Fermentation problems

Causes of sluggish (slow) and stuck (stopped) fermentations include nutritional deficiency, inhibitory substances, processing difficulties, as well as bacterial antagonism (Section 6.6.2). Although various factors can contribute to a sluggish or stuck fermentation, the exact cause (s) of a particular occurrence cannot always be identified. 

Unlike bacteria, Saccharomyces can accumulate large intracellular concentrations of amino acids. Depending upon the particular amino acid, stage of growth, and activity of necessary transport enzymes, these amino acids may be (a) directly incorporated into proteins, (b) degraded for either their nitrogen or carbon components, or (c) stored in vacuoles or cytoplasm for later use (Bisson, 1991). 

Another nutrient important for yeast is the presence of oxygen. Without some initial oxygen, fermentation can slow because Oj is needed for sterol synthesis (Section 1.4.2). Owing to the activity of grape oxidases (Section 7.5.1), the oxygen content of unsulfited musts can rapidly decrease. Aeration or pumping-over musts are methods employed by winemakers to supply additional oxygen to a deficient must, and addition of SO2 at crush can help limit the activity of oxidases. 

Must ripeness (°Brix) Yeast-assimilable nitrogen (mg N/L) 

20% sugar. Nishino et al. (1985) attributed the slower fermentation rates of high-sugar musts to increases in osmotic pressure on yeasts. 

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Zamora 2004). For that reason, aerating the grape juice is recommended during the exponential growth phase of the fermentation in order to encourage yeast to build their membranes and avoid fermentation problems (Barre et al. 1998 Ribdreau-Gayon et al. 2000b). 

In the case of P. putida KT2440, a so-called one-step fed batch can be carried out where the cells are supplied with a highly concentrated medium to limit volume increase (Sun et al. 2009). This is possible because this sfiain does not reqnire a nntrient limitation as mentioned previously. A significant problem with this strategy is the oxygen supply (Kellerhals et al. 2000). Other approaches follow more sophisticated control systems to rednce the fermentation problems and have been nicely reviewed recently (Sun et al. 2007b). 

Thus the fermentation model addressed is represented by a set of five differential equations and two algebraic equations. Table 5.1 provides the base values of the parameters used in this Z. mobilis fermentation problem (see all variables definitions and units in the Nomenclature section). 

The fermentation problem of section 4.3.2 used the old ode45 routine... 

Another potential post-alcoholic fermentation problem is Zygosaccharomyces. This yeast causes spoilage by forming gas, sediment, and/or cloudiness in bottled wines (Loureiro and Malfeito-Ferreira, 2003). Synthesis of other compounds, namely succinic, acetic, and lactic acids, as well as acetaldehyde and glycerol has also been reported (Rankine, 1967 Oura, 1977 Zeeman et al., 1982 Nykanen, 1986 Herraiz et ak, 1990 Moreno et al., 1991 Mateo etal., 1992 Lema etak, 1996). Thomas (1993) estimated that for yeasts such as Z. bailii, as few as one viable cell in a bottle of wine is sufficient for spoilage. 

As with dry white winemaking, overclarification can lead to large fermentation problems and increased acetic acid production. Must turbidity should not be as low as in dry white winemaking (100-200 NTU) 500-600 NTU or even a slightly higher turbidity is perfectly acceptable. Moreover, botrytized sweet wines are not subject to the same problems related to insufficient clarification as dry white wines—the development of reduction odors and vegetal tastes, oxidability, etc. 

Lipids present in the diet may become rancid. When fed at high (>4-6%) levels, Hpids may decrease diet acceptabiUty, increase handling problems, result in poor pellet quaUty, cause diarrhea, reduce feed intake, and decrease fiber digestion in the mmen (5). To alleviate the fiber digestion problem, calcium soaps or prilled free fatty acids have been developed to escape mminal fermentation. These fatty acids then are available for absorption from the small intestine (5). Feeding whole oilseeds also has alleviated some of the problems caused by feeding Hpids. A detailed discussion of Hpid metaboHsm by mminants can be found (16). 

A fermented-egg product (EEP), patented as an attractive bait for synanthropic flies, has been shown to be attractive to coyotes and repeUent to deer (79). Its components are variable, with relative concentrations of 77% fatty acids, 13% bases, and 10% (primarily) neutrals composed of at least 54 volatiles such as ethyl esters, dimethyl disulfide, and 2-mercaptoethanol. Synthetic formulations have been evaluated to find a replacement for a patented fermented-egg protein product that attracts coyotes and repels deer. Ten aUphatic acids (C-2 to C-8), four amines (pentyl, hexyl, heptyl, and trimethyl), dimethyl disulfide, 2-mercaptoethanol, and 54 more volatiles (C-1 to C-5 esters of C-1 to C-8 acids) have been tested as synthetic fermented egg (SEE) (80) in approximately the same proportions that are present in EEP. Weathering was a problem that caused decreased efficacy, which suggests trials of controUed-release formulations. Eourteen repeUents have been examined against white-taU deer in Peimsylvania in choice tests when treated onto sheUed com (81). 

Fermentation. Much time and effort has been spent in undertaking to find fermentation processes for vitamin C (47). One such approach is now practiced on an industrial scale, primarily in China. It is not certain, however, whether these processes will ultimately supplant the optimized Reichstein synthesis. One important problem is the instabiUty of ascorbic acid in water in the presence of oxygen it is thus highly unlikely that direct fermentation to ascorbic acid will be economically viable. The successful approaches to date involve fermentative preparation of an intermediate, which is then converted chemically to ascorbic acid. 

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