Bacteriological Study of Chocolate Milk

A bacteriological study was made of chocolate milks containing, respectively, three brands of cocoa powder and three of chocolate syrup. A preliminary experiment showed that two of the powders and one of the syrups had high bacterial counts, and the others had low counts. Growth of the bacterial flora of the milk itself was inhibited when powder or syrup was added, except with one syrup (S) which was flavored with an extract of cocoa powder. Growth of pure cultures of bacteria was inhibited in the chocolate milks, excepting growth of Streptococcus lactis. Other species of bacteria employed were Escherichia coli, Aerobacter aerogenes, Staphylococcus aureus, and Bacillus cereus. Syrup S was not used in this experiment. Tannic acid added to milk inhibited the growth of the same bacterial species as those mentioned in the preceding paragraph, and the growth of Streptococcus lactis was not hindered. Parallel groups of samples were stored at room temperature (72°F.) and at refrigerator temperature (43°F.) for the experiments on the milk flora and pure cultures in chocolate milk. For the experiment with tannic acid, oxalic acid, and theobromine, only room-temperature storage was used. The inhibition of bacterial growth at room temperature was more marked because of the greater growth-rate of bacteria at that temperature as compared with refrigerator temperature. Oxalic acid and theobromine did not have any noticeable effect on the growth of bacteria. The palatability of the chocolate milks, and especially of those samples stored at room temperature, was directly comparable with the bacterial counts of the milks. The conclusion seems to be justified that cocoa powders or chocolate syrups (excepting those flavored with cocoa extracts) added to milk definitely inhibit the growth of bacteria likely to be found in milk, with the exception of Streptococcus lactis, and that the tannic substances of the cocoa products are the agents responsible for inhibiting bacterial growth.