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In the Winery


Yeast: Much more than just a pretty face


If you require a


combination of traits that occur in different strains, it’s not difficult to breed yeast.


By Gary Strachan


haven’t done any yeast breeding for many years, but I’m still fascinated by the things that can be done with this tiny beast. Yeast is the simplest of the eukaryotes. That means that it has a nucleus to contain its DNA and a cell that is similar in structure to ours. Yeast (Saccharomyces cerevisiae) is important enough to geneticists that its genome was mapped before the human genome. It’s a great little fermentation factory for wine, beer, bread and industrial alcohol, but the list doesn’t stop there. If you look on the ingredient list of foods, you will often see “hydrolyzed yeast”. These are dead yeast cells that have ruptured and released their contents. They contribute B vitamins, protein, flavour and texture to processed foods. It’s a versatile ingredient. In winemaking we use hydrolyzed yeast as a nutrition supplement to assist fermentations.


I


Over the years there have been thousands of yeast strains adapted to special conditions, such as low or high temperatures, high sugar contents, release of aroma compounds or efficient alcohol production.


All of these situations can be accommodated by selecting yeast from various strains that are naturally occurring. If your application requires some combination of traits that occur in different strains, it’s not difficult to breed yeast. Just as with plants and higher animals, yeast have an equivalent of male and female, called a and a. These are haploid cells (only a single strand of DNA in the nucleus) similar to our own sperm and egg cells. After you isolate the haploids of two yeast strains, they fuse


together easily and you then have a new strain that contains the traits of both parents.


It’s an easy process. If you’re careful you can do it in an average kitchen.


The diploid cells (two strands of DNA in the nucleus) may or may not perform as you wish. If a trait is dominant, you’re in luck. If it’s recessive, the trait will be hidden.


GARY STRACHAN


This is a 400X magnification of sporulating wine yeast, a Lallemand strain purchased at a local health food shop. The dark cells are stained with malachite green, which is specific for spores. The red cells are counterstained with safranin. Commercial yeast often lose their ability to sporulate and the yield of spores can be low.


The way around this is to ‘sporulate’ your new strain and select haploids again. You can actually test haploid strains. They grow and reproduce just a diploids do.


Picture an egg that can reproduce itself. Yeast have this weird gene that allows them to spontaneously switch mating types. If you’re lucky, this could happen to your experimental haploid culture and you could end up with a stable diploid strain that exhibits the trait you want. Simple, right? Much has been made of genetically modified organisms (GMOs). In my mind the line is crossed when you actually introduce genes from another species. The process that I described above is similar to conventional plant breeding in which plants from the same species are crossed and the progeny selected for particular traits. It you feel you must go over the line, there are kits available online to facilitate gene splicing. Yeast are ideally suited to the manufacture of pharmaceutical products. Take, for example, human insulin. The human gene sequences have already been spliced into yeast and “human” insulin is being produced in large quantities by yeast. I visualize a wine made for diabetics. A glass a day keeps your blood sugar in balance. Other yeasts have already been created with special traits that could be useful during winemaking. The only problem is that winemaking is considered to be a “natural” technology and GMO yeast in


your wine would not be a good marketing ploy. Consider the backlash that occurred when Arctic, non- browning apples were introduced. One of the common processes of winemaking is to complete the yeast fermentation with Saccharomyces cerevisiae and then lower acidity and create complexity by fermenting with a lactic acid bacterium, Leuconostoc oenos. The bacterial genes have already been inserted into a yeast strain. This enables the two fermentations to be carried out simultaneously.


Another potential modification is the insertion of genes for flavour compounds. Yeast are already being used for the production of flavour compounds, especially those that are difficult to isolate from nature, in commercial quantities. One of these is blackberry flavour. If you wish to enhance the nose on your Merlot, no problem. We can do it!


The perception of GMOs is similar in many respects to the pharmaceutical industry. A great many drugs in use target a specific disease, but their effect doesn’t end with the target action. Every time you get a prescription it is accompanied by a warning of potential side effects. It’s frightening.


I think GMOs are in the same category. Yes, we potentially could make a wine that contains insulin, but what would be the side effects? Just because we can do it doesn’t make it morally acceptable, and what about the unknown side effects?


British Columbia FRUIT GROWER • Spring 2017 29


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