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Flavour improvement and changes in production of volatile compounds and organic acids by adjunct cultures of lactobacilli in soft cheeses
Simposio; III Simposio Internacional de Bacterias Lácticas; 2009
Adjunct cultures of mesophilic lactobacilli are generally aimed at improving cheese quality indirectly, by control of non starter lactic acid bacteria and their potentially undesirable biochemical activities. However, some strains displaying beneficial enzymatic abilities can contribute directly to cheese flavour, via peptidolysis and production of aroma compounds. In this work we assessed the influence of two adjunct cultures on aroma biogeneration in a soft cheese model. For that purpose, control cheeses were manufactured with a primary lactic starter of Streptococcus thermophilus, without added lactobacilli; experimental cheeses were made with the same starter, plus adjunct cultures of either Lactobacillus casei I90 or Lactobacillus plantarum I91. Cheeses were prepared under controlled microbiological conditions on different days, with different milk batches, and sampled during ripening at 3, 30 and 60 days. Analyses consisted of microbiological counts of lactic and non lactic microorganisms, volatile compounds, free amino acids and organic acids, diacetyl and acetoin. A sensory triangular test (“sniffing”) was applied to compare the aroma of samples with and without added lactobacilli. Streptococci counts were ca. 109 UFC ml-1 in all samples during ripening, while lactobacilli increased to 108 UFC ml-1 in adjunct-treated cheeses at 3 days, then remained constant. Lactobacilli counts in control cheeses were very low; non lactic microorganisms (coliform bacteria and molds and yeasts) were slightly higher in controls than in lactobacilli-added cheeses. Total content of free amino acids in cheeses with L. casei I90 or L. plantarum I91 was significantly higher than in control cheeses; aspartic acid was one of the amino acids increased. Citrate was the second organic acid in decreasing order of concentration in all cheeses, after lactic acid. Acetoin production was notably higher in the adjunct-treated cheeses: this result was confirmed by the analysis of either the aqueous extract of the cheeses or the volatile compounds in the headspace of the samples. Diacetyl content was also much higher in cheeses with L. casei I90 or L. plantarum I91 than in the controls, which was evidenced by the profile of volatile compounds. Results suggested that increased production of diacetyl and acetoin in lactobacilli-added cheeses is due to aspartate catabolism, which can be produced during all ripening. The strains assayed showed the ability to provide this precursor by peptidolysis, and to produce derived aroma compounds. Although diacetyl and acetoin can also be generated from citrate, which was available in all cheeses, this catabolic pathway seems less likely, as citrate concentration was not higher in adjunct-treated cheeses, and acetic acid, a compound typically derived from citrate, was not increased by the addition of lactobacilli. Differences in aroma between control and adjunct-treated cheeses were highly significant and were described as an increase in the fresh and buttery aroma in cheeses with lactobacilli.