ReviewLactic acid bacteria as functional starter cultures for the food fermentation industry
Introduction
A starter culture can be defined as a microbial preparation of large numbers of cells of at least one microorganism to be added to a raw material to produce a fermented food by accelerating and steering its fermentation process. The group of lactic acid bacteria (LAB) occupies a central role in these processes, and has a long and safe history of application and consumption in the production of fermented foods and beverages (Caplice & Fitzgerald, 1999, Ray, 1992, Wood, 1997, Wood & Holzapfel, 1995) (Table 1). They cause rapid acidification of the raw material through the production of organic acids, mainly lactic acid. Also, their production of acetic acid, ethanol, aroma compounds, bacteriocins, exopolysaccharides, and several enzymes is of importance. In this way they enhance shelf life and microbial safety, improve texture, and contribute to the pleasant sensory profile of the end product.
The earliest production of fermented foods was based on spontaneous fermentation due to the development of the microflora naturally present in the raw material. The quality of the end product was dependent on the microbial load and spectrum of the raw material. Spontaneous fermentation was optimised through backslopping, i.e., inoculation of the raw material with a small quantity of a previously performed successful fermentation. Hence, backslopping results in dominance of the best adapted strains. It represents a way, be it unconsciously, of using a selected starter culture to shorten the fermentation process and to reduce the risk of fermentation failure. Backslopping is still in use, for instance in the production of sauerkraut and sourdough, and particularly for products for which the microbial ecology and the precise role of successions in microbial population are not well known (Harris, 1998). Today, the production of fermented foods and beverages through spontaneous fermentation and backslopping represents a cheap and reliable preservation method in less developed countries, whereas in Western countries the large-scale production of fermented foods has become an important branch of the food industry. Moreover, the Western consumer appreciates traditionally fermented products for their outstanding gastronomic qualities.
The direct addition of selected starter cultures to raw materials has been a breakthrough in the processing of fermented foods, resulting in a high degree of control over the fermentation process and standardisation of the end product. Strains with the proper physiological and metabolic features were isolated from natural habitats or from successfully fermented products (Oberman & Libudzisz, 1998). However, some disadvantages have to be considered. In general, the initial selection of commercial starter cultures did not occur in a rational way, but was based on rapid acidification and phage resistance. These starters are not very flexible with regard to the desired properties and functionality of the end product. Originally, industrial starter cultures were maintained by daily propagation. Later, they became available as frozen concentrates and dried or lyophilised preparations, produced on an industrial scale, some of them allowing direct vat inoculation (Sandine, 1996). Because the original starter cultures were mixtures of several undefined microbes, the daily propagation probably led to shifts of the ecosystem resulting in the disappearance of certain strains. Moreover, some important metabolic traits in LAB are plasmid-encoded and there is a risk that they are lost during propagation. It is further likely that loss of genetic material occurred due to adaptation to the food matrix. The biodiversity of commercial starters has therefore become limited. This often leads to a loss of the uniqueness of the original product and the loss of the characteristics that have made the product popular (Caplice & Fitzgerald, 1999).
In contrast, the fermentation of traditional fermented foods is frequently caused by natural, wild-type LAB that originate from the raw material, the process apparatus, or the environment, and that initiate the fermentation process in the absence of an added commercial starter (Bocker et al., 1995, Weerkamp et al., 1996). Moreover, many traditional products obtain their flavour intensity from the non-starter lactic acid bacteria (NSLAB), which are not part of the normal starter flora but develop in the product, particularly during maturation, as a secondary flora (Beresford, Fitzsimons, Brennan, & Cogan, 2001). Pure cultures isolated from complex ecosystems of traditionally fermented foods exhibit a diversity of metabolic activities that diverge strongly from the ones of comparable strains used as industrial bulk starters (Klijn, Weerkamp, & de Vos, 1995). These include differences in growth rate and competitive growth behaviour in mixed cultures, adaptation to a particular substrate or raw material, antimicrobial properties, and flavour, aroma, and quality attributes. Wild strains need to withstand the competition of other microorganisms to survive in their hostile natural environment, so that they often produce antimicrobials such as bacteriocins (Ayad, Verheul, Wouters, & Smit, 2002). In addition, they are more dependent on their own biosynthetic capacity than industrial strains and harbour more amino acid converting enzymes that play a key role in flavour formation. Such findings underline the importance of the Designation of Protected Origin (DPO) of many of these products, which is crucial from an economical point of view since they contribute to the survival of small-scale fermentation plants in a world of ongoing globalisation. A recent trend exists in the isolation of wild-type strains from traditional products to be used as starter cultures in food fermentation (Beukes et al., 2001, De Vuyst et al., 2002, Hebert et al., 2000).
Section snippets
Definition
Nowadays, the consumer pays a lot of attention to the relation between food and health. As a consequence, the market for foods with health-promoting properties, so-called functional foods, has shown a remarkable growth over the last few years (Nutrition Business Journal, 2002). Also, the use of food additives is regarded as unnatural and unsafe (Ray, 1992). Yet, additives are needed to preserve food products from spoilage and to improve the organoleptic properties. The demand for a reduced use
Food preservation and safety
Chemical food additives such as nitrite, sulphite, propionic acid, sorbic acid, and benzoic acid are commonly applied in food preservation technology (Smith, 1993). As an alternative, the antimicrobial activity displayed by LAB strains may help to combat microbial contamination (Holzapfel et al., 1995, Lucke, 2000). LAB produce several natural antimicrobials, including organic acids (lactic acid, acetic acid, formic acid, phenyllactic acid, caproic acid), carbon dioxide, hydrogen peroxide,
Selection and construction of suitable strains
Selecting for strains with interesting properties to be used as new, functional starter cultures may lead to an improved fermentation process and an enhanced quality of the end product. However, as it has been shown for bacteriocin-producing LAB, the success of using functional starter cultures in a particular food is strongly strain dependent (Leroy, Verluyten, Messens, & De Vuyst, 2002). The kinetics of the applied strains have to be adapted to the process conditions and the intrinsic factors
Conclusion
Novel insights into the metabolism of LAB offer perspectives for the application of a new generation of starter cultures. Functional LAB starter cultures may offer several health, marketing, and technological advantages. They may be obtained by genetic engineering or as wild-type organisms. Bioinformatics will be available soon to search in genomes for specific genes, gene clusters or functionalities. However, fundamental and applied research is still needed to optimally implement functional
Acknowledgements
The authors acknowledge their financial support from the Research Council of the Vrije Universiteit Brussel, the Fund for Scientific Research-Flanders (FWO), the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT), in particular the STWW project ‘Functionality of novel starter cultures in traditional fermentation processes” and the GBOU project ‘Development of a fast, non-invasive technological tool to investigate the functionality and effectiveness of pro- and
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