Yeasts as adjunct starters in matured Cheddar cheese

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Abstract

Debaryomyces hansenii and Yarrowia lipolytica are typical foodborne yeast species frequently associated with dairy products and capable of predominating the yeast composition in such systems. The two species fulfil a number of criteria to be regarded as co-starters for cheesemaking. They are known for their proteolytic and lipolytic activity as well as their compatibility and stimulating action with the lactic acid starter cultures when co-inoculated. Recent studies indicated that yeasts could be included as part of starter cultures for the manufacturing of cheese, enhancing flavour development during the maturation. The potential of D. hansenii and Y. lipolytica as agents for accelerated ripening of matured Cheddar cheese has been evaluated during four cheese treatments. The interaction between the two yeast species and the lactic acid bacteria was surveyed incorporating (i) D. hansenii, (ii) Y. lipolytica, (iii) both species as adjuncts to the starter culture and (iv) a control cheese without any additions for the production of matured Cheddar cheese. The physical and chemical properties of the cheeses were monitored in order to evaluate the contribution of the yeasts to cheese maturation. The yeasts grew in association with the lactic acid bacteria without any inhibition. The yeasts species when individually added contributed to the development of bitter flavours despite accelerated development of strong Cheddar flavours. When both species were incorporated as part of the starter culture, the cheese, however, had a good strong flavour after a reduced ripening period. The cheese retained this good flavour and aroma after 9 months of production. The simultaneous application of D. hansenii and Y. lipolytica as part of the starter culture for the production of matured Cheddar cheese is proposed.

Introduction

Reports on the occurrence of yeasts in cheeses date back to the early part of this century, but it is still not widely appreciated that yeasts can be an important component of many, if not all, cheese varieties Fleet and Mian, 1987, Walker, 1988, Devoyod, 1990, Fleet, 1990. High numbers of yeasts are frequently observed in cheeses and are believed to make a significant contribution to the maturation process Viljoen and Greyling, 1995, Welthagen and Viljoen, 1998, Welthagen and Viljoen, 1999. Their occurrence may be attributed to the yeast's ability to grow at low temperatures, the assimilation/fermentation of lactose, the assimilation of organic acids like succinic, lactic and citric acid, their proteolytic and lipolytic activities, resistance against high salt concentrations and resistance to cleaning compounds and sanitizers Fleet, 1990, Fleet, 1992, Jakobsen and Narvhus, 1996, Laubscher and Viljoen, 1999a. Furthermore, yeasts have the ability to tolerate low pH and water activity values Wyder and Puhan, 1999a, Wyder and Puhan, 1999b.

Yeasts frequently occur in dairy products. They can either cause spoilage or effect desirable biochemical changes Seiler and Busse, 1990, Eliskases-Lechner, 1998. Yeasts are involved in the ripening process of cheese and partake in microbial interactions and contribute to texture changes and the biosynthesis of aromatic compounds like volatile acids and carbonyl compounds Fleet and Mian, 1987, Roostita and Fleet, 1996, Rossi et al., 1998, Welthagen and Viljoen, 1999. Due to features such as high proteolytic and lipolytic activities, some yeast species play an important role in the formation of aroma precursors such as amino acids, fatty acids and esters (Lenoir, 1984). They can inhibit undesired microorganisms (Kaminarides and Laskos, 1992) and excrete growth factors like B-vitamins, pantothenic acid, niacin, riboflavin and biotin Purko et al., 1951, Lenoir, 1984, Fleet, 1990, Jakobsen and Narvhus, 1996. The main contribution of yeasts to the cheese maturation process is the utilisation of lactic acid which in turn increases the pH and therefore favouring bacterial growth and initiates the second stage of cheese ripening Fleet, 1990, Rhom et al., 1992.

The possibility of using Debaryomyces hansenii and Yarrowia lipolytica as starter cultures for cheese production was proposed due to their positive attributes to cheese ripening Devoyod, 1990, Fleet, 1990, Suzzi et al., 2001. D. hansenii, the perfect form of Candida famata, predominated in most studies of yeasts associated with dairy products (Walker and Ayres, 1970, Seiler and Busse, 1990, Eliskases-Lechner, 1998, Welthagen and Viljoen, 1998, Wyder and Puhan, 1999a. The reasons for the high numbers of D. hansenii in cheeses are due to the species' ability to grow at low temperatures (Davenport, 1980), high salt concentrations (Mrak and Bonar, 1939), low aw values (Tilbury, 1980) and also to their lipolytic and proteolytic activity Fleet and Mian, 1987, Wyder and Puhan, 1999a. D. hansenii also inhibits the germination of undesired microorganisms, like Clostridium butyricum and C. tyrobutyricum in cheese brines (Fatichenti et al., 1983). Furthermore, a synergistic effect between lactic acid bacteria and D. hansenii, with a resulting prolonged survival in the cheese of the lactic acid bacteria, has been reported by Yamauchi et al. (1975). Welthagen and Viljoen (1998) and Laubscher and Viljoen (1999b) suggested further research on the possibility of including D. hansenii as part of cheese starter cultures due to its great resistance against high salt concentrations, low temperatures and ability to multiply in cheese systems.

Y. lipolytica occurs frequently in milk products and the species has the ability to predominate over the naturally occurring yeasts (Guerzoni et al., 1998). Its compatibility with starter cultures and possible stimulating action when co-inoculated have been indicated by Guerzoni et al. (1998) and Van den Tempel and Jakobsen (2000). The species is known for its strong proteolytic and lipolytic activity Roostita and Fleet, 1996, Wyder and Puhan, 1999b, Guerzoni et al., 2001, Suzzi et al., 2001. In a study by Wyder and Puhan (1999a), Y. lipolytica was indicated as the species with the strongest proteolytic activity. This yeast is considered as the most predominant species contributing to lipolytic activity (Choisy et al., 1987). It was possible to accelerate cheese ripening and to improve the quality of cheese by the addition of this yeast species Lenoir et al., 1985, Devoyod, 1990.

Debaryomyces hansenii and Y. lipolytica have been regarded as good candidates for ripening agents in cheese (Guerzoni et al., 1998) fulfilling specific criteria to be regarded as co-starters for cheesemaking Guerzoni et al., 2001, Suzzi et al., 2001. However, despite the frequent references to the presence of yeasts and the lactic acid starter cultures in cheese, few studies refer to these interactions. Ripening studies at laboratory level with a model system, for example, with cheese curd slurries, are usually not sufficient to reproduce the conditions in the cheese and can only be seen as a preliminary approach (Wyder et al., 1999). It is always necessary to perform cheese treatments at a pilot plant level and therefore, in this study, we endeavoured to study the interaction between the two yeast species and the lactic acid bacteria by incorporating (i) D. hansenii, (ii) Y. lipolytica and (iii) both these species together as part of the starter culture for the production of matured Cheddar cheese. The physical and chemical properties of the cheeses were monitored in order to evaluate the contribution of the yeasts to cheese ripening.

Section snippets

Starter culture preparation

The D. hansenii culture (Product LAF3) was obtained from Chr. Hansen (Hørsholm, Denmark). Two units (each comprising of 2×109 cells) of this culture were incorporated together with the starter culture for the production of matured Cheddar cheese as recommended by the manufacturer. A strain of Y. lipolytica, previously isolated from Cheddar cheese (Welthagen and Viljoen, 1999), was cultured in 400 ml YM-broth (Wickerham, 1951) under agitation at 30 °C for 96 h. Cells were counted by means of a

Microbial populations

Yeasts and other microorganisms present in dairy products may interact in different ways. The yeasts may inhibit or eliminate undesired microorganisms that cause quality defects or potential pathogenic characteristics. They may inhibit the starter culture or contribute positively to the maturation process by supporting the function of the starter culture (Jakobsen and Narvhus, 1996). Studies on the yeast diversity associated with Cheddar and Gouda cheeses by Welthagen and Viljoen, 1998,

Conclusions

D. hansenii and Y. lipolytica grew and competed with other naturally occurring yeasts in the cheese and with the starter bacteria without any inhibition of the starter culture. The species contributed to the accelerated development of a strong Cheddar flavour, although bitter and fruity flavours were detected when the yeasts were inoculated individually. When both species were incorporated as part of the starter culture, the cheese had a good strong flavour after a ripening period of 4 months.

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