HYNES Erica Rut
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Production of flavour compoundsby individual or mixed cultures of lactic acid bacteria
Simposio; IV International Symposium on Lactic Acid Bacteria; 2013
The cooperation of lactic acid bacteria with different enzymatic abilities involved in the catabolism of amino acids (AA) has been proposed as a possible approach to increase flavour formation in cheese. In this work, we studied the influence on fermentation and volatile compounds production of two strains of lactobacilli, L. paracasei 90 (Lp90) and L. casei 72 (Lc72), and their potential cooperation with a strain of S. thermophilus (St). The thermophilic culture was assayed both as viable cells (Stv) or attenuated culture, in which case ethanol (Sta) or ultrasound waves were applied (Stu). Experiments were carried out in a sterile extract which consisted of the aqueous phase obtained from soft cheeses of 15 days of ripening, standardized to pH 5.2 and NaCl 1.5%. Experimental extracts were inoculated with Lp90 or Lc72, which possess aminotransferase (AT) activity towards different AA, and Stv, Sta or Stu, all derived from a strain with high glutamate dehydrogenase (GDH) activity. Control extracts without inoculation were also obtained. The pH of all the experimental extracts decreased significantly during incubation, due to fermentation of lactose and galactose, with the consequent production of lactic acid. As for amino acids metabolism, significant differences were observed in α-ketoglutarate concentration. This key intermediate compound is consumed as amino group acceptor in the first step of the catabolism of AA and it is replenished from glutamic acid by GDH activity. Concentration of α-ketoglutarate was lower than the controls in the extracts containing Lp90+Stv and Lp90+Sta, but higher than in the controls in extracts with Lc72, Lc72+Sta and Lc72+Stu. Diacetyl and acetoin were significantly higher in all extracts inoculated with Lp90 in comparison with the control; evidence of some cooperation with attenuated cells of S. thermophilus was found, as both volatile compounds were highest when Lp90 was combined with Sta o Stu. In extracts containing Lc72, diacetyl was higher than in the controls; the highest production was found when Lc72+Stv were added. Extracts with Lp90 single or mixed with Stv, Sta and Stu always contained more diacetyl and acetoin than those with Lc72. This result correlates with the aminotransferase profile of Lp90, which was highest towards Asp, as diacetyl and acetoin may derive from Asp transamination in cheese. On the other hand, 3-methyl butanal and 3-methyl butanol, derived from the catabolism of Leu, were higher in all extracts with Lc72 than in the controls. This finding also matches with aminotransferase specificity of Lc72 towards branched chain AA. In extracts with Lp90, these compounds were only slightly higher than in the controls. In the present work, we found that fermentation of lactose and galactose and formation of volatile compounds were led by the strain of Lactobacillus tested. Only minimal changes were observed which could be attributed to the combination of these strains with St in different physiological states, and they always reinforced the influence detected for the Lactobacillus strains when used alone.