Abstract
The advent of highly efficient, environmentally friendly and cost effective fermentation technology has given impetus to research in the field of optimizing nutritional parameters for optimum yeast fermentative performance. Very high gravity fermentation is a novel fermentation technology that provides an increased production capacity from same size fermentation facilities, with outstanding benefits that includes: high ethanol yield per fermentable mash, considerable savings in energy and process water usage, and effluents with low biological oxygen demand amongst others. Limitations to full commercialization of the technology have been attributed to deleterious effects of the fermentation condition on yeast physiology which include high osmotic stress and ethanol toxicity amongst others. The impact of these physiological stresses on yeast cells during high substrate fermentation manifest as sluggish and incomplete fermentation with high residual sugars in beer, reduced ethanol yield, disproportionate synthesis of esters and generation of respiratory deficient yeast crop. However, compelling evidence has implicated magnesium ions with numerous biological processes and more importantly, with the role of curtailing the impact of these stress conditions. Hence, this review highlights two potential stress conditions of very high gravity fermentation; their mechanism of inhibition versus yeast stress response mechanism, role of magnesium ions in yeast physiology and its impact on fermentation processes. The knowledge emphasized herein will be of practical importance to industrial fermentation processes, as it provides feasible clues to improving yeast fermentative performance under high substrate conditions - with perspectives to precise magnesium regulation in yeast. Key words: Very high gravity fermentation, osmotic stress, ethanol stress, yeast stress tolerance, magnesium ion.
Highlights
Alcoholic fermentation technology has a long history, yet it is still at the forefront of biotechnological development (Linko et al, 1998; Bamforth, 2003; Puligundla et al, 2011)
The stress-induced expression, conferred by stress response element (STRE) is negatively regulated by the Ras cyclic adeno-sine monophosphate-protein kinase A (Ras-cyclic adenosine monophosphate (cAMP)-PKA) pathways whose activity is important in the biosynthesis and degradation of glycogen (Verbelene et al, 2009), and under conditions of zinc deficiency or stress (Gauci et al, 2009)
It is regulated by enzymatic reaction(s) which involves the activity of glycogen synthase phosphatase on glucose, under the control of the cyclic adenosine monophosphate-dependent protein kinase A (PKA) pathway - having glucose 6phosphate and glucose 1-phosphate as precursors for its biosynthesis (Boulton, 2000)
Summary
Role of magnesium ions on yeast performance during very high gravity fermentation Henry Okwudili Udeh* and Tsietsie Ephraim Kgatla. Limitations to full commercialization of the technology have been attributed to deleterious effects of the fermentation condition on yeast physiology which include high osmotic stress and ethanol toxicity amongst others. The impact of these physiological stresses on yeast cells during high substrate fermentation manifest as sluggish and incomplete fermentation with high residual sugars in beer, reduced ethanol yield, disproportionate synthesis of esters and generation of respiratory deficient yeast crop. This review highlights two potential stress conditions of very high gravity fermentation; their mechanism of inhibition versus yeast stress response mechanism, role of magnesium ions in yeast physiology and its impact on fermentation processes.
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