Search

The ability of lactic acid bacteria to produce lactic acid from various sugars plays an important role in food fermentations. Lactic acid is derived from pyruvate, the end product of glycolysis and thus a fast lactic acid production rate requires a high glycolytic flux. In addition to lactic acid, alternative end products—ethanol, acetic acid and formic acid—are formed by many species. The central role of glycolysis in lactic acid bacteria has provoked numerous studies aiming at identifying potential bottleneck(s) since knowledge about flux control could be important not only for optimizing food fermentation processes, but also for novel applications of lactic acid bacteria, such as cell factories for the production of green fuels and chemicals. With respect to the control and regulation of the fermentation mode, some progress has been made, but the question of which component(s) control the main glycolytic flux remains unanswered. [Copyright &y& Elsevier]

The effect of leucine and α-ketoglutarate addition on transamination of branched-chain amino acids was studied in model minces inoculated with Pediococcus pentosaceus and Staphylococcus carnosus. Leucine addition changed the ratio of volatile breakdown products of leucine, isoleucine and valine but not the total amount of the volatiles and it was concluded that the amount of free amino acids does not limit transamination of amino acids. The addition of α-ketoglutarate resulted in increased levels of methyl-branched aldehydes and insignificant positive changes in methyl-branched acid production. The results were verified in real fermented sausages with no, low (0.09% w/w) and high (0.36% w/w) addition of added α-ketoglutarate since the levels of the flavour intensive methyl-branched aldehydes and acids were drastically increased in sausages added α-ketoglutarate. The catabolism of phenylalanine was also induced by α-ketoglutarate and there were further indications of increased transamination of aspartate. A triangular test showed that the flavour of sausages with no and low addition of α-ketoglutarate could be clearly distinguished from one another. Altogether the results presented in this paper point to glutamate dehydrogenase, the enzyme catalyzing regeneration of α-ketoglutarate, as a key enzyme in catabolism of amino acids and thereby also in aroma formation during sausage processing. [Copyright &y& Elsevier]

Three sausage batches inoculated with normal inoculation level of Pediococcus pentosaceus (5×106 CFU/g) and with low, intermediate, and high inoculation levels of Staphylococcus carnosus (105, 5×106, 5×107 CFU/g, respectively) were produced. Cell counts and formation of volatiles were followed throughout a ripening period of three weeks. The staphylococci exhibited the fastest growth in sausages with a low inoculation level, whereas growth was only moderate in sausages with a high initial level. Analysis of volatiles showed that methyl-branched aldehydes and acids, phenylacetaldehyde, 2-methyl-1-butanol, dimethyldisulphide and dimethyltrisulphide were produced in higher amounts in sausages with a high inoculation level of S. carnosus, whereas a low inoculation level correlated with high amounts of diacetyl, ethanol and ethyl esters. The levels of most compounds increased over time, but the amount of diacetyl was negatively correlated to ripening time. A negative interaction effect between inoculation level and ripening time was observed for the amounts of methyl-branched aldehydes. [Copyright &y& Elsevier]

A fermented meat model system was developed, by which microbial formation of volatiles could be examined. The model was evaluated against dry, fermented sausages with respect to microbial growth, pH and volatile profiles. Fast and slowly acidified sausages and models were produced using the starter cultures Pediococcus pentosaceus and Staphylococcus xylosus. Volatiles were collected and analysed by dynamic headspace sampling and GC–MS. The analysis was primarily focused on volatiles arising from amino acid degradation and a total of 24 compounds, of which 19 were quantified, were used for multivariate data analysis. Growth of lactic acid bacteria was comparable for model and sausages, whereas survival of S. xylosus was better in the model. Multivariate analysis of volatiles showed that differences between fast and slowly acidified samples were identical for model and sausage. For both sausage and model, fast-acidified samples had a high content of ketones, sulphides and methyl-branched acids, whereas slowly acidified samples had the highest content of methyl-branched alcohols, aldehydes, their ethyl esters, phenylacetaldehyde and methional. Furthermore, model repeatability with respect to pH, microbial growth and volatile profiles was similar to sausage production. Based on these findings, the model system was considered valid for studies of aroma formation of meat cultures for fermented sausage. [Copyright &y& Elsevier]