In vitro evaluation of Saccharomyces cerevisiae cell wall fermentability using a dog model

Six Saccharomyces cerevisiae cell wall samples were tested by the in vitro gas production technique using dog faeces as inoculum. In particular, the substrates resulted from three different production processes (alcoholic_A, bakers_BA and brewers_BR) and were characterized by two different carbohydrates (mannans + glucans) concentrations nitrogen-free extract (NFE high and low). Gas production of fermenting cultures was recorded for 72 hr to estimate the fermentation profiles. The organic matter degradability (OMD), fermentation liquor pH, short-chain fatty acids (SCFA) and ammonia (NH3) productions were also measured. All substrates presented a high percentage of OMD (>92%) and moderate fermentability in terms of cumulative volume of gas related to incubated OM (OMCV >50 ml/g) and short-chain fatty acids production (>25 mmol/g), proving their functional properties. Comparing the substrates, it seems evident that the production process affects the chemical composition of the yeast cell wall in terms of crude protein, ether extract and ash content. Consequently, the in vitro fermentation process was significantly different among substrates for volume of gas, SCFA and ammonia production. Regarding the fermentation rate profiles, the production process influenced mainly the curve shape, whereas the NFE concentration affected the quantity of gas produced per hour. In particular, both S. cerevisiae_BR showed very high percentage of OM degradability, gas and SCFA productions and a fast fermentation process due to their high content of fermentable carbohydrates. On the contrary, both S. cerevisiae_BA yeast cell walls appeared to be less degradable and fermentable, probably due to their high content of ether extract. Regarding both S. cerevisiae_A, the high protein content of these substrates could explain the contrasting in vitro results (high degradability with low gas and SCFA production).