There are sugars whose job involves much more than being sweet or providing the organism with energy. Trehalose, known to anybody who has studied brewers’ yeast, is one such bonbon. It is a disaccharide, which means that one molecule is made up of two joined-up basic sugar molecules (in this case a pair of glucose molecules) and it is found throughout the microbial, plant and animal kingdoms. Its role in yeast is the most studied, although it is probably best known to the man and woman on the street (and cosmetics scientists!) as the sugar that allows the resurrection plant (Selaginella) come back to life after months or even years of drought.
And it is not far off the mark to think of trehalose as the resurrection sugar — or at least the sugar that keeps organisms from drying out or dying from stress. Trehalose’s functions in yeast include protecting cells from the stressors which they encounters as they beaver away at producing the beer, wine and cider that we love so: high alcohol concentrations; heat; dehydration; oxidation; low pH; and osmotic stress. In a nutshell, if it weren’t for each yeast cell having thousands of trehalose molecules, these budding babies would never get very far into a fermentation: the conditions in the initial wort and during the first few hours of fermentation would see off 99.999% of old Saccharomyces cerevisiae. Imagine how long your cells would survive in a 12% sugar solution? Or in beer or wine?
So, how exactly does trehalose work in de-stressing cells? It’s all down to chemistry and how the two glucose molecules that make up trehalose shape up to one another and the outside world. Because of the 1,1-glycosidic bond (a bond between carbon atom 1 of the first sugar and carbon atom 1 of the second) between each glucose, trehalose forms a closed ring, and thanks to this tends to self-associate when in solution. In such associations, trehalose demonstrates an avidity for water that few other molecules match. And because of its binding properties the sugar can enhance yeast cell membranes by forming a stable matrix and sticking to charged groups of lipids and proteins, thereby preventing the disruption of proteins and undesirable reactions between them. From this buttressing of the cell membrane comes its usefulness when levels of ethanol (a solvent, let us remember) are high in the environment.
The stress caused to cells by a too-concentrated solution is called osmotic stress. And by golly are yeast cells under osmotic stress at the beginning of fermentation, where they are thrown into a 12% sugar solution (or more, in the case of high-gravity brewing). Trehalose would seem to counteract osmotic stress by coating proteins and stopping them turning into mush (denaturing). Stable proteins mean solid, correctly folded and associated cell contents.
So important is trehalose to brewing yeast that during fermentation it composes up to 5% of cell dry weight. In high-gravity fermentations, where there can be as much as 18 g of sugar per 100 ml, much higher levels are formed — up to 25% of the dry weight. Similar levels are found in dried yeast preparations where its presence and concentration correlate with the ability to withstand the rigours of rehydration.
What about what is known of trehalose’s role in other organisms? In the insect world, trehalose is the rocket fuel used for flight, chosen by Mother Nature to power the pullulation of butterflies and the buzzing of bees because of its efficiency as a storage carbohydrate (twice that of starch). The sugar is not found in abundance though in the plants and animals that humans regularly eat. Even though we have the necessary enzyme — trehalase — to break it down, it is only very recently that trehalose has become a regular part of the diet of non-shiitake mushroom and -oyster eaters (both of these are high in trehalose). Since the invention of an efficient method of trehalose extraction from starch, the sugar has been making its way into our diet through its addition to processed foods, especially frozen foods, where its cryoprotectant properties are used to lower the freezing point of things like ice cream. There is a theory out there that the rise in cases of Clostridium difficile infections may be down to our newfound ingestion of trehalose. This sweet-toothed (sweet-flagella’d?) bacterium seems to have quite a penchant for trehalose!