Bacteriophages

Bacteriophages (or phages, for short) are arguably the number one problem in cheese pro-duction.They are certainly the main reason why starter culture activity is sometimes inhibited. Infection of starter cultures by phages may result in slow or sluggish fermentations; occasionally the fermentation may fail completely. Phage infections ultimately can lead to prolonged make times and inferior or poor quality cheese, both of which cause substantial economic hardships on the manufacturer. Although phages were once thought to be primarily a problem for cheeses made using mesophilic lactococcal cultures, it is now recognized that thermophilic cultures (especially S. thermophilus) are also susceptible to attack by phages.

As discussed in Chapter 3, classification of lactic phages is based mainly on morphological criteria (i.e., head shape and size and tail

Box 5-8. Adding Flavor With Adjunct Cultures (Continued)

ing cheese aging.The aminopeptidases released by cell lysis are then able to degrade the bitter peptides that accumulate in cheese as a result of partial casein hydrolysis by chymosin and starter culture proteinases. Thus, the cheese becomes less bitter, and the released amino acids can be metabolized to yield desirable end products.

The latter point deserves emphasis, because de-bittering is not the only function of culture adjuncts.There appears to be an association, for example, between strains with high glutamate dehydrogenase activity (which forms a-ketoglutarate from glutamate) and formation of desirable cheese flavor and aroma (Tanous et al., 2002). Similarly, lactic acid bacteria that metabolize glutathione release cysteine into the cheese, which serves as precursor for sulfur-containing flavor compounds.

Recently, the use of non-lactic acid bacteria as cultures adjuncts has been proposed, based largely on the ability of these organisms to metabolize amino acids and generate various flavor compounds (Weimer et al., 1999). Specifically, strains of Brevibacterium linens (the organism used for Limburger and other surface-ripened cheese) have been reported to enhance flavor development via production of sulfur-containing volatiles, such as methanethiol, and fatty acids, such as isovaleric (Ganesan et al., 2004).

References

El Soda, M., S.A. Madkor, and P.S.Tong. 2000.Adjunct cultures: recent developments and potential significance to the cheese industry. J. Dairy Sci. 83:609-619. Fenelon, M.A.,T.P. Beresford, and T.P. Guinee. 2002. Comparison of different bacterial culture systems for the production of reduced-fat Cheddar cheese. Int.J. Dairy Technol. 55:194-203. Ganesan, B., K. Seefeldt, and B.C.Weimer. 2004. Fatty acid production from amino acids and a-keto acids by

Brevibacterium linens BL2.Appl. Environ. Microbiol. 70:6385-6393. McGugan W.A., D.B. Emmons, and E. Larmond. 1979. Influence of volatile and nonvolatile fractions on intensity of Cheddar cheese flavor. J. Dairy Sci. 62:398-403. Midje, D.L., E.D. Bastian, H.A. Morris, F.B. Martin,T. Bridgeman, and Z.M.Vickers. 2000. Flavor enhancement of reduced fat Cheddar cheese using an integrated culturing system. J.Agric. Food Chem. 48:1630-1636. Reiter, B.,T.F. Fryer,A. Pickering, H.R. Chapman, R.C. Lawrence, and M.E. Sharpe. 1967.The effect of microbial flora on the flavor and free fatty acid composition of Cheddar cheese. J. Dairy Res. 34:257-272. Tanous, C.,A. Kieronczyk, S. Helinck, E. Chambellon, and M.Yvon. 2002. Glutamate dehydrogenase activity: a major criterion for the selection of flavour-producing lactic acid bacteria.Antonie van Leeuwenhoek 82:271-278.

Weimer, B., K. Seefeldt, and B. Dias. 1999. Sulfur metabolism in bacteria associated with cheese.Antonie van Leeuwenhoek 76:247-261.

length). Importantly, they vary widely with respect to host range and virulence. Some phages have a narrow host range, infecting only a few host strains, whereas others are able to attack many different strains. Although temperate phages that lysogenize their hosts may occasionally become virulent, lytic phages are, by far, the more serious.

Following infection, lytic phages may replicate and release as many as 100 or more new infective phage particles from a single host cell within thirty to forty minutes. Even if the initial phage concentration in the milk is low, phage replication may be so rapid that enough phages are produced to significantly inhibit or decimate the starter culture before that culture does its job.In the example shown in Table 5-3, a single phage with a latent period of thirty minutes and a burst size of fifty will outnumber even a fast-growing culture in less than three hours. In a cheese operation, that means the curd pH may never reach the desired level, leaving the manufacturer with a "dead" vat and a huge headache. Phages that are more virulent

1This simulated fermentation profile is based on the following assumptions:

Culture generation time = 0.5 hours

Phage replication time = 0.5 hours

Final desired pH after 3 hours = 5.2

Average phage burst size = 50

Table 5.3. The phage problem during industrial cheese fermentations1.

Time

Phage absent

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