Dissertation committee:
ΛΙΟΥΝΗ ΜΑΡΙΑ, Καθηγήτρια Τμήμα Χημείας ΕΚΠΑ,
ΧΑΤΖΗΛΑΖΑΡΟΥ ΑΡΧΟΝΤΟΥΛΑ, Καθηγήτρια ΠΑΔΑ,
ΙΑΤΡΟΥ ΕΡΜΟΛΑΟΣ, Καθηγητής Τμήμα Χημείας ΕΚΠΑ,
ΜΑΥΡΟΜΟΥΣΤΑΚΟΣ ΘΩΜΑΣ, Καθηγητής Τμήμα Χημείας ΕΚΠΑ,
ΝΤΟΥΡΤΟΓΛΟΥ ΕΥΘΑΛΙΑ, Αναπληρώτρια Καθηγήτρια ΠΑΔΑ,
ΠΡΟΕΣΤΟΣ ΧΑΡΑΛΑΜΠΟΣ, Καθηγητής Τμήμα Χημείας ΕΚΠΑ
ΔΑΣΕΝΑΚΗ ΜΑΡΙΛΕΝΑ, Επίκουρη Καθηγήτρια ΕΚΠΑ
Summary:
The food sector is responsible for generating a huge amount of waste and by-products, estimated at around 95 to 98 million tonnes per year. In particular, the lees, which make up 2 to 6% of the total volume of the wine, are rich in ethanol, tartaric acid, yeast cells, polysaccharide complexes, polyphenols and inorganic matter. This specific waste is a challenge for countries where winemaking occupies a key position in their economy, as its effective management and utilization is of the utmost importance. The total volume of lees produced worldwide in 2023 amounts to 10.32 million hectoliters.
The objective of this thesis is the thorough study of the ability of the bacterial strain Komagataeibacter sucrofermentans DSM 15973 to synthesize bacterial cellulose utilizing as a carbon substrate the by-products of nine different Greek red and white grape varieties as well as the brewery by-products. Through carefully controlled batch fermentations, carried out in Hestrin-Schramm (HS-glucose) culture medium as a base, a sugar concentration of 20 g/L and a nitrogen concentration level of amino acids and peptides, especially free amino groups, were maintained at 385 milligrams per liter.
In the initial stage, the commercial monosaccharides, glucose, fructose, galactose, the disaccharide lactose, and their potential in the synthesis of bacterial cellulose were studied. The primary goal during this critical phase was to determine the most efficient carbon source that could be adequately metabolized by the microbial strain. After careful analysis, it was found that the glucose substrate exhibited the highest performance compared to the rest of the commercially available sugars.
In the second part of the experimental study, the impact of six different carbon-nitrogen ratios, in ground brewer's grains (BSG), on the amount of bacterial cellulose produced was examined. The ratio C/FAN=32.8 showed the maximum biopolymer production and the dependence of carbon and nitrogen availability. It is noteworthy that on the 13th day of fermentation, the highest concentration of bacterial cellulose was reached with a value of 1.79 g/L, and the sugar consumption reached 82%.
Ph regulation is the most critical factor controlling the oxidative fermentation of bacterial cellulose (BC) synthesis. For successful BC production, it is necessary to stabilize the pH at slightly acidic or near neutral values. For this reason, in the third part the performance of bacterial cellulose was studied in a different range of pH values ranging from 3 to 8 and it was found that the pH value=5 creates the optimal conditions for the growth of microorganisms and the fermentation of sugars to produce bacterial cellulose and resulted in the highest performance and productivity. More specifically, the resulting film presented a compact structure and retained its opacity even after the drying process. At the end of the fermentation, the microorganism had consumed a significant amount of the free nitrogen of the amino group and recorded a yield of 0.13 (g/g).
Finally, after extensive research, it was found that the production of bacterial cellulose in the by-products of winemaking is mainly influenced by the pH of the by-products and the type of grape used. When analyzing the data collected, it was observed that there was a constant consumption of sugars in the popular red varieties Cabernet Sauvignon, Merlot and Xinomavro that had a pH between 5.1 and 5.4. Notably, the utilization of sugars in this pH range showed extremely high rates, ranging from 81% to 83%. In addition, nitrogen levels showed a significant decrease during the fermentation process, which eventually led to the formation of stable films.
Keywords:
wine by-products, brewing by-products, bacterial cellulose, circular economy
