Impact of NSLAB on cheese quality

The impact of NSLAB on cheese quality has been the subject of much investigation. The approach taken in this research has (i) endeavoured to control the entry and growth of NSLAB in cheese and (ii) added a selected strain in an effort to ensure that these strains dominate cheese during ripening and can thus be associated with any subsequent impact on quality. This research is complicated by the fact that NSLAB are adventitious microorganisms and few studies have succeeded in gaining full control of the population during extended ripening.

Many studies have used stringent hygienic approaches to control NSLAB entry to cheese, in particular in laboratory and pilot-scale trials. Such approaches are successful for controlling leuconostocs and enterococci. However; control of non-starter lactobacilli has proved more difficult. The fact that non-starter lactobacilli are ubiquitous microorganisms and can readily grow in cheese presents particular hurdles to this type of research. The approaches involved sterilisation of all cheesemaking utensils prior to cheese manufacture, more stringent milk pasteurisation procedures and attempts to avoid contamination from the environment during manufacture. Interpretation of the outcome of this research is complicated by the fact that full control of wild non-starter lactobacilli was rarely achieved; however, many of the studies concluded that non-starter lactobacilli have a role to play in cheese quality.

Control of non-starter flora through the addition of antibiotics to the cheese curd at the end of the manufacturing process or through the use of bacteriocin-producing starter cultures has also been attempted. In studies with antibiotics, growth of non-starter lactobacilli was greatly reduced, though not totally inhibited. While the overall patterns of proteolysis, considered an important quality indicator, in the cheese was not impacted upon by the antibiotic treatment, lower levels of amino acids were reported in cheese containing antibiotics. Bacteriocins produced by the starter culture within the cheese, are an alternative to antibiotics to control growth of secondary flora. The broad spectrum bacteriocin, lacticin 3147, can be produced by starter cultures in cheese during manufacture and has the capacity to inhibit growth of non-starter lactobacilli. As with the use of antibiotics, non-starter lactobacilli grew in cheese containing lacticin 3147 albeit at reduced rates and final population size. In these studies little impact of reduced non-starter Lactobacillus populations was noted.

Control of NSLAB through manipulation of the ripening temperature has also been attempted. In all such studies to date while reducing the ripening temperature inhibits growth of non-starter lactobacilli, no cheese was maintained free of lactobacilli using this technology, even when the temperature was reduced to 10C.

The effect of adding adjunct cultures [18] of non-starter lactobacilli to milk for cheesemaking has been studied for several decades. The results of these studies are equivocal, with some studies showing positive effects while others report negative effects on flavour development. The reason for the equivocal nature of these findings probably results from the potential of the isolates selected to generate flavour combined with growth of adventitious strains during ripening. However, most of the more recent studies on this topic have indicated that non-starter Lactobacillus adjuncts exert a positive effect on flavour and may help to control some defects such as gas production by heterofermentative lactobacilli during ripening.

The potential impact of pediococci on cheese quality has not been well studied, due primarily to the infrequency with which they have been encountered. Enzymes, including protease, peptidase and lipase activities [23], with potential to promote cheese ripening have been identified in various strains. Further investigation regarding the prevalence of pediococci and their growth and survival during cheese ripening is required prior to defining their role, if any, in flavour development.

The impact of enterococci on cheese quality is also unclear; indeed, their presence in cheese may be considered an indicator of insufficient sanitary conditions during the production and processing of milk. While most strains studied have relatively low extracellular proteolytic activity, many have significant lipolytic activities in addition to the capacity to metabolise citrate, which can result in the production of cheese flavour compounds.

The potential of leuconostocs to impact on cheese ripening is well documented. They produce acetate, CO2, diacetyl, acetoin and 2,3-butanediol resulting from citrate metabolism. The CO2 produced is responsible for small eye formation in Dutch cheeses such as Edam and Gouda, while diacetyl and acetate contribute to the flavour of products such as Quarg, Fromage Frais and Cottage cheese. They also contain intercellular proteolytic and esterase activities which are likely to contribute to the ripening process.

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