Research pick: Exergy boost for energy drink - "Exergetic growth efficiency of Kluyveromyces fragilis on complex organic and minimal inorganic media"

Research in the International Journal of Exergy has implications for the food and beverage industry, dieticians, and health-conscious consumers who are interested in the production and nutritional value of the fermented milk drink kefir.

Kefir is thought to have originated in the Caucasus region of Eastern Europe, it is made by fermenting milk with kefir grains, small, gelatinous clusters of bacteria and yeast. The drink is tangy and slightly fizzy with a creamy consistency similar to yogurt but with a distinct taste. The health benefits of kefir have been discussed for many years. It is a source of so-called probiotics, beneficial bacteria thought to support digestive health. It also contains protein, calcium, and other nutrients.

Mustafa Özilgen of the Department of Food Engineering at Yeditepe University in Istanbul, Turkey, has turned to the concept of exergy to study efficiency and cultivation in this organism with a view to optimising the growing media and food supply for the yeast to obtain maximum growth and the shortest period of time.

Exergy is a term from thermodynamics, related to energy, enthalpy, and entropy. It represents the maximum useful work that a system can do as it reaches equilibrium with its surroundings, taking into account thermal and mechanical components. It is usually described as a measure of the capacity of a system’s energy to do useful work and is commonly discussed in the context of energy management, efficiency, and sustainability.

In this work, Özilgen evaluated how efficiently Kluyveromyces fragilis can be grown on different types of system containing one of the sugars glucose or lactose on an organic or minimally inorganic medium. He found that although the highest amount of cellular mass was produced in the complex organic media, the cultivation of K. fragilis was more exergy-efficient in the minimal inorganic media. This difference might be explained by a phenomenon known as the Crabtree effect. The Crabtree effect is seen in various yeast species and other microorganisms, where they exhibit reduced respiration and increased fermentation rates in the presence of high levels of glucose or other fermentable sugars. This leads to the preferential use of fermentation as a metabolic pathway for energy production, even in the presence of oxygen, which would otherwise be used for respiration, rather than the production of cellular mass.

In Özilgen’s work, the Crabtree effect reduces exergy efficiency in the presence of glucose in organic media, but not with the minimal inorganic media. Indeed, the highest exergetic efficiency, a measure of how effectively energy is used, was 61.2% in the minimal inorganic medium with glucose, while the lowest efficiency of 24% was observed in the minimal organic medium with glucose. This would suggest that K. fragilis cultivation for use in kefir production would benefit from using glucose and minimal inorganic material in the culture medium to reduce overall energy costs. The drive towards sustainability in food production is important in the context of pollution and climate change. It might be suggested that is especially the case for foods aimed at improving health.

Özilgen, M. (2023) ‘Exergetic growth efficiency of Kluyveromyces fragilis on complex organic and minimal inorganic media’, Int. J. Exergy, Vol. 40, No. 3, pp.336–346.