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Traditional methods for characterizing beer-spoiling yeasts

Identifying yeasts using traditional methods is a complex and inexact process. Direct microscopy is useful in determining cell structure, budding patterns, the presence of pseudohyphae, and the shape of spores (if present). This can be supplemented with analysis of phenotypic traits including fermentation of sugars, production of acids, and a range of biochemical tests to determine nutritional dependencies. The latter can be facilitated by the use of commercially available API strips (Biomerieux, France). However, although they have been widely used for analysis of clinical isolates, their use within the brewing industry to date has been limited. Other methods that are more accurate but have yet to be fully translated to the brewing industry include matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) (Usbeck et al., 2014; see Chapter 7), as well as pyrolysis mass spectroscopy and Fourier transform infrared spectroscopy (FTIR) (Timmins et al.,

1998), which allow species differentiation based on mass fingerprinting. These offer interesting opportunities since production strain data can be stored in databases, allowing quick and precise comparisons between unknown samples to be made. Other methods include profiling of total fatty acids based on determination of fatty acid methyl ester (FAME) compounds (Timke et al., 2008), protein fingerprinting (Kobi et al., 2004), and immunological-based methods, which employ species-specific monoclonal antibodies to detect organisms by enzyme-linked immunosorbent assays (ELISA) (Kuniyuki et al., 1984). In addition, techniques can be applied to characterize unknown yeasts by determining spoilage characteristics based on phenotype analysis. Examples include testing for acetic acid or

Table 11.5 Media for the detection of wild yeasts. For the majority of media types, it should be noted that other yeasts may show limited growth over an extended period of time

Method

Specificity and details

Reference

CLEN

Non-Saccharomyces wild yeast

Multinitrogen media containing cadavarine, lysine, ethylamine, and nitrate.

Supports the growth of multiple wild yeasts

Martin and Siebert (1992)

Lins Wild Yeast

Medium

(LWYM)

Saccharomyces wild yeasts

Contains crystal violet and fuschin-sulfite to suppress brewing yeast growth

Lin (1981)

Lysine

Non-Saccharomyces wild yeasts

Saccharomyces strains are unable to use lysine as a sole nitrogen source

Walters and Thiselton (1953)

MYGP + Copper

Range of wild yeasts

Production strains are inhibited by copper in the range of 100200 mg/l

Taylor and Marsh (1984)

XMACS

Range of wild yeasts

Multicarbon media containing xylose, mannitol, adonitol, cellobiose and sorbitol

De Angelo and Siebert (1987)

phenolic compounds, both of which are obvious by performing a basic sniff test. Although care should always be taken when sniffing ‘unknown' laboratory samples, by inoculating a suspected beer-spoiling yeast into media comprising ferulic acid, POF+ yeast can be detected due to the production of a medicinal-like (phenolic/clove) aroma, typical of 4-vinylguiacol (Cowley et al., 2016).

 
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