An investigation into the fructose block associated with the brewing process

The uptake of fructose in Saccharomyces cerevisiae 2036 is via a biphasic transport system, in which the first component is a high affinity, low capacity, dry weight, proton symport which does not transport glucose and is independant of the maltose proton symport. The presence of glucose has no effect on the uptake of fructose via the symport. The stoichiometry of uptake is one proton per molecule of fructose. Maltose and ethanol non-competitively inhibit fructose uptake via the proton symport. The second component is a lower affinity, higher capacity facilitated diffusion system which transports both glucose and fructose. Glucose uptake is monophasic and has the highest affinity, Km = 1.3 mM, of all sugars for this transport system. In the fermentations containing glucose and fructose together, glucose competitively inhibits fructose uptake causing preferential utilization of glucose over fructose. The methods of experimentation then include the use of tritium-labelled glucose and 14C-labelled fructose. Ethanol non-competitively inhibits glucose uptake of the facilitated diffusion system. A consequence of slower fructose utilization results in residual fructose concentrations remaining at the end of fermentation when sucrose adjuncts are used, hence causing the 'fructose block'. Amelioration of the 'fructose block' is multifaceted. The residual fructose concentrations in wort for the last three days of fermentation are inversely proportional to the pitching rate