Fermentation threshold values

Table 10.1 gives a summary of the main by-products of fermentation by yeasts and other microbiological activities which can be identified in distilled spirits from different raw materials, like fruits, wine, grain, sugar cane, or other carbohydrate-containing plants. Since the sensory relevance of a flavour compound is related to its odour thresholds and odour quality. Table 10.1 presents also odour qualities and a review of threshold values of the fermentation by-products in ethanol solutions [9-10] and/or water [11-14] (Christoph and Bauer-Christoph 2006, unpublished results). 

Table 10.1 Odour quality, odour threshold value in water and/or ethanol solution, and concentration range of single volatile compounds in distilled spirits produced during alcoholic fermentation from carbohydrates by yeasts and other microorganisms ...

To estimate the sensory contribution of the 42 odorants to the overall flavor of the wine samples, their OAV s were calculated (Table II). To take into account the influence of ethanol, the odor threshold values of wine odorants were determined in a mixture of water/ethanol (9+1, w/w) and were used to calculate the OAV s for each compound. According to the results in Table A, 4-mercapto-4-methylpentan-2-one, ethyl octanoate, ethyl hexanoate, 3-methylbutyl acetate, ethyl isobutyrate, (E)-fi-damascenone, linalool, cis rose oxide and wine lactone showed the highest OAV s in the Scheurebe wine. With exception of 4-mercapto-4-methylpentan-2-one the above mentioned odorants also showed the highest OAV s in Gewurztraminer wine. Differences in the OAV s of ethyl octanoate, ethyl hexanoate, 3-methylbutyl acetate and ethyl isobutyrate between the two varieties are probably caused by differences in the maturity of the fruit at harvest and/or by the fermentation process. 

The Oil of Javanicus was of high purity and passed all toxicity tests It was superior in safety evaluations to conventional plant oils, which always contain very small residual amounts of pesticides, insecticides, and herbicides from the various sprays that are used on such crops. These levels, though, are always below the recommended threshold values laid down by regulatory authorities. Being cultivated in fermenters, M. circinelloides does not, of course, need to be sprayed with any chemical pesticide, herbicide, or fungicide. 

So far, chemical processes have been discussed as well as fermentation reactors. These systems can be characterized by the fact that there are usually no discontinuities in the model equations. In physiological systems, however, there is often a threshold value that has to be succeeded before certain phenomena take place. This causes a threshold non-linearity in the model and therefore these systems can be analyzed well through simulation but theoretically they are difficult to analyze. It is not the purpose of this chapter to discuss different physiological models the two systems that will be selected for illustration of system dynamics are the modeling of glucose-insulin dynamics and cardiovascular modeling approaches. 

After optimization of a number of parameters, highest cellular aminopeptidase activities are obtained with E. coli cells containing the construct pTrpLAP. As already mentioned by its inventors [38], use of the tryptophan promoter in this construct can be further characterized by a rather simple induction strategy the expression of the pepA gene automatically switches on when the tryptophan concentration in the medium drops below a certain threshold value. Because of these aspects, it was decided to further develop the fermentation of E. co/i/pTrpLAP on lab scale. An example of a typical batch fermentation... 

The presence of diacetyl at any stage of the process does not necessarily iadicate an infection by pediococci, because diacetyl is normally formed duting fermentation by oxidation of the precurser 2-acetolactate, which reaches a peak (1—1.2 ppm) at 24—36 h fermentation. The concentration of 2-acetolacetate is usually reduced to values of 0.01 ppm or less, and the diacetyl is reabsorbed by the yeast cells and en2ymatically transformed through acetoia to butanediol. It is extremely important that 2-acetolactate as diacetyl is reduced below the threshold of 0.05—0.10 ppm (ia terms of diacetyl). 

For each fermentation product, the Pq value was plotted against the value as shown in Figure 4. The threshold concentration was found to increase linearly with an increasing value. The equation of this line was determined to be ... 

In beer, flavanoids could be also responsible for astringency [41, 42]. Catechin and epicatechin thresholds lie between 1 and 20 ppm [41,42], with higher values for the beer matrix (20 ppm). Astringency is intensified at low pH, especially near 4.0-4.2 [43], but a higher astringency has been measured by Franfois et al. [44] in beers with a pH close to 5. In this case, it was suspected that the pH of the samples fell in the mouth before polyphenol/protein interactions occurred. Sensory analyses applied to top-fermented beers have shown that storage (20°C or 40°C with air in the head-space) decreases bitterness and post-bitterness but intensifies astringency [45]. On the... 

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