Starter Culture Functions
Currently, food industries face the challenge of meeting the demand of consumers for safe food with functional effects (e.g. probiotics), with long shelf life, which are minimally processed and produced in the concept of a clean label. Thus, the use of compounds produced by these microorganisms can provide these features to the fermented food with an attractive technology option to replace the use of additives.
Some fermentations require the use of SC composed of different microorganisms with different growth requirements. In such operations, associative action by components in the starter mixtures may be desired, and in other cases, the conditions need to be manipulated to curtail the growth and activity of one component, but favour the other or promote a balanced growth and activity of both components. All these events have to be carefully controlled to obtain consistently superior products.
The native or wild cultures are those present in foods and exhibit interesting characteristics, since they survive adverse environment conditions and predominate in this ecosystem. In order to survive in the environment, they need to resist in competition with other microorganisms, generally by the production of antimicrobial substances. They are cultures with greater ability to colonize a matrix of origin. Several studies have also reported the production of enzymes, flavouring compounds, vitamins and many physiological biomodulation therapeutic effects in humans (e.g. production of probiotic bioactive peptides) in isolated native cultures of different foods.
The cultures that produce antimicrobial metabolites may be employed in order to improve the preservation of food. Such substances are recognized, for example, as hydrogen peroxide, organic acids and bacteriocins. The use of bacteriocins as natural substitutes for chemical preservatives in foods has been extensively studied, since they are a biopreservative effective in the control of pathogenic and spoilage microorganisms (Pratush et al. 2012).
The basic role of dairy SC composed of LAB is to drive the fermentation process. They are applied in the production of a variety of dairy products (cheese, fermented milks and cream butter). A SC can provide more control and predictability in milk fermentation. The LAB SC are used because they are able to produce lactic acid from lactose. The pH decrease during the fermentation affects a number of aspects of the process: quality, texture and composition. The other important function of LAB SC is the production of aroma compounds which determine the specific identity of the cultured dairy product.
Yogurt manufacture exemplifies a process where synergism between two different starter components is desired. In cultured buttermilk, the conditions are manipulated to prevent dominance by acid-producing bacteria, so that the flavour-producing components in the starter mixture can function, assuring a balanced growth and activity of both components. Typically, the culture is a combination of microorganisms such as, for example, in the case of yogurt, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. The most important characteristics for yogurt cultures are rapid acidification, production of characteristic balanced flavour and ability to produce the desired texture. Excessively rapid acidification can result in overacidification and a harsh flavour. Acidification of yogurt is controlled by refrigeration, but the culture may continue to acidify slowly at cold temperatures.
Microorganisms are an essential component of all natural cheese varieties and play important roles during both cheese manufacture and ripening. The microflora of cheese may be divided in two groups: starter LAB and secondary microorganisms. The first are involved in acid production during manufacture and contribute to the ripening process, while the latter do not contribute to acid production during manufacture, but generally play a significant role during ripening (De Dea Lindner et al. 2008). While the manufacture of cheese can frequently involve fermentation by naturally present non starter microflora, there are many cheeses in which SC are used. Nowadays, multiplex real time PCR (mRT-PCR) is useful to rapidly screen microbial composition of SC for cheeses and to compare samples with potentially different technological properties. In the study of Bottari et al. (2013), the developed mRT-PCR was found to be effective for analysing species present in the samples with an average sensitivity down to less than 600 copies of DNA and, therefore, sensitive enough to detect even minor LAB community members of thermophilic SC. This type of study can add to a major increase in understanding that the SC contribution to cheese ecosystem could be harnessed to control cheese ripening and flavour formation.
The yeasts associated with the Kefir grains generate the needed alcohol and carbon dioxide in Kefir that provide essential flavour notes. In dahi, yeasts acquired through chance contamination and carried over by back-slopping practice are attributable to the yeastiness. The SC used in Viili contain a mould, Geotrichum candidum, which forms a layer or mat on the surface of the product (aerobic growth). The mould metabolizes lactic acid, induces a “layered mildness” to the product and also imparts a “musty” aroma.
A controlled coffee fermentation process by the use of SC may guarantee a standardized quality and reduce the economic loss for the producer. Only few studies have been reported the use of SC for coffee fermentation, although the attempt to control coffee fermentation has existed for over 40 years. Early studies reported the use of residual waters from a previous fermentation as starter. Only in 1966, a study conducted by Agate and Bhat (1966) effectively introduced a SC for coffee fermentation. They demonstrated that the incorporation of a mixture of three pure cultures of the Saccharomyces species accelerates the process of modifying flavours. In this case, an enzyme preparation from the Saccharomyces species was observed to hasten the mucilage-layer degradation. Evangelista et al. (2014) evaluated the use of yeasts as SC in coffee semi-dry processing. Arabica coffee was inoculated with one of the following starter cultures: Saccharomyces cerevisiae UFLA
YCN727, Saccharomyces cerevisiae UFLA YCN724, Candida parapsilosis UFLA YCN448 and Pichia guilliermondii UFLA YCN731. A total of 47 different volatiles compounds have been identified. The coffee inoculated with yeast had a caramel flavour that was not detected in the control. They cited that the use of SC during coffee fermentation is an interesting alternative for obtaining a beverage quality with distinctive flavour.
Cocoa beans (Theobroma cacao L.) are the raw material for chocolate production. Fermentation of cocoa pulp by microorganisms is necessary to develop chocolate flavour precursors. Yeasts conduct an alcoholic fermentation within the bean pulp that is essential for the production of good quality beans, giving typical chocolate features. However, the roles of bacteria such as LAB and acetic acid bacteria (AAB) in contributing to the quality of cocoa bean and chocolate are not fully understood. Ho et al. (2015), using controlled laboratory fermentations, investigated the contribution of LAB to cocoa bean fermentation. The yeasts Hanseniaspora guilliermondii, Pichia kudriavzevii, Kluyveromyces marxianus and Saccharomyces cerevisiae, the LAB Lactobacillusplantarum, Lactobacilluspentosus and Lactobacillus fermentum and the AAB Acetobacter pasteurianus and Gluconobacterfrateurii were the major species found in control fermentations. They demonstrated that beans fermented in the presence or absence of LAB were fully fermented, had similar shell weights, and gave acceptable chocolates with no differences in sensory rankings. The conclusion was that LAB may not be necessary for successful cocoa fermentation.
In wine production, the contribution of yeasts has also been studied. High alcohol concentrations reduce the complexity of wine sensory properties. Wine industry is actively seeking technologies that facilitate the production of wines with lower alcohol content. However, commercially available wine yeasts produce very similar ethanol yields. Contreras et al. (2015), in order to achieve this aim, used yeast strains which are less efficient at transforming grape sugars into ethanol. Non-conventional yeasts, in particular non-Saccharomyces species, have shown potential for producing wines with lower alcohol content. These yeasts are present in the early stages of fermentation and in general are not capable of completing alcoholic fermentation. Among 48 non-Saccharomyces strains evaluated, two (Torulaspora delbrueckii AWRI1152 and Zygosaccharomyces bailii AWRI1578) enabled the production of wine with reduced ethanol concentration under limited aerobic conditions.