The Citrate Metabolism in Homo-and Heterofermentative LAB:A Selective Means of Becoming Dominant over Other Microorganisms in Complex Ecosystems

The citrate metabolism has been extensively studied in lactic acid bacteria (LAB) for its aroma compound production. Among the 4-carbon (C4) by-products obtained from citrate fermentation, diacetyl is one of the better known products for its contribution to the buttery aroma of dairy products. A lot of documents deal with ways to improve diacetyl concentration in food matrices. Apart from these organoleptic advantages, in a microbial ecosystem, the citrate metabolism gives selective advantages to citrate positive microorganisms. Citrate metabolism allows the LAB to use another carbon source for their growth, withstand acidic conditions and generate a “proton motive force” (PMF). Moreover, the citrate/glucid co-metabolism leads to the fast release of organic compounds known for having bacteriostatic effects. Under specific conditions, the C4?pathway liberates diacetyl which is bacteriostatic. In this review we first describe the citrate metabolism and the enzymes involved in the two homo- and heterofermentative LABLc diacetylactisandLeuconostocspp. Moreover, the way to shift the metabolic pathway toward the production of aromatic compounds is discussed for both of these fermentative types of bacteria. Finally, the selective advantages of citrate metabolism for LAB in complex microbial ecosystems are delineated.

Usefulness of Natural Starters in Food Industry: The Example of Cheeses and Bread

Natural starters have been extensively used for many centuries to make many different fermented food products from different raw materials: Milk, meat, roots, vegetables, etc. The industrialisation of food production at the end of the 19th century necessitated the use of regular selected starters to standardize the organoleptic characteristics of the final product. As a consequence, during the 20th century, there was a decline in the use of natural starters in Western countries except in the production of local cheeses or sourdough breads. The beginning of this new millennium has witnessed a deep change in consumer demand, in pursuit of quality, safety and pleasure. In this context, natural starters could, in the future, play an important role in the development of fermented products. However, food producers and researchers have first to cope with fundamental problems in the understanding of these complex ecosystems. The dynamic evolution of the microbial population inside the natural starter (its resilience, its genetic and physiological aptitudes) and the consequences on the product are still partially unknown. This document reviews a broad range of articles concerning the use of natural starters with a specific focus on cheeses and breads, and discusses the major stakes for local food production and the consumption of typical products.

Utilisation of a Portable Electronic Nose, NeOse Pro, to Follow the Microbial Fermentation of a Yoghurt

Yoghurt is fermented milk which results from the exclusive action of Lactobacillus delbrueckii subsp bulgaricus and Streptococcus thermophilus. However, a disruption of the bacterial growth can lead to significant industrial losses. Currents monitoring methods, based on the checking of the pH during the fermentation step are not always satisfactory. The NeOse Pro system, a portable electronic nose, is a mean to obtain immediately aromatic profiles. In this work, we applied this technology to the yoghurt ecosystem, a highly hydrated product. The profiles obtained allowed us to discriminate yoghurts before and after fermentation. In detail, the discrepancy between the two bacteria cultured alone was slight. It was also the case when we compared different initial bacterial ratios. However, two different mixes of bacteria led to clearly distinguishable profiles. A GC/MS analysis performed on products fermented 7 h allowed us to explain the detection of acetaldehyde by the stimulation of some captors of the apparatus. NeOse Pro is so convenient to study the fermentation of yogurt.

Selective Lactococcus Enumeration in Raw Milk

The Lactococcus diversity in cow and goat raw milk was investigated. To do so, a protocol had to be established for the specific enumeration of lactococci. Eight agar media and one control medium were analysed to compare their proficiency in evaluating the Lactococcus population in raw milk: M17 Nal, Elliker, modified Elliker, PCA + milk, modified KCA, modified Chalmers, Turner, FSDA. The M17 medium was used as reference. Eighteen pure strains were tested on these media for their selectivity towards lactococci: six Lactococcus species or subspecies, three Leuconostoc, three Enterococcus, two Lactobacillus, one Streptococcus thermophilus, one Pseudomonas fluorescens, one Escherichia coli and one Staphylococcus aureus. All these bacteria were chosen for their regular presence in raw milk. The KCA medium proved to be the most selective towards lactococci, on condition that 1) we discriminated the colonies using the catalase test and 2) we subtracted the Enterococcus population counted on BEA. However, it was not possible to separate the Streptococcus from the Lactococcus colonies on KCA. The “Lactococcus-like” population including these two genera was estimated at a mean level of 3.18 log(cfu)/mL and 4.14 log(cfu)/mL in cow and goat raw milk respectively. This is consistent with the data already published.

Influence of the Backslopping Practice on the Microbial Diversity of the Lactococcus Population in a Model Cheesemaking

The objective of this work was to study the microbial diversity of natural whey starters (NWS), with a special focus on Lactococcus strains, during 10 successive days of backslopping. Each day, whey samples were enumerated for their Lactococcus, Enterococcus, Leuconostoc and Lactobacillus levels. The diversity and dynamics of the Lactococcus population in whey were investigated using phenotypic methods such as acidifying aptitude and enzymatic activities. Molecular approach using REP-PCR, ERIC-PCR, plamid profiles, phages and prophages research and PFGE was also applied. Lactococci were the main population in whey. Strains from raw milk were sub-dominant. Based on PFGE and phenotypic results lactococci in whey displayed a more heterogeneous phenotype and pulsotype which may reflect greater variations than previously observed within starter. No phages and prophages were spotted on. Plasmids did not seem to be exchanged from strain to strain. The backslopping practice seemed to allow the strains of the starter to rapidly acquire a specificity of their own. The changes observed presumed a slow adaptation of the strains to the “back- slopping environment”. The study of NWS diversity constitutes an important step for the comprehension of acidification defects that recurrently occur in cheesemaking technologies using the backslopping practice.