the molds showed the greatest decrease in numbers. Bacillus subtilis and the spore former from strawberries showed the greatest resistance to prolonged freezing. The presence of acid seemed to have considerable effect on the destruction of bacteria by freezing. Microorganisms suspended in cherry juice and strawberry juice with a pH of about 3.8 and plain broth with a pH of 4.0 and 5.5 showed a greater decrease than in other menstra. While this same effect was noticeable with the yeasts and molds, it was not as pronounced as with the bacteria. Suspensions in strawberry and cherry juice, either sweetened or unsweetened, behaved very much like those in plain broth. Reactions on the alkaline side of neutrality were not as harmful as those on the acid side. High concentrations of sodium chloride seemed to cause a more rapid decrease in numbers of living cells especially in the higher concentrations used (6%).