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  • Chapter 10

    Yeasts

    Yeasts are fungi which reproduce vegetatively by

    means of single cells which bud, or less commonly,

    divide by fission. This property enables yeasts to

    increase rapidly in numbers in liquid environments,

    which favour the dispersal of unicellular microor-

    ganisms. Many yeasts grow readily under strictly

    anaerobic conditions, again favouring their growth

    in liquids. On the other hand, reproduction as single

    cells restricts spreading on, or penetration into,

    solid surfaces, where filamentous fungi are at an

    advantage. Being eukaryotic organisms, yeasts

    reproduce more slowly than do most bacteria, and

    hence do not compete in environments which

    favour bacteria, i.e. at pH values near neutral or at

    very high temperatures. In common with filamen-

    tous fungi, many yeasts are tolerant of acid condi-

    tions. In broad terms, then, yeasts are more likely to

    be active in acidic, liquid environments than else-

    where. However, many yeasts also appear to be

    highly resistant to sunlight and desiccation and

    occur widely in nature on the surfaces of leaves,

    fruits and vegetables.

    When defined as above, i.e. as budding, nonpho-

    tosynthetic eukaryotes, yeasts are a heterogeneous

    assembly of often quite unrelated fungi. Barnett

    et al. (2000) recognised almost 680 species of yeasts

    in all, divided into over 90 genera: 57 were classified

    as Ascomycetes, 36 as Basidiomycetes with 2

    unclassified. It is not surprising, then, that yeasts

    possess diverse properties.

    As with other fungi, yeasts are classified into

    genera primarily on the type and appearance of

    spores, in this case ascospores or basidiospores.

    Because yeasts possess limited morphological varia-

    bility, classification at species level has traditionally

    relied on biochemical tests, principally the utilisa-

    tion of various carbon or nitrogen sources, and

    vitamin requirements. Some physiological proper-

    ties, i.e. growth at high temperature and reduced

    water activity, are used in a secondary role.

    The 2000 edition of Yeasts: characteristics and

    identification listed 90 separate tests, including

    utilisation of 47 carbon sources, 10 nitrogen

    sources, vitamin requirements, fermentation pat-

    terns, growth at various temperatures and in the

    presence of elevated glucose and NaCl levels and

    morphological characteristics. Clearly, taxonomy

    of that kind is beyond the scope of the present work.

    In recent years, DNA sequencing has become a

    major classification tool and is now commonly used

    for yeast identification. Molecular methods have

    clarified some of the bases for taxonomy of yeasts,

    but at the same time have led to increasing complex-

    ity, with a seemingly random reassortment of spe-

    cies into genera with each new monograph.

    Teleomorphanamorph connections. As with

    other fungi of industrial significance, ascomycetous

    and basidiomycetous yeasts frequently reproduce in

    nature or in the laboratory as their anamorphic

    states, and hence possess anamorphic names. Dili-

    gent study has in recent years enabled yeast specia-

    lists to linkmany anamorphs with teleomorphs, and

    in many cases the anamorph name has fallen into

    disuse. As with some filamentous fungi, however, a

    case for dual nomenclature exists because establish-

    ing the anamorphteleomorph connection is often

    difficult. Sometimes too the link is tenuous, with

    only a few isolates carrying the life cycle to comple-

    tion. Dual names have been retained here for most

    species discussed.

    J.I. Pitt, A.D. Hocking, Fungi and Food Spoilage, DOI 10.1007/978-0-387-92207-2_10, Springer ScienceBusiness Media, LLC 2009

    357

  • Yeasts in foods. By comparison with many

    strictly filamentous fungi, yeasts possess limited

    biochemical pathways and quite fastidious nutri-

    tional requirements. Foodstuffs, generally substrates

    rich in the hexose sugars, minerals and vitamins,

    which many yeasts require for growth, are an ideal

    substrate for yeasts. Their association with the phyl-

    losphere of many crops ensures their presence on

    fruit and vegetables, and their entry into food proces-

    sing plants.

    Relatively few yeast species cause significant

    spoilage in processed foods, most being adventi-

    tious contaminants from natural sources. Deak

    and Beuchat (1996) list 99 yeast species which

    occur in foods including fruit, beverages, wine,

    beer, meat, dairy products, low aw products and

    low pH products. Many of these yeasts grow poorly

    if at all in properly formulated processed foods,

    as they are intolerant of reduced water activity,

    heat processing or preservatives. Even if limited

    metabolism by such yeasts does occur, it is usually

    of little consequence unless there is significant gas

    production. It is doubtful whether any yeasts pro-

    duce mycotoxins, and few produce even marginally

    unacceptable off-odours. There are, of course,

    exceptions. Certain species must be classified as

    spoilage yeasts because they possess one or more

    undesirable properties. These commonly occurring

    spoilage species form the basis for most of the sub-

    ject matter of this chapter.

    Spoilage yeasts. In our experience, only about ten

    species of yeasts are responsible for spoilage of

    foods which have been processed and packaged

    according to normal standards of good manufac-

    turing practice. These species are responsible for

    major losses of processed foods around the world

    every year. We believe that relatively simple tests

    can be used to identify them when taken in conjunc-

    tion with their spoilage habitats. These species are

    listed in Table 10.1, with their principal undesirable

    properties. Two species which do not cause spoi-

    lage, but are of widespread occurrence in foods, are

    also included in Table 10.1, and in subsequent

    descriptions in this chapter.

    It is important to note that if good manufactur-

    ing practice is neglected, i.e. if factory hygiene is

    poor, if preservatives are omitted, either deliber-

    ately or unintentionally, if pasteurising tempera-

    tures are inadequate, or filling machinery or factory

    premises are unsanitary, raw materials are of poor

    quality, brining or syruping procedures are poorly

    controlled, etc., many other adventitious yeast con-

    taminants can develop in a product. In such cases

    the following text will be of little value and neither

    will identification of the yeasts concerned. The cor-

    rect approach, and often the only recourse in such

    Table 10.1 Spoilage yeasts

    Yeast Important properties

    Brettanomyces bruxellensis Production of off-odours in beer, cider and soft drinks

    Candida krusei Preservative resistant; film formation on olives, pickles and sauces

    C. parapsilosis Lipolysis, fermentative; causes spoilage in a wide range of foods including cheese, margarines,dairy and fruit products

    Debaryomyces hansenii Growth at low water activities in foods preserved with NaCl, especially salt meat

    Kazachstania exigua Moderately preservative resistant; of common occurrence in olive brines, relevantly rarely thecause of spoilage of sauerkraut or of juices, dairy products or soft drinks

    Kloeckera apiculata Spoilage of fresh and processed fruit

    Pichia anomala Spoilage of processed fruit and yoghurts containing fruit

    P. membranaefaciens Preservative resistant; film formation on olives, pickles and sauces

    Rhodotorula mucilaginosaR. glutinis

    Common food contaminants; spoilage of dairy products; occasional spoilage of fresh fruits

    Saccharomyces cerevisiae Common food contaminant; sometimes fermentative spoilage of soft drinks; some strainspreservative resistant

    Schizosaccharomyces pombe Preservative resistant; relatively rare spoilage yeast

    Zygosaccharomyces bailii Preservative resistant; fermentative spoilage of acid, liquid preserved products such as juices,sauces, ciders and wines

    Z. bisporus Preservative resistant; properties intermediate between Z. bailii and Z. rouxii

    Z. rouxii Growth at extremely low water activities; fermentative spoilage of juice concentrates, honey,jams, confectionery, packaged dried fruits, etc.

    358 10 Yeasts

  • cases, is to pay attention to manufacturing guide-

    lines, so that this kind of problem is positively

    eliminated.

    Identification of spoilage yeasts. Considerable

    work has been carried out in recent years to develop

    simplified systems for identification of foodborne

    yeasts, as yeast identification procedures based on

    biochemical reactions are too complex and take too

    long to be of value in the food or industrial micro-

    biology laboratory. Several automated systems are

    now commonly used, as well as simpler laboratory-

    based systems, and these are discussed in some

    detail in Chapter 4. Identification using DNA

    sequencing is increasingly becoming the method of

    choice, as extensive databases such as GenBank are

    freely available for identification purposes. The

    600650 nucleotide D1/D2 region of the large sub-

    unit (26S) ribosomal DNA is the most widely tar-

    geted section of the genome, and sometimes the ITS

    region may also be used (Kurtzman et al., 2003).

    However, a word of caution. Results presented by a

    computer identif

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