Growth and metabolism of sugar and acids ofLeuconostoc oenosunder different conditions of temperature and pH

The Role of Yeasts and Lactic Acid Bacteria on the Metabolism of Organic Acids during Winemaking

The main role of acidity and pH is to confer microbial stability to wines. No less relevant, they also preserve the color and sensory properties of wines. Tartaric and malic acids are generally the most prominent acids in wines, while others such as succinic, citric, lactic, and pyruvic can exist in minor concentrations. Multiple reactions occur during winemaking and processing, resulting in changes in the concentration of these acids in wines. Two major groups of microorganisms are involved in such modifications: the wine yeasts, particularly strains of Saccharomyces cerevisiae, which carry out alcoholic fermentation; and lactic acid bacteria, which commonly conduct malolactic fermentation. This review examines various such modifications that occur in the pre-existing acids of grape berries and in others that result from this microbial activity as a means to elucidate the link between microbial diversity and wine composition.

Leuconostoc citreum TR116: In-situ production of mannitol in sourdough and its application to reduce sugar in burger buns

A marketing study revealed that commercially available burger buns can contain up to 10% (w/w) of added sugar. In order to reduce sugar and maintaining the product quality at the same time, functional ingredients and alternative sweetening agents have to be incorporated. In this study, the sourdough lactic acid bacteria Leuconostoc citreum TR116, selected for its ability to produce high amounts of mannitol, was used to produce wheat sourdough and its biochemical characteristics (cell count, pH, TTA, sugar- and acid profile, as well as mannitol production) were monitored over 48 h. The so produced sourdough was then incorporated, as a functional ingredient, into a sugar reduced burger bun system and the quality characteristics of the dough and the final product were determined. Sourdough incorporation counteract the negative effects of sugar reduction on dough properties and resulted in the same viscoelastic properties (0.423 ± 0.008) and gluten-network-development (PMT: 160 ± 12.6 s; TM: 44.0 ± 2.6 BU) as the full-sugar control dough. Furthermore, the investigation of specific volume, crumb hardness and chewiness revealed no significant differences between sugar reduced sourdough burger buns and its control. It is noteworthy that sourdough contributed to browning reaction resulting in darker crumb and crust colour (-8.2%; -9.6%) and it extended microbial shelf life of the burger buns significantly (+3.5 days). Sensory evaluation showed no significant differences in sweetness and sourness between sugar reduced buns containing sourdough and the full-sugar control. In conclusion, the incorporation of mannitol-rich sourdough fermented by Leuconostoc citreum TR116 represents a novel technological approach in the field of sugar reduction and showed high potential as a functional ingredient to ameliorate the losses of important quality parameters. Especially sourdough containing higher amounts of mannitol and lower amounts of lactate improved significantly the dough and burger bun quality.

Biotechnological production of mannitol and its applications

Mannitol, a naturally occurring polyol (sugar alcohol), is widely used in the food, pharmaceutical, medical, and chemical industries. The production of mannitol by fermentation has become attractive because of the problems associated with its production chemically. A number of homo- and heterofermentative lactic acid bacteria (LAB), yeasts, and filamentous fungi are known to produce mannitol. In particular, several heterofermentative LAB are excellent producers of mannitol from fructose. These bacteria convert fructose to mannitol with 100% yields from a mixture of glucose and fructose (1:2). Glucose is converted to lactic acid and acetic acid, and fructose is converted to mannitol. The enzyme responsible for conversion of fructose to mannitol is NADPH- or NADH-dependent mannitol dehydrogenase (MDH). Fructose can also be converted to mannitol by using MDH in the presence of the cofactor NADPH or NADH. A two enzyme system can be used for cofactor regeneration with simultaneous conversion of two substrates into two products. Mannitol at 180 g l(-1) can be crystallized out from the fermentation broth by cooling crystallization. This paper reviews progress to date in the production of mannitol by fermentation and using enzyme technology, downstream processing, and applications of mannitol.

Influence of ethanol and pH on the gene expression of the citrate pathway in Oenococcus oeni

The consumption of citrate by the malolactic bacterium Oenococcus oeni changes the aromatic profile of wines due to the production of volatile compounds such as diacetyl and acetic acid. In this study, the expression of genes related to citrate utilization in the O. oeni strain PSU-1 was investigated to further understand the role of this metabolic pathway in the adaptation to wine environment and its impact on organoleptic qualities. Different conditions of ethanol content (0% and 10%) and pH (3.5 and 4.0) were assayed to evaluate the transcriptional response to both these stress factors. In the presence of ethanol, metabolic and transcriptional behavior was different than the observed when ethanol was absent. The expression of citrate pathway genes was mainly affected by ethanol, while pH showed a lower effect. Among the studied genes, citE, ackA and alsD were the genes revealing a distinctive transcriptional response. The differences observed in gene expression were in correlation with the different content of end products such as acetic acid and diacetyl. The increment of gene expression observed in the presence of ethanol at low pH suggests the participation of citrate metabolism in the response to stress conditions.