This is the final article in our series of fungal spoilage of food products, originally written by Doreen Ting from RMIT University in 2008. As her project facilitator, I have made some changes to her piece below, but it is largely as per her original submission.

Source of Spoilage Yeast

Food may be susceptible to spoilage with fungal contamination from a variety of sources. This may be from the environment (for example soil, air, and water) or from poor manufacturing process while intrinsic characteristics of the food (such as pH, water activity, and sugar concentration) can increase the likelihood of growth of these fungal contaminants (Ray, 2004; Tournas & Katsoudas, 2005). The most commonly food spoiled by yeast, as mentioned earlier are fruits. Fruits, as received at processing plants, are often contaminated with large amounts of yeasts. In some cases, fruits that have been damaged before reaching processing plants, due to poor handling or damaged by birds or insects may contain significantly higher yeast populations (Tournas & Katsoudas, 2005). Due to the exposed tissues of these damaged fruits, yeasts are introduced, thus enabling them to use the naturally occurring sugar and nutrients in the fruit to support their growth. 

     Physiological characteristic of the food itself may also encourage the growth of yeast thus leading to spoilage. This may be due to several reasons, which includes: (i) pH value of food; (ii) water activity; and (iii) sugar concentration. Favourable growth conditions for yeast are in foods with low pH values (Martorell et al., 2006; Praphailong & Fleet, 1997;). As mentioned earlier, several species of the yeast genus Zygosaccharomyces, were reported growing at a water activity level down to 0.62 (Frazier & Westhoff, 2005; Martorell et al., 2004) and some at a pH value as low as 2.5 (Steels et al., 1999). It is reported that food with a high concentration of sugars are also particularly at risk besides food with low pH (Tilbury 1980, cited in Steels et al. 1999; Thomas & Davenport, 1985; Tournas & Katsoudas; 2005).  As stated by Tournas & Katsoudas (2005), fruits in general contain high concentration of sugar favourable to yeast, which was mentioned earlier (i.e. glucose and fructose), the ideal water activity level with their low pH level thus making them particularly susceptible to spoilage by yeast. 

      Poor manufacturing process may also contribute to spoilage of foods. Processing of food in a non-hygienic environment may lead to contamination of the food and thus leading to spoilage. Food produced in a non-hygienic environment introduces hazards, which can adversely affect the stability of the food product (Vasconcellos, 2004). Introduction of food spoilers at the very start of production increases the risk of food spoilage. Although most spoilers may be killed during processing especially during heat treatment, some yeast strains have the ability to survive in heat-stressed environment (Raso et al., 1998; Tran & Farid, 2004).  

     As stated previously, the low pH of many fruits is the major factor that places them at risk of spoilage by yeast. Despite having most yeasts removed during early stages of processing (i.e. washing and peeling), recontamination can subsequently occur wherever there is opportunity for yeast to growth in later processing stages (Worobo & Splittstoesser, 2005). The use of contaminated equipments for processing (i.e. conveyor belts, slicers, fillers) are potential sources of contamination. Research conducted by Martorell et al. (2007) on yeast strains isolated from a high sugar environment (i.e. candied fruits and nougats) demonstrated resistance to preservatives however, did not show exceptional resistance to biocides such as peracetic acid and hypochlorite. The authors indicated that yeast spoilers might best be prevented through the use of biocidal agents in the factory rather than treating the food with preservatives. Food manufacturers are required to comply with the Codex Alimentarius: Recommended International Code of Practice – General Principles of Food Hygiene (Codex Alimentarius, 2003). In Australia, food manufacturers are required to operate in accordance with the Food Standards Australia New Zealand, which aims to control food safety hazards during production, manufacture, and handling of food (FSANZ, 2008). 

     According to Tapia and Welti-Chanes (2002), Good Manufacturing Practice (GMP) and Hazard Analysis Critical Control Point (HACCP) systems are important tools used by the food industry for production of microbiologically safe food. Good Manufacturing Practice (GMP) involves general principles of hygiene whereas the Hazard Analysis Critical Control Point (HACCP) system involves identification of possible hazards and preventative measures necessary for the control of hazards (Vasconcellos, 2004).

Control/Prevention Methods

According to Loureiro (2000), the consequence of food spoilage is a severe economical loss to the food industry because of the large scale at which food products are made. Due to its detrimental effects to the food industry, food microbiologists have continued to research on various methods in terms of controlling the growth of yeasts. As stated by van der Vossen & Hofstra (1996), it is advantageous to know the identity of the spoilage organism present in the product before designing strategies to prevent spoilage. Amongst the method used for identification and characterisation of spoilage yeast are: (i) analysis of long-chain fatty acids (Loureiro, 2000), and (ii) DNA based identification technology including polymerase chain reaction (PCR) (Casey & Dobson, 2003; Rawsthorne & Phister, 2006; Renard et al., 2008; van der Vossen & Hofstra, 1996) (iii) monitoring of isoenzymes patterns (Duarte et al., 2004; Loureiro, 2000).

     As mentioned earlier in the review, several yeast species are able to survive in stressed environments, which include presence of weak-acid preservatives. One species in particular known for its exceptional ability to resist weak-acid preservatives is Zygosaccharomyces bailii (Andrews et al., 1997; Steels et al., 1999; Thomas & Davenport, 1985). Inactivation of this particular species was found only in environments with high levels of preservatives (El Halouat et al., 1998). However, due to legal limitations, researchers opted for combination of hurdles to achieve this. Though the mechanism of this has not been thoroughly understood, researchers have continued to carry out experiments on a combination of hurdles in preventing the growth of yeast in food. Weak-acid preservatives used along with pH adjustments (Cole & Keenan, 1986; Nielsen & Arneborg, 2007; Quintas et al., 2004), use of heat with combination of high hydrostatic pressure and pulsed electric field (Raso et al., 1998), and use of different type and amount of carbon source (Levya et al., 1999; Merico et al., 2003) were amongst the methods used in controlling the growth of yeast. 

     Nevertheless, there has been increasing demand for foods with reduced amounts of chemical additives and less physical damage (Palou et al., 1997), in other words minimally processed food. This has lead researchers to investigate the use of modified atmospheric packaging as another alternative in preserving food products from yeast contaminants. Lucas (2003) view modified atmospheric packaging as a technique of shelf-life extension, which involves a mixture of gases including oxygen, carbon dioxide, and nitrogen. Restuccia et al., (2006) reported that modified atmospheric packaging in combination with hurdles (i.e. temperature), work synergistically in maintaining freshness, extending shelf life and ensuring the safety of food. El Halouat & Debevere (1996), who conducted a research on the influence of modified atmosphere and preservatives on the growth of Zygosaccharomyces rouxii isolated from dried fruits, reported on the synergistic effects of inhibiting factors such as low water activity level, preservatives and carbon dioxide. El Halouat et al. (1998) also found that modified atmospheric packaging enhanced the inhibitory effects of weak-acid preservatives used in preventing growth of yeast. 

Links to prior articles in this series and to the reference list of literature cited in this and/or prior articles.

Food spoilage, with special reference to fruit products Blog article published on 21 April 2022.

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 spoilage yeast reference list