This is a (barely edited) extract of a lengthly literature review written by a 3rd-year food science and technology student, Doreen Ting, at RMIT University in 2008 – I (Dr Philip Button) was her project facilitator. The overall literature review is excellent, and despite its age, at 14 years old now, it will still serve a purpose as a review of the fundamental elements of key food spoilage yeasts. On 5 January 2020, we published a blog article as part 1 in this series, giving an overview of the significance of yeasts in this context. On 13 February 2022 we published about the physiology and biochemistry of yeasts in the context of food spoilage. We won’t label this and subsequent articles as parts, but shall provide links to them, and to a separate document which contains all the literature cited. In this article, we cover of the overall significance of yeast in a food spoilage context, as well as providing insights into the specific physiological system of membrane transport. To be successful on numerous fronts, microbial membrane transport systems are key, and here we provide an introduction to these.
Significance of yeast in food spoilage
Yeast are known to be one of the major spoilers in the food industry. Apart from their outstanding role in processes such as leavening of bread and fermentation of wine, yeast has caused vast losses to the food industry. It is able to grow in a wide range of food; high (syrups) to low (beverages) viscosity food, high sugar to high salt content food, and solid food such as fruits.
As mentioned earlier, the yeast genus Zygosaccharomyces is most notable in the food industry for having three species that poses serious economic spoilage risks to food manufacturers. Some yeast are able to thrive in harsh environments including environments with high levels of preservatives. The three species are Zygosaccharomyces bailii, Zygosaccharomyces rouxii and Zygosaccharomyces bisporus. These three species are therefore regarded as osmotolerant (El Halouat & Debevere, 1996).
Among the three species, Z. bailii possess the most distinct and diversified antimicrobial resistance characteristics. A major yeast contaminant spoiling foods and beverages such as mayonnaise, pickles, dried fruits (Verma & Joshi, 2000; Deak & Beuchat, 1996), fruit concentrates (Thomas & Davenport, 1985), soft drinks and wines (Thomas & Davenport, 1985), Z. bailii is known for its exceptional tolerance to preservatives, high sugar, low acid and pasteurisation regimes (Thomas & Davenport, 1985).
Yeast have created undesirable properties in food products including off flavours (Rodrigues et al., 2001), hazing (Rodrigues et al., 2001), emulsion breakage (Encyclopaedia of Food Microbiology 1999, p. 2362), and brown film development on surfaces of food products (Encyclopaedia of Food Microbiology 1999, p. 2362). Z. bailii is known to metabolise much aggressively compared to the other two species, Z. rouxii and Z. bisporus. This not only has caused spoilage of food but also has lead to explosion of canned and bottled foods and beverages on the shelf (Levya et al., 1998; Oda et al., 2006). In terms of Occupational Health & Safety, explosion of canned and bottled food products on the shelf are significant as there is a risk of injury (Grinbaum et al. 1994, cited in Steels et al. 1999), which may also cause the company to be exposed to contentious liability issues.
Yeast membrane transport
The purpose of yeast membrane transporters can be divided into several aspects: (i) nutrient uptakes (sugar, amino acids, and vitamins) (Belle & Andre, 2001); (ii) importing and exporting of proteins or peptides (Andre, 2004); (iii) exporting of toxic compounds from the cell to prevent any deleterious reaction (Demidchik, Macpherson & Davies, 2004). According to Munn (2000), Saccharomyces cerevisiae, the baker’s yeast, has been widely used as the model organism for the study of many cellular processes, which includes the yeast membrane transport system.
The ability of certain yeasts to grow in a low pH environment with the presence of weak-acid preservatives has inflicted heavy losses to the food and beverage industry. Macpherson et al. (2005) believes that understanding the mechanisms of this resistance is the key in developing more effective food and beverage preservation methods. El Halouat & Debevere (1996) stated in their findings that at low pH, an active transport system is induced to remove both hydrogen ions and preservative anions from the cell. Both the hydrogen ions and preservative anions in the product of the dissociation of weak-acid preservatives in the cell (Pampulha & Loureiro-Dias, 1989).
Macpherson et al. (2005) conducted a research on the weak-acid preservative resistance of both Saccharomyces cerevisiae and Zygosaccharomyces bailii. Their research aims to investigate the uptake of potassium cation (K+) by both strains on long-term exposure to benzoic acid. Macpherson et al. (2005) concluded in their findings that long-term exposure to benzoic acid leads to accumulation of potassium cation (K+) by both Saccharomyces cerevisiae and Zygosaccharomyces bailii, which improved the growth of both strains in the presence of preservative. In the presence of weak-acid preservatives, the plasma membrane H+-ATPase pumps protons out of the cell, which contributes to the inhibition of weak-acid preservatives from toxicating cells.
According to Pribylova et al. (2008), K+ is required for many physiological functions whereas a high concentration of sodium cation (Na+)is toxic for most cells. A research on the plasma membrane of salt tolerant yeast Zygosaccharomyces rouxii has shown that in order to maintain an optimal cytoplasmic concentration of K+ and a stable high intracellular ratio of K+/Na+, the yeast cells possess a transport system, which participate in both detoxication from excess sodium cations and the maintenance of potassium homeostasis and intracellular pH (Pribylova et al., 2008). As mentioned previously, K+ assists in the growth of yeast cells by stabilising their intracellular pH despite the presence of weak-acid preservatives. This is also known as the Na+/H+-antiporter system (Pribylova et al., 2008). Watanabe et al. (1999) reported that the Na+/H+-antiporter system of Zygosaccharomyces rouxii has enabled it to survive in environments with high salt concentration as the system extrudes Na+ across the plasma membrane of the cell.
Links to related articles and to the reference list of literature cited in this and/or related articles.
Physiology and biochemistry of food spoilage yeasts Blog article published on 13 February 2022.
Overall importance and general characteristics of food spoilage yeast Blog article published on 5 January 2020.