The Mystery of Stubborn Milk: What Factors Can Inhibit the Fermentation of Milk?

Overview

Objectives/Goals The production of yogurt from milk using live milk-fermenting bacteria as a starter culture does not always give reproducible results. I wanted to find out what factors could inhibit the fermentation of milk. I hypothesized that non-organic milk produced with antibiotics and pesticides may contain traces of these compounds that can inhibit the growth of lactic bacteria and thus prevent milk from fermentation. Methods/Materials In my first experiment I investigated the speed of milk fermentation. 150mL samples of 5 varieties of organic and non-organic milk were inoculated with equal amounts of live milk-fermenting bacteria. The samples were incubated at 37ºC. The pH and thickness of milk were checked at 0, 8, 20 and 25 hours of incubation. The time to obtain the final product (yogurt) was also recorded. The experiment was repeated 3 times. In my second and third experiments I studied the inhibition of bacterial growth by milk. Since lactic bacteria required special growth conditions, I used Escherichia coli instead. I inoculated the best and the worst fermenting varieties of milk with 2 strains of E. coli and after incubation I plated the dilutions of milk on a growth agar. The next day I counted the number of colonies. Results After 20 hours of incubation at 37ºC only the organic milk samples were fermented. After 25 hours all milk brands were fermented except non-organic Ralphs milk. Organic milk had a quicker and greater decrease in pH (from 6.5 to 4.83) during fermentation than non-organic (from 6.5 to 5.33). The antibiotic-sensitive strain of E. coli grew better in organic (3.9x108 bacteria/mL) than in non-organic milk (2.7x108 bacteria/mL). The penicillin-resistant E. coli also grew faster in organic milk (9.4x107 bacteria/mL) when compared with non-organic (2.8x107 bacteria/mL). Conclusions/Discussion These results support my hypothesis. Beneficial lactic bacteria such as Lactobacillus acidophilus ferment organic milk better than non-organic. Non-organic milk inhibits the growth of bacteria including E. coli. This inhibition in case of Ralphs milk can not be explained by the presence of penicillin alone. Organic milk, with its better ability to support the growth of beneficial bacteria, should be used for young children, for people with weak immune system, and for cancer patients who take probiotics (live beneficial microorganisms) to restore their bacterial population after radio- or chemotherapy.

Competition history

• CSEF 2005 Microbiology · Entry J1329

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