As biotechnology advances, the making of fermentation products by use of bioreactors utilizing immobilized microorganisms is under study in the fields of brewing for malt alcohol beverages (beers), wines, sake, vinegar, soy sauce, and the like. When a bioreactor is used as such, the following are expected:
1) Since a high concentration of yeast is immobilized to carry out fermentation, the brewing is completed rapidly, so that the brewing period can be shortened, whereby the number of manufacturing tanks and the cost of construction can be lowered.
2) Since continuous fermentation is possible, it is unnecessary to charge and collect yeast.
Conventionally, when a beer or the like is manufactured by use of a bioreactor, however, the amount of amino acids and diacetyl (DA) has become greater, and the amount of ester has become smaller in the product as compared with a product manufactured by a traditional batch fermentation method. As a result, the product obtained by use of the bioreactor has been problematic in that it is disadvantageous in flavor, and this problem has been remarkable in particular when the bioreactor is used for primary fermentation (main fermentation).
For overcoming such a problem, studies have conventionally been carried out. Japanese Patent Application Laid-Open Gazette No. HEI 7-123969 discloses a method in which a fermentation liquid is circulated in a continuous fermentation method using a fluidized bed type reactor. Though the consumption of amino acids in the fermentation process is ameliorated by this method, the amount of diacetyl which causes raw odor or immature odor is not lowered sufficiently, whereby the resulting product still has room for improvement in terms of flavor. Also, manufacturing methods in bioreactors including this method have been problematic in that the number of floating yeast cells upon the end (completion) of primary fermentation is small in general, the fermentation rate in the process of fermentation is unstable, and they are hard to control.
Meanwhile, it has conventionally been known that yeast for making a fermentation product such as beer is required to have flocculation ability (agglutinability) to a certain extent. Here, flocculation ability refers to a property in which yeast cells flocculate as a mass upon the end of fermentation and are separated from the liquid so as to float up or sediment. If the flocculation ability is too high, then the flocculation occurs at an earlier stage, so that the contact between the fermentation liquid and yeast is broken, whereby fermentation becomes insufficient. If the flocculation ability is too low, by contrast, then yeast floats in the liquid for a long time, so that the amount of recovery of yeast decreases, which takes unnecessary time and labor for separating and collecting yeast by centrifuge or the like, for example. Therefore, it has conventionally been common knowledge to use yeast having a certain degree of flocculation ability (flocculent yeast) for making a fermentation product. Such flocculent yeast (agglutinative yeast) is used in the above-mentioned method disclosed in Japanese Patent Application Laid-Open Gazette No. HEI 7-123969 as well.
In view of the above-mentioned problems in the prior art, it is an object of the present invention to provide a method in which, when making a fermentation product by use of a bioreactor utilizing an immobilized microorganism, the fermentation rate in the fermentation process can be held constant, and the number of floating yeast cells upon the end of fermentation can stably be maintained at a level higher than the conventional one, whereby, when making a malt alcohol beverage by use of a bioreactor in particular, the amount of diacetyl in the fermentation liquid and final product can be lowered sufficiently and so forth, so as to improve the flavor of final product.