As discussed in Tuesday’s post about stipend-ready meals, yogurt can be a delicious, nutritious, and very cheap meal component. Although making yogurt is fun and easy, the science behind yogurt production is far from finished, and scientists are still uncovering interesting clues about optimizing the growth and fermentation of the microbes responsible in yogurt production. Although S. thermophilus has been found to provide formate and carbon dioxide to L. bulgaricus, which in turn provides peptides and amino acids to S. thermophilus, there are still many aspects of the symbiotic relationship that have yet to be understood. For example, Sasaki et al., previously found that yogurt fermentation by S. thermophilus and L. bulgaricus was less efficient at higher concentrations of dissolved oxygen in the milk (yet another reason to boil the milk when making yogurt, since heat reduces dissolved oxygen). Following up on this finding, Sasaki et al. investigated the how the presence of S. thermophilus might contribute to yogurt fermentation in terms of reducing the concentration of dissolved oxygen. In particular, Sasaki et al. found that the NADH oxidase of S. thermophilus was primarily responsible for the reduction of dissolved oxygen in the milk, promoting yogurt fermentation and the production of acids. You can read more about their interesting work, here since it’s an open access piece.
Although their research took an interesting detour, Sasaki et al. (like many scientists) was initially looking to optimize the taste and texture of yogurt. A recently published open access paper by Wu al. examined the ability of S. thermophilus to produce exopolysaccharides which could affect the perceived texture (and creaminess) of the yogurt as well as the bacteria’s able to survive and serve as a probiotic. To study exopolysaccharide production by S. thermophilus, Wu et al sequenced the entire genome of a strain of S. thermophilus: ST 1275. Once they determined the gene cluster essential for EPS production, they compared this gene cluster from ST 1275 to that of five other strains of S. thermophilus. Additionally, Wu et al. found important proteases and membrane transporters important for enabling S. thermophilus to thrive in milk (which has an abundance of proteins, but considerably less sugars.) Interestingly enough, Wu et al. also found stress response genes which are potentially responsible for the bacteria’s ability to thrive at very warm temperatures (remember, ~40C for making yogurt), and to survive under more acidic and cold temperatures (hence the bacteria’s ‘live and active’ status in refrigerated yogurt.) Thankfully, Wu et al.’s fascinating findings can be found on PubMed Central (PMC) where anyone can access it.
For more intriguing recent research on yogurt bacteria check out Ferdoisi et al.’s evaluation of probiotic survivability in yogurt exposed to cold chain interruptions, which reveals how temperature/storage conditions affect the ability of the yogurt bacteria to survive before you consume it. Again, these excellent researchers have made their findings available at PubMed Central
Bottom line. When nature gives us things we don’t understand like milk spoilage/fermentation, we can use science to understand, improve, and direct the process so we can better appreciate the wonders of nature.