Zygosaccharomyces bailii – азық-түлік өнімдерін бұзғыш саңырауқұлақ

Species of fungus

Zygosaccharomyces bailii is a species in the genus Zygosaccharomyces. It was initially described as Saccharomyces bailii by Lindner in 1895, but in 1983 it was reclassified as Zygosaccharomyces bailii in the work by Barnett et al. Spoilage resulting from growth of the yeast Zygosaccharomyces is widespread, which has caused significant economic losses to the food industry. Within this genus, Z. bailii is one of the most troublesome species due to its exceptional tolerance to various stressful conditions. A wide range of acidic and/or high sugar products such as fruit concentrates, wine, soft drinks, syrups, ketchup, mayonnaise, pickles, salad dressing, etc., are normally considered to be shelf stable, i. e. they readily inactivate a broad range of food borne microorganisms. However, these products are still susceptible to spoilage by Z. bailii.

Zygosaccharomyces bailii vegetative cells are usually ellipsoid, non motile and reproduced asexually by multilateral budding, i. e. the buds can arise from various sites on the cells. During the budding process, a parent cell produces a bud on its outer surface. As the bud elongates, the parent cell's nucleus divides and one nucleus migrates into the bud. Cell wall material is filled in the gap between the bud and the parent cell; eventually the bud is separated to form a daughter cell of unequal size. Z. bailii cell size varies within a range of (3.5 6.5) x (4.5 11.5) μm and the cells exist singly or in pair, rarely in short chain. It has been observed that the doubling time of this yeast is approximately 3 hours at 23 °C in yeast nitrogen base broth containing 20% (w/v) fructose (pH 4.0). In more stressful conditions, this generation time is significantly extended. Besides the asexual reproduction mode, under certain conditions (e. g. nutritional stress) Z. bailii produces sexual spores (ascospores) in a sac called ascus (plural: asci). It should be mentioned that the ascospores are rarely observed as it is difficult and may take a long time to induce their formation; besides many yeast strains lose the ability to produce ascospores on repeated sub cultures in the laboratory. For the detection of acid resistant yeasts like Z. bailii, acidified media are recommended, such as MEA or TGY with 0.5% (v/v) acetic acid added. Plating with agar media is often used for counting of yeasts, with surface spreading technique is preferable to pour plate method because the former technique gives a better recovery of cells with lower dilution errors. The common incubation conditions are aerobic atmosphere, temperature 25 °C for a period of 5 days. Nevertheless, a higher incubation temperature (30 °C) and shorter incubation time (3 days) can be applied for Z. bailii, as the yeast grows faster at this elevated temperature.

An outstanding feature of Z. bailii is its exceptional resistance to weak acid preservatives commonly used in foods and beverages, such as acetic, lactic, propionic, benzoic, sorbic acids and sulfur dioxide. In addition, it is reported that the yeast is able to tolerate high ethanol concentrations (≥ 15% (v/v)). The ranges of pH and aw for growth are wide, 2.0 7.0 and 0.80 0.99, respectively. Notably, individual cells in any Z. bailii population differ considerably in their resistance to sorbic acid, with a small fraction able to grow in preservative levels double that of the average population. In some types of food, the yeast is even able to grow in the presence of benzoic and sorbic acids at concentrations higher than those legally permitted and at pH values below the pKa of the acids. however Z. bailii can grow in soft drinks containing 0.05% (w/v) of this acid (pKa 4.8). Particularly, there is strong evidence that the resistance of Z. bailii is stimulated by the presence of multiple preservatives. Hence, the yeast can survive and defeat synergistic preservative combinations that normally provide microbiological stability to processed foods. It has been observed that the cellular acetic acid uptake was inhibited when sorbic or benzoic acid was incorporated into the culture medium. Similarly, ethanol levels up to 10% (v/v) did not adversely influence sorbic and benzoic acid resistance of the yeast at pH 4.0 5.0. have proved that in Z. bailii, ethanol plays a protective role against the negative effect of acetic acid by inhibiting the transport and accumulation of this acid intracellularly. Like other microorganisms, Z. bailii has the ability to adapt to sub inhibitory levels of a preservative, which enables the yeast to survive and grow in much higher concentrations of the preservative than before adaptation. Some studies have revealed the negligible effects of different sugars on preservative resistance of Z. bailii, e. g. comparable sorbic and benzoic acid resistance was observed regardless whether the cells were grown in culture medium containing glucose or fructose as fermentable substrates. However, the preservative resistance of the yeast is influenced by glucose level, with maximum resistance obtained at 10 20% (w/v) sugar concentrations. The highest tolerance to salt has been observed at low pH values, e. g. the maximum NaCl allowing growth was 12.5% (w/v) at pH 3.0 whereas this was only 5.0% (w/v) at pH 5.0. Moreover, the presence of either salt or sugar has a positive effect on the ability of Z. bailii to initiate growth at extreme pH levels, e. g. the yeast showed no growth at pH 2.0 in the absence of NaCl and sucrose, but grew at this pH in 2.5% (w/v) NaCl or 50% (w/v) sucrose. Most facultatively fermentative yeast species cannot grow in the complete absence of oxygen. That means limitation of oxygen availability might be useful in controlling food spoilage caused by fermentative yeasts. However, it has been observed that Z. bailii is able to grow rapidly and ferment sugar vigorously in a complex medium under strictly anaerobic condition, indicating that the nutritional requirement for anaerobic growth was met by the complex medium components. Therefore, restriction of oxygen entry into foods and beverages, which are rich in nutrients, is not a promising strategy to prevent the risk of spoilage by this yeast. Besides, Leyva et al. (1999) have reported that Z. bailii cells can retain their spoilage capability by producing a significant amount of gas even in non growing conditions (i. e. presence of sugars but absence of nitrogen source).

Different strategies have been suggested in accounting for Z. bailii resistance to weak acid preservatives, which include: (i) degradation of the acids, (ii) prevention of entry or removal of acids from the cells, (iii) alteration of the inhibitor target, or amelioration of the caused damage. which indicates the induction of a system whereby the cells can reduce the intracellular acid accumulation. According to Warth (1977), Besides, it is unlikely that an active acid extrusion alone would be sufficient to achieve an unequal acid distribution across the cell membrane. Instead, Z. bailii might have developed much more efficient ways of altering its cell membrane to limit the diffusional entry of acids into the cells. This, in turn, will dramatically reduce any need for active extrusion of protons and acid anions, thus saving a lot of energy. Indeed, Warth (1989) In addition, Cole and Keenan (1987) This is because in Z. bailii, fructose is transported by a specific high capacity system, while glucose is transported by a lower capacity system, which is partially inactivated by fructose and also accepts fructose as a substrate. The slow fermentation of sucrose is directly related to fructose metabolism. According to Pitt and Hocking (1997), Therefore, the use of sucrose as a sweetener (instead of glucose or fructose) is highly recommended in synthetic products such as soft drinks. As sugars are common components of foods and beverages, fermentation is a typical feature of the spoilage process. Principally, these sugars are converted to ethanol and CO2, causing the products to lose sweetness and acquire a distinctive alcoholic aroma along with gassiness. Besides, many secondary products are formed in small amounts, such as organic acids, esters, aldehydes, etc. Z. bailii is noted for its strong production of secondary metabolites, e. g. acetic acid, ethyl acetate and acetaldehyde. In high enough concentrations, these substances can have a dominant effect on the sensorial quality of the products. The higher resistance of Z. bailii to weak acids than S. cerevisiae can partly be explained by its ability to metabolize preservatives. It has been demonstrated that Z. bailii is able to consume acetic acid in the presence of fermentable sugars, whereas the acetate uptake and utilization systems of S. cerevisiae are all glucose repressed. In addition, Z. bailii can also oxidatively degrade sorbate and benzoate (and use these compounds as a sole carbon source), while S. cerevisiae does not have this capability.

According to Thomas and Davenport (1985), Furthermore, the spoilage by this yeast has been expanding into new food categories such as prepared mustards, fruit flavoured carbonated soft drinks containing citrus, apple and grape juice concentrates. Spoilage by Z. bailii often occurs in acidic shelf stable foods, which rely upon the combined effects of acidity (e. g. vinegar), salt and sugar to suppress microbial growth. The spoiled foods usually display sensorial changes that can be easily recognized by consumers, thus resulting in significant economic losses due to consumers' complaints or product recalls