MITOCHONDRIAL RELEVANCE TO YEAST FERMENTATIVE PERFORMANCE: A REVIEW

It is well known that dissolved oxygen fulfils critical roles in brewing yeast physiology and overall fermentative performance. The major and minor roles that have been identified are briefly discussed and another role, that of providing for minimal mitochondrial development and functionality, is suggested. The long accepted theory that mitochondria are irrelevant to fermentative performance is reviewed as to its basis and the evidence in support of it. However, minimal mitochondrial development is required to provide the cell with critical metabolic intermediates and components. These are identified and reviewed and finally, evidence is presented that mitochondria are critical to brewing yeast fermentative performance. The review concludes that when assessing the role of mitochondria, concern should be broader than simply for the energetic function of these organelles.     

THE EFFECTS OF GLUCOSE ADJUNCT IN HIGH GRAVITY FERMENTATION BY SACCHAROMYCES CEREVISIAE 2036

Laboratory fermentations of 16°Plato glucose adjunct worts by Saccharomyces cerevisiae 2036 demonstrated the absence of “glucose repression” of maltose and maltotriose uptake. However, when compared to worts in which maltose syrup was employed as an adjunct, residual glucose was present at the end of fermentation, maltose and maltotriose uptake rates were enhanced, fructose uptake was blocked and the sequence of sugar uptake was changed. These findings partially explain residual glucose and fructose that sporadically appear in commercial beers. Further research suggests that the physiological quality of the yeast is of prime importance in carbohydrate metabolism, and that critical concentrations of glucose vary with different physiological conditions for this brewing strain in 16°P wort .     

THE SENSITIVITY OF DIFFERENT BREWING YEAST STRAINS TO CARBON DIOXIDE INHIBITION: FERMENTATION AND PRODUCTION OF FLAVOUR-ACTIVE VOLATILE COMPOUNDS

The sensitivity of brewing yeast strains, with different oxygen demands, to carbon dioxide inhibition was investigated. Laboratory fermentations were performed with, and without, protein-based “yeast foods” to lower dissolved CO₂ during fermentation. Differences were observed in yeast fermentative performance in the presence and absence of “yeast foods” for all yeast strains tested. Fermentation performance was improved with the addition of “yeast foods”. There was improved carbohydrate utilisation and amino acid uptake, while acetaldehyde levels at the end of fermentation were decreased. There was an increase in fusel oil production and acetate ester levels at the end of fermentation. Sulphur dioxide levels at the end of fermentation were unaffected by “yeast food” addition. Different yeast strains displayed differing sensitivity to CO₂ inhibition for all parameters tested. Sensitivity to CO₂ was not found to be related to oxygen demand of the yeast strains.     

On the thermal hydraulics of Magnox reactor pressure vessel insulation: Part 2. Numerical simulations of laminar forced and mixed convection in an insulation passage

The paper presents a computational study of laminar flow and heat transfer in a layer of the multiple plate insulation used to line the interior surface of ‘Magnox’ reactor pressure vessels. The flow passage consists of a 2 mm deep channel, which is plane on its upper surface and has raised ‘dimples’ on its lower surface. Under forced convection conditions strong flow ‘streaming’ is found in the unobstructed regions of a channel, and Nusselt numbers there rapidly attain full development. Between adjacent dimples heat transfer levels are characterized by single- and double-peaking distributions which are shown to be due to convective influences. The effect of buoyancy is examined in mixed convection flows with the passage oriented at various angles between horizontal and vertical. Complex non-monotonic trends of heat transfer enhancement and impairment are found, and these are also shown to be attributable to changing patterns of convection.     

YEAST PROTEIN MEASUREMENT USING NEAR INFRARED REFLECTANCE SPECTROSCOPY

The measurement of total protein in yeast cells is useful for fermentation and yeast research as well as for byproduct improvement studies. The protein content of the cell changes with alterations in several fermentation and medium parameters and during the course of the cell cycle. In general, the protein content of the harvested cells is maximised under conditions ideal for yeast growth. Brewers often sell excess yeast as a byproduct and the quality of the yeast, as well as the associated price per unit weight, are often determined by the protein content. Many methods to measure the protein content in yeast cells exist but can be impractical for various reasons or prone to error. In this research, a method was developed to measure the protein content of yeast cells by near infrared reflectance spectroscopy (NIR) - a technique which is finding many applications in brewing and is simple, rapid and amenable to on-line control systems. With this latest application for NIR, it is now possible to measure yeast glycogen, trehalose and protein contents on the same sample in less than 30 minutes including a sample preparation time of 25 minutes .     

STUDIES ON THE BINDING BETWEEN YEAST AND A MALT POLYSACCHARIDE THAT INDUCES HEAVY YEAST FLOCCULATION

A factor which causes heavy yeast flocculation of Saccharomyces cerevisiae 2036 was found to be associated with the malt husk by Axcell et al.¹ In the present work it was established that the factor is a polysaccharide and immunological techniques were used to show that the factor binds to the yeast cell surface during fermentation. It was shown that the factor is present at significantly higher concentrations in wort causing premature yeast flocculation than in normal wort. A particular malt husk extract, obtained using a mild aqueous extraction procedure, induced premature flocculation when added to fermentations in normal wort. In sodium-dodecyl-sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of the malt husk extract, 4 protein bands (42,600; 17,500, 15,100 and 13,100 daltons) and a high MW (molecular weight) polysaccharide were identified. Antibodies were raised against electroeluted proteins as well as against the homogenized polyacrylamide gel containing the polysaccharide band. Enzyme-linked immunosorbent assays (ELISA) showed that the protein components were present in similar concentrations in premature flocculent and normal wort. In contrast, the high MW polysaccharide occurred at a significantly higher concentration in wort inducing premature flocculation than in normal wort. Immunogold electron microscopy showed that the high MW polysaccharide bound extensively to the surface of flocculent cells grown in premature flocculent wort. There was markedly less labelling on yeast cells grown in normal wort. Negligible labelling occurred when the yeast cells were incubated with antibodies against the different protein components of malt husk extract.     

A Novel Approach for Resolving Reverse Emulsions in SAGD Production Systems

The standard method for water clarification in SAGD operations involved the injection of latex polymers to break produced reverse emulsions. Operationally, this resulted in large quantities of oil in the water downstream of the first oil and water separation vessel. Problems occurred because this generated large amounts of oily solids and emulsion. This slop material represented a significant additional expenditure. Champion developed a novel approach to treat these systems and reduce slop. This program has been very successful in removing oil from the water stream during primary separation, thus reducing costs associated with reprocessing slop oil, and resulting in greater oil recovery.     

Evidence of Antimycin‐Insensitive Respiration in a Commercial Brewing Yeast

The presence of antimycin‐insensitive respiration in an industrial brewing yeast strain (Saccharomyces cerevisiae AJL 2036) was confirmed by studying the effects of antimycin A (inhibits electron transfer from cyt b to c₁) on brewing yeast performance in the presence and absence of oxygen. The inhibitor sodium azide was also used to assess whether blocking electron transfer further downstream of the antimycin‐effective site eliminated the enhanced fermentation parameters observed in the presence of antimycin A. Oligomycin, an inhibitor of the mitrochondrial membrane ATPase, was also used to determine the importance of intramitochondrial ATP synthesis on the observed effects. Fermentations were monitored for overall performance and beer quality indicators. The measured parameters showed no changes due to oligomycin addition indicating that the major source of energy in the cells was cytoplasmically‐generated via glycolysis. Results from the fermentations in the presence of antimycin A confirmed the existence of an alternative respiratory pathway, the stimulation of which resulted in improved fermentative performance. It is postulated that antimycin A enhanced the fermentation rate by increasing the amount of yeast growth. This caused a direct pull on yeast metabolism including fermentative activity. The mechanism was postulated to involve an increased supply of critical growth intermediates in the presence of antimycin A rather than an increased intramitochondrial energy supply per se.     

On the thermal hydraulics of Magnox reactor pressure vessel insulation: Part 1: Experimental determination of the thermal performance of multiple plate insulation

The thermal conductivity of multiple plate insulation used in ‘Magnox’ reactor pressure vessels has been investigated experimentally. In the reactor application the fluid within the insulation pack is carbon dioxide at 20 bar, however, in this experimental investigation the insulation was tested in air at lower pressures. The conductivity of the insulation was measured using a test facility based on the guarded hot plate method for a range of temperatures, pressures, and inclinations. Measurements were made both with the fluid within the insulation pack nominally stationary and with an imposed flow through it, simulating leakage through the insulation pack. The values of thermal conductivity obtained were similar to those for conventional high temperature insulation. The conductivity increased with increases in average temperature, pressure, and inclination, in all cases due to enhanced convection within the insulation. The introduction of forced flows into the insulation pack increased the effective conductivity still further.