Polisac® ExtraWhite is an organic improver based on yeast cell walls and characterized by high antioxidant properties. Best used during fermentation and re-fermentation of white wines.
Thanks to its content in GSH – unsaturated fatty acids – sterols, yeast cells are able to assimilate, from the must or wine to which it has been added, high quantities of these molecules that act as “altruistic or sacrificial metabolites”. This is because they participate in the metabolic responses when the cell suffers from oxidative stress; they are also considered as “survival factors” because they grant the cell a greater chance of surviving when the environment becomes extreme.
The effects that can be verified by using Polisac®ExtraWhite are: enhancing both kinetics and fermentative “cleanliness” and a richer aromatic expression. As far as sparkling wines are concerned, an improvement in the quality of the foam and in taste persistence is definitely noted and appreciated.
Polisac® ExtraWhite contains inactive yeast obtained through a process that allows endogenous enrichment of reduced glutathione (GSH, Fig. 1) within the cells during their propagation. The resulting product has a very high content of reduced glutathione (GSH) and readily available.
The optimization of the production process also allows obtaining a high concentration of fatty acids, in particular the monounsaturated ones (about 7%), and a good quantity of total sterols (about 2%), ergosterol in particular (about 1%).
In the image shown above, you can have an overview of the glutathione metabolism within the yeast cell.
Positive effects of GSH
GSH plays an essential role in the detoxification from free radicals and reactive forms of oxygen (ROS), group to which ozone and hydrogen peroxide, for example, belong.
Yeasts that have little or no GSH at all are sensitive to metabolic stress of oxidative nature.
It acts as a strong cell buffer, protects the cells by eliminating free radicals through the thiols fraction, thus eliminating peroxides thanks to “glutathione-peroxidase” and/or complexing heavy metals (copper) by means of the “glutathione S-transferase” activity.
Strains that have the ability to over-express the genes of GSH’s biosynthetic path are also more efficient in converting sugar into ethanol.
The increase in GSH in yeast cells impart a greater tolerance towards those inhibiting substances that adversely affect the overall metabolic efficiency of the microorganism: resilience, growth capacity, multiplication rate, longer cellular chronological age, resistance to stress, fermentative rate.
It counteracts the negative effects of heavy metals (copper), responsible for oxidative stress. Copper excess reduces cell viability by compromising the integrity of the membrane and negatively affecting cellular enzymatic activity.
Besides being produced by the cell through a two-step mechanism, involving two genes (GSH1 and GSH2), it can be assimilated by the surrounding environment thanks to the presence of specific transporters (Hgt1p, Ycf1p, Bpt1p) present in the cell membrane (HGT1 – gene that encodes a high affinity glutathione transporter of the plasma membrane, Fig. 2).
It is involved in basic cellular functions such as maintaining mitochondrial and membrane integrity. It also assumes fundamental roles in the response to the condition of sulfur and nitrogen “starvation”.
Positive effects of unsaturated fatty acids
The main monounsaturated fatty acids of the yeast are palmitoleic acid (C16:1) and oleic acid (C18:1), which also are the main monounsaturated fatty acids that Polisac® ExtraWhite is equipped with.
The production of fatty acids depends mainly on the conditions and composition of the development medium; being the synthesis of unsaturated fatty acids in Saccharomyces cerevisiae an aerobic process (such as the ergosterol synthesis), prolonged anaerobic conditions may have extremely negative effects on the viability and the fermentative performance of the yeast.
Ethanol produced during fermentation is a source of stress for the cell and, as such, it greatly affects the composition in fatty acid of membrane’s lipids. The content of unsaturated fatty acid in the phospholipids of membranes increases in presence of ethanol and determines increased fluidity.
The most common fatty acid in phospholipids molecules is palmitic acid (16:0). Its presence decreases as the yeast gets closer to multiplication. Under anaerobic conditions (alcoholic fermentation), the yeast is not able to produce unsaturated fatty acids starting from their relevant saturated fatty acids. In fermentation context, the only way that yeast has to increase its supply of unsaturated fatty acids is to absorb them from the surrounding environment. When the yeast is subject to temperature changes, adaptation is guaranteed by the increase of unsaturated fatty acids level in membrane’s phospholipids.
Cells grown under anaerobic conditions have lower levels of unsaturated fatty acids than cells grown in aerobic conditions. Cells grown under aerobic conditions have higher levels of palmitoleic acid (16:1) and show greater resistance to temperature and oxidative stress.
In fermentation condition, since palmitic acid (C16:0) cannot be transformed into palmitoleic acid (C16:1), the only chance to increase the concentration of palmitic acid is to take it from the surrounding substrate.
Positive effects of sterols
Resistance to ethanol is higher in cells that have higher values of ergosterol (and unsaturated fatty acids). Ergosterol is essential for membrane structure and integrity. Variations in levels and composition of sterols have effects on membrane functions.
Ergosterol is important for the membranes structure and it also affects the dynamics and activities of trans-membranes proteins.
Sterol biosynthesis is involved in maintaining membrane efficiency when the yeast cell is subject to stress conditions. In particular, a yeast cell subject to ethanol stress, increases the saturation index of fatty acid chains of the phospholipids, and increases the ergosterol content of the membranes.
Anaerobiosis condition adversely affects the synthesis of lipids and sterols.
In absence of O2 and exogenous lipids, yeast cells reduce the surface area of the organelles membranes and dilute their lipid content in order to limit the loss of viability. Under these conditions the cells go into anaerobic stress, resulting in loss of lipid biosynthesis intermediates and oxidative damage to cellular structures.
Ergosterol plays an important role in response to oxidative stress.
Ergosterol production is of crucial importance in promoting cell growth, regulating the fluidity and permeability of the membrane, and in the activity of enzymes related to the membrane (in particular transport proteins).
Experiences
Here following is an example of fermentation kinetics; the operating conditions were the same in terms of starting matrix (experience on white wine Chardonnay), temperature and nutrition.
Here are the two theses in comparison:
- Thesis No 6: Addition of Polisac® ExtraWhite;
- Thesis No 9: Conventional procedure, without use of Polisac® ExtraWhite.
In the reported experience, as the graph shows, the differences in fermentation kinetics are clear and evident between the thesis using the organic improver Polisac® ExtraWhite and the one without it. The latter shows a longer latency phase and a lower exponential phase.
Conclusions
Polisac® ExtraWhite is a polyhedral product, capable of acting positively on many aspects concerning fermentation, sparkling wine production and refining of white wines.
Its use works great for a Winemaker who wants to improve both the pleasantness of wines and their longevity, but also to get a better performance by the yeast used in fermentation.
As for its application, it is recommended to use it in the early stage of fermentation at a dosage between 10 and 30 g/hl; alternatively, and at the same doses, at the end of fermentation.
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