Introduction
Starter cultures have a basic role: to drive the fermentation process. Concomitantly, they contribute to all the characteristics of products, as well as to their sensorial and safety characteristics. Therefore, the introduction of starter cultures has undoubtedly improved the quality of products and the standardization of the industrial process.
A very important aspect is to have a good knowledge of the metabolic properties required to improve a specific product and to select useful microbial strains. Nevertheless, the limited number of already selected and studied strains that are also able to possess highly technological properties, as well as the constant risk of bacteriophage attacks, are stimulating research into new starter strains, in order to obtain higher quality and product diversification, in response to more and more aware consumers.
General aspects of starter cultures
The production of fermented foods today is based on the use of starter cultures, for example lactic acid bacteria (LAB), which initiate fast acidification of raw material. The great advantage of starter cultures is that they can provide controlled and predictable fermentation.
Starter cultures of LAB can contribute to microbial safety or offer one or more technological, organoleptic, nutritional or health advantages. Examples are LAB that produce antimicrobial substances, sugar polymers, sweeteners, aromatic compounds, vitamins or useful enzymes, or that have probiotic properties (Leroy and De Vuyst 2004).
While starter cultures, chosen on the basis of their good safety and âfunctionalâ characteristics, can benefit the consumer, they must first be able to be manufactured under industrial conditions (Saarela et al. 2000). Safety aspects of LAB include specifications such as origin, nonâpathogenicity, certain metabolic activities (e.g. deconjugation of bile salts), toxin production, haemolytic potential, side effects in human studies (i.e. systemic infections, deleterious metabolic activities, excessive immune stimulation in susceptible individuals and gene transfer) and epidemiological surveillance of adverse incidents in consumers (postâmarket). Functional aspects can be related to viability and persistence in the gastrointestinal (GI) tract, survival at low and high pH and in the presence of bile salts, hydrophobic properties, antibiotic resistance patterns, immunomodulation, and antagonistic and antimutagenic properties. Technological aspects concern growth at different sodium chloride (NaCl) amounts, temperatures, pH values, acidifying ability and metabolism (arginin deamination, esculin hydrolysis, acetoin production) and the ability to produce adequate flavour/texture.
With regard to the effect of salting, the addition of NaCl is a common practice in most fermented dairy foods, and also affects the growth of starter bacteria. Most LAB are partially or fully inhibited by levels of NaCl higher than 5%. However, it is evident that salt tolerance is a strainâdependent characteristic, thus this criterion is important in starter selection (Powell et al. 2011).
LAB starters are primarily used because of their ability to produce lactic acid from lactose and for consequent pH reduction, leading also to important effects like inhibition of undesirable organisms, improvement of sensorial and textural properties, as well as contribution to health benefits. A major role of starter cultures in dairy production is the degradation of peptides generated by the coagulant to small peptides and amino acids. Starter cultures are also capable of degrading caseins and converting amino acids to a range of flavour compounds. However, since many of the proteolytic enzymes are intracellular, flavour development in maturing cheese also depends on the release of the enzymes from starter cultures into the cheese matrix through cell lysis. Cell lysis, and the consequent release into the cheese matrix of intracellular enzymes, particularly peptidases and amino acidâdegrading enzymes, is an important characteristic for both general protein degradation and also the control of bitterness. Autolysis results from the enzymatic degradation of the bacterial cell wall by indigenous peptidoglycan hydrolases released into the growth medium, although it is still unclear how this process is controlled in the cell. The process is highly strain dependent and is also influenced by factors such as the nutrient status of the growth medium and environmental conditions (Lortal and ChapotâChartier 2005).
Generally, in maturing cheese there is a positive relationship between the period of starter culture autolysis and the flavourâforming reactions, involving not only proteolysis but also lipolysis. Consequently, various screening assays using buffers or model cheese and milk solutions have been proposed to select highly autolytic strains for use in cheese manufacture. Lysis positively influences the ripening and flavour of the cheese, but the type of peptidases is also very important, in particular since low peptidase activities and low lytic properties produce bitter cheese. One of the most successful strategies to counteract this defect involves the use of LAB with high peptidase activities, particularly Pep N.
For these reasons, the use of good starter cultures can ensure the safety, quality and acceptability of both traditional and innovative fermented dairy products.
Types of starter cultures
In practice starter cultures may be categorized as mesophilic or thermophilic, according to the incubation and manufacturing temperatures under which they are used. Mesophilic cultures grow and produce lactic acid at optimal levels, at a moderate temperature (about 30 °C), whereas thermophilic cultures optimally function at a higher temperature (about 42 °C). Examples of mesophilic dairy starter cultures are the species Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Leuconostoc mesenteroides subsp. cremoris and Leuconostoc lactis. On the other hand, the most thermophilic LAB species are Streptococcus thermophilus, Lactobacillus delbrueckii and Lactobacillus helveticus.
Nevertheless, the most common classification of starter cultures is based on the complexity of the culture and the way it is reproduced. All starter cultures available today ar...
