BIOVINO PROJECT HAS OBTAINED LACTIC ACID FROM VINE SHOOTS AND WINE LEES
What is lactic acid?
Lactic acid is a three-carbon organic acid which is present in nature and in several foods, such as youghurt, bread dough, pickles and wine. This acid is formed through fermentations performed mainly by bacteria of the orders Lactobacillales and Bacillales. In this fermentative process, sugars are transformed into lactic acid.
What is lactic acid used for?
Lactic acid is essentially used in the food industry as an acidulant, preservative, flavouring, emulsifier and pH regulator. Additionally, it is employed in the cosmetic industry due to its humectant, exfoliant and antimicrobioal properties. Its use in the chemical industry as a building block allows the synthesis of acetaldehyde, acrylic acid or ethyl lactate, among other compounds. Similarly, lactic acid is the feedstock to produce a bioplastic: polylactic acid (PLA).
Vine shoots: a lignocellulosic winery by-product
Vine shoots are the most important winery by-product in terms of volume, since they represent 93% of the solid wastes originated by viticulture and their production is in the range of 1.4-2.0 t/ha. According to FAO, the area devoted to vines in 2019 was 936,890 ha in Spain and 178,780 ha in Portugal. Spain and Portugal rank first and tenth worldwide, respectively, in terms of surface devoted to viticulture.
Vine shoots, as lignocellulosic vegetal biomass, is mainly composed of cellulose, hemicellulose and lignin. Cellulose and lignin are polysaccharidic fibres, that is to say, they are long chains of sugar molecules. The links of these chains can be separated by a physico-chemical or enzymatic pretreatment, thus releasing the sugars that they contain (glucose, xylose, arabinose, etc.).
What are wine lees?
Wine lees are a liquid and viscous waste composed of water and materials decanted during wine fermentation. Their weight is equivalent to 1.5% of processed grape and they contain an important amount of exhausted yeasts. As their total nitrogen (TN) content is about 10 g/L, they could be used as a source of organic nitrogen for various microbial processes.
Lactic acid production from vine shoots and wine lees
Biovino project assessed the use of vine shoots as a sugar source for lactic acid fermentation, while using wine lees as the single nutritional supplement for microorganisms during this fermentative process.
Vine shoots contained 49 % carbohydrates, including 33 % cellulose and 11 % hemicellulose. Firstly, several acidic and alkaline pretreatments, followed by an enzymatic hydrolysis, were assessed to release the sugars from the biomass. A pretreatment with H2SO4 was adopted, which – after a statistical optimisation process – led to a broth with 40 g/l sugars and low concentrations of fermentation inhibitors (acetic acid, formic acid or furfural).
Then, this broth was fermented with three different bacterial strains: one belonging to the species Bacillus coagulans and two belonging to the species Geobacillus stearothermophilus. All of them are characterised by their ability to grow at high temperatures (50-60 ºC) and by their undemanding nutritive requirements.
The best results were obtained with Bacillus coagulans. To begin with, yeast extract was used as a nitrogen source and it was observed that this microorganism obtained 29.8 g/l lactic acid, which means a yield of 93 %. Finally, yeast extract was replaced by a concentrated of red wine lees and white wine lees. Both types of wine lees were very efficient as nitrogen sources for the fermentation of vine-shoot hydrolysates, because B. coagulans produced 29.2 g/l lactic acid with red wine lees and 29.1 g/l lactic acid with white wine lees after a 24-h fermentation.
Although the obtained concentrations of lactic acid (~30 g/l) are not very attractive from the industrial point of view, the optimisation process of vine-shoot pretreatment enabled to obtain a readily fermentable broth. The thermotolerant strain B. coagulans has interesting features, such as its ability to grow in aerobic and unsterile environments, its ease of using wine lees as a nitrogen source, or the high proportion of the isomer L(+) of acetic acid produced (this isomer is the one used by the food industry).
Link to journal article: https://doi.org/10.1007/s00253-021-11693-1.
Link to open access repository (preprint version): https://doi.org/10.5281/zenodo.5723325.
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