Alcoholic fermentation starter cultures

Commercially available yeast extracts are made from brewers yeast, from bakers yeast, from alcohol-grown yeast (C. utilis) and from whey grown yeast (K fragilis). Extracts are used ia fermentation media for productioa of antibiotics, ia cheese starter cultures, and ia the productioa of viaegar. They are also exteasively used ia the food iadustry as condiments to provide savory flavors for soups, gravies and bouillon cubes, and as flavor intensifiers ia cheese products. 

To limit this possibility, Torulaspora delbrueckii, in mixed or sequential culture with S. cerevisiae, has been proposed for fermentation (Bely et al., 2008 Ciani et ah, 2006 Lafon-Lafourcade et ah, 1981 Renault et ah, 2009). Although this yeast generates little alcohol, its application in mixed starter cultures is promising. It may also reduce the accumulation of other undesirable by-products, such as acetaldehyde, ethyl acetate, and acetoin. 

In many countries, alcoholic fermentation is induced by inoculation with a yeast starter culture of Saccharomyces selected for its desirable winemaking qualities (Kunkee, 1984 Kunkee and Bisson, 1993 Rainieri and Pretorius, 2000 Reed and Chen, 1978 Reed and Nagodawithana, 1988). Starter cultures of S. cerevisiae strains are generally used because of to their increased ethanol and sulfur dioxide resistance and production of desirable aromas and flavors (Boulton et al., 1996 Ebeler, 2001 Nykanen, 1986 Reed and Chen, 1978 Reed and Nagodawithana, 1988). 

MLF most commonly occurs after alcoholic fermentation but may occur simultaneously with the primary fermentation. Because relying on natural microflora can be unpredictable and difficult to control, starter cultures of pure strains of bacteria have been developed (Henick-Kling, 1993 Krieger et al., 1993 Kunkee et al., 1964 Nielsen et al., 1996 Pilone, 1995). Although selected strains of Lactobacillus can be inoculated, O. oeni is the primary species to conduct MLF because of acid tolerance and the flavor profile produced (Guzzo et al., 1994 Krieger et al., 1993 Kunkee et al., 1964 Liu, 2002 Nielsen et al., 1996 Wibowo et al., 1985). 

The yeast responsible for alcoholic fermentation in winemaking is usually introduced into the must from the surface of the grapes, the surface of winery equipment, or from specifically prepared cultures. The fermentation process can occur either naturally, without inoculation, or by inoculating the must with selected starters. The use of locally selected yeast strains (usually belonging to the species Saccharomyces cereoisiae), with strain-specific metabolic characteristics can positively affect the final quality of the wine (Regodon et al., 1997 Romano et al., 2003). Several studies have clearly shown the effects of indigenous and inoculated yeast populations on the wine volatile composition (Mateo et al., 2001 Nurgel et al, 2003). 

Techniques for growing/expanding LAB starters vary somewhat but can generally be divided into either pure culture or coculture (LAB and wine yeast) methods. In the latter case, most utilize the same yeast strain that is used to carry out alcoholic fermentation. In either case, a properly prepared and expanded LAB starter should yield a stationary-phase inoculum of >10 CFU/mL. 

One commercial LAB starter (Vinflora LP, available through Chris Hansen Laboratory) is marketed as a freeze-dried culture for prefermentation addition to juice/must. In this case, MLF is initiated and partially carried out in advance of alcoholic fermentation by the addition of a high-titer inoculum of the homofermenter Lactobacillus plantarum. Being homofermentative, any utilization of glucose at this stage yields lactic acid solely. However, the strain is relatively susceptible to alcohol thus, activity... 

Hansen and Lund, 1987). In rye bread, compounds such as alcohols, esters and carbonyls have been identified (Hansen et al., 1989). Choice of fermentation temperature, dough yield, flour quality and starter culture all influence the sensory properties of the final bread. Free amino acids formed during fermentation increase Maillard reaction products, thus intensifying the taste. Sensory analysis has shown that sour dough rye breadcrumbs had the most intense and bread-like flavour compared with chemically acidified doughs (Hansen et al., 1989). 

This technique is very discriminating and not too expensive, but it is long and requires several complex manipulations. It is useful for the subtle characterization of a small number of sfiains. Inoculation effectiveness can also be verified by this method. To verify an inoculation, a sample is taken during or towards the end of alcoholic fermentation. In the laboratory, the lees are placed in a liquid medium culture. The mtDNA restriction profile of this total biomass and of the yeast starter strain are compared. If the restriction profile of the sample has no supernumerary bands with respect to the yeast starter strain profile, the yeast starter has been properly implanted, with an accuracy of 90%. In fact, in the case of a binary mixture, the minority strain must represent around 10% of the total population to be detected (Hallet et al., 1989). 

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