This blog article is the fourth with have published, that showcases the brilliant work of Doreen Ting from the School of Applied Sciences at RMIT University, in 2008. I had the privilege of guiding her on the preparation of this substantial piece of work, from which this is extracted from. In this article, we provide an introduction to bacteria and other types of food spoilage, then delve deeper into spoilage of fruit products. Such products, due to their origin and intrinsic conditions, tend to be, by and large, spoiled by non-bacterial organisms, almost exclusively fungi, which can be either yeasts and/or moulds. This article provides some insights into that aspect.
Bacterial vs. Non-bacterial Spoilage
Bacteria are the largest group of microorganism present while yeast is a member of the fungi kingdom. Besides spoilage caused by yeast, food may also be spoiled by several forms of bacteria such as lactic acid bacteria, acetic acid bacteria and spore-forming bacteria (Worobo & Splittstoesser, 2005). Amongst the bacteria listed, lactic acid bacteria and yeast possess similarities in terms of the role they play, both beneficial and detrimental, in the food industry. Lactic acid bacteria and yeast both act as fermentors and both are also associated with spoilage of food. According to Vermeulan et al., (2008), microbial spoilage of acidified sauces stored at ambient temperature are predominantly caused by lactic acid bacteria and yeast. Regardless, both are still very different organisms. Lactic acid bacteria are able to proliferate when exposed to a given substrate that specifically suits their nutritional requriements and under storage conditions with low gas permeability (Farag & Korashy, 2006). Yeast on the other hand, especially the genus Zygosaccharomyces, grow and survive in the presence of high concentrations of sugar (Cole & Keenan, 1986; Ludovico et al., 2002; Steels et al., 1999) and under optimum conditions, which includes low pH values (Cole & Keenan, 1986; Vermeulane et al., 2006), and low water activity level (Steels et al., 1999). Nevertheless, Levya et al. (1999) reported that different species of Zygosaccharomyces favour different types of sugar. Z. bailii was found to be fructophilic (Levya et al. 1999; Vermeulan et al., 2008), favouring fructose and therefore metabolising fructose more rapidly compared to glucose. The ability of yeast to thrive in high salt or sugar concentrations environment sets it apart from bacteria. Therefore, they are capable of spoiling certain foods, in which bacteria are not able to grow.
Spoilage caused by bacteria may result in organoleptic changes such as souring, slime formation, off-odour, off-flavours, and gas production (Farag & Korashy, 2006). This is most often seen in spoilage of raw meat by lactic acid bacteria. Yeast causes undesirable properties in food by producing off-flavours (Rodrigues et al., 2001), hazing (Rodrigues et al., 2001), emulsion breakage (dressings) (Vermeulan et al., 2008), gas production (Vermeulan et al., 2008), and ethanol production (Vermeulan et al., 2008).
Zygosaccharomyces species
The yeast genus Zygosaccharomyces is noted for its infamous relationship with food spoilage. According to Steels et al. (1999), this particular genus includes some of the most osmotolerant organisms known, yeasts which are able to vastly resist concentrations of food preservatives. The osmotolerant species, comprising of Zygosaccharomyces bailii, Zygosaccharomyces rouxii and Zygosaccharomyces bisporus are not only known for their fermentative spoilage in food with high sugar concentration, low water activity, and low pH value but also known for their heat resistance profiles (Chen & Tseng, 1996, Makdesi & Beuchat, 1996). According to Raso et al. (1998), Z. bailii has the ability to form ascospores, which are more resistant to physical and chemical agents than vegetative cells, thus are also more resistant to heat.
Food Products
Foods are considered habitats for microbial growth. Food products commonly spoiled by yeast range from fresh (raw) fruits to soft drinks. Fruit juices and soft drinks impose an adverse environment for most microorganisms. This is due to their low pH value and low nitrogen and oxygen contents (Deak & Beuchat, 1996). These beverages however have been found to be excellent substrates for growth of yeasts. Fruit juices in particular are found to be most susceptible to yeast spoilage (Keyser et al., 2007; Elez-Martinez et al., 2004; Tournas & Katsoudas, 2005). Tran & Farid (2004) stated in their research that yeast is one of the most common spoilers found in fruit juices (i.e. orange juice). Other high sugar foods such as honey, syrups, jams, jelly, and dried fruits are also susceptible to spoilage by yeast (Deak & Beuchat, 1996).
Fruit products
According to Deak & Beuchat (1996), yeast form a part of the natural microflora of most fruits, which is usually found on the external surface of fruits. Fruits are high in nutrients and moisture content. Its low pH (3 to 5) sets them apart from vegetables in terms of susceptibility to spoilage by yeast. In addition, fruits are high in soluble carbohydrates, increasing their desirability to yeast as compared to vegetables. Therefore, weak-acid preservatives are widely used in high-sugar, low pH products such as fruit juices in which spoilage are most often caused by yeast. Weak-acid preservatives commonly used in the food industry such as sorbic acid, benzoic acid, propionic acid, acetic acid, and sulphur dioxide (Martorell et al., 2006) has been found to be of little use in preserving food products from spoilage unless a significant level of preservative is added to the food (Martorell et al., 2006). However, a constraint faced in this case is the legal limitation on level of preservative addition.
Despite its benefits in reducing risk of obtaining chronic diseases, fruits are still consumed less especially by the younger generation (Ness & Powles, 1997; Steinmetz & Potter, 1996). Studies have shown that not everyone is interested in consuming the recommended dietary intake of fruits (Betts et al., 2003; Padula et al., 2003). Therefore, for many decades, health professionals have collaborated with food manufacturers in developing food products, in hopes of helping people change their dietary habits. This has lead to creation of different food products consisting of fruits, such as fruit juices and fruit yoghurt. However, due to this fast-paced world we currently live in, consumer demands has also moved to a different level. Consumers now demand fruit products, which are convenient and ready-to-eat, however, still containing sufficient nutrients for the recommended dietary intake. Consumer demand has lead to the production of minimally processed fruits. According to Alzamora et al. (2000), minimally processed fruits are products that maintain their attributes and quality similar to those of fresh products, which include dried fruits. The purpose of minimally processed fruits according to Laurila & Ahvenainen (2002) is: (i) to keep produce fresh, without losing its nutritional quality and; (ii) to ensure a product shelf life is sufficient to make distribution feasible within a region of consumption.
Dried Fruits
Dried fruits are one of the most notable minimally processed food products on the market. A high concentration of sugar, low water activity level and low pH value of dried fruits causes it to be one of the food products susceptible to yeast spoilage (El Halouat, 1996). With its high concentration of sugar and its low water activity level, dried fruits are particularly at risk of spoilage by the genus Zygosaccharomyces. As sulphur dioxide is most commonly used in preservation of dried fruits (Florin et al., 1993), some species of the genus Zygosaccharomyces have been found to be resistant to this preservative (Martorell et al., 2005; Thomas & Davenport, 1985).
Fruits are dried to reduce the water activity level to less than 25% (Worobo & Splittstoesser, 2005), as fruit product usually will not spoil when the water activity level is below 0.65 (Corry 1987, cited in Worobo & Splittstoesser 2005; Pitt & Hocking, 1985). El Halouat & Debevere (1997) found that Zygosaccharomyces rouxii were among the dominant contaminants isolated from high moisture prunes and raisins. High moisture according to Worobo & Splittstoesser (2005) is having a water activity level of at least 35%.
Links to prior articles in this series and to the reference list of literature cited in this and/or prior articles.
Significance of yeast in food spoilage and their membrane transport systems Blog article published on 19 February 2022.
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.
Food Microbiology Academy 
