#14 Dear Yeast, Thank You Very Much
Professor Emeritus at Hokkaido University
Advisor at Japan Food Research Laboratories
Yeast has been an indispensable microorganism for human diet since antiquity. It still plays an important role in present dietary life. Making sake, miso, and soy sauce requires yeast. Sake brewing is said to have a several-thousand-year history, but the word “kobo”, a Japanese translation for yeast, started to be used around the middle of Meiji Period (1868-1912). In sake brewing, words such as “moto” and “syubo” (mixture of rice malt, water, boiled rice, and yeast) have been used since a long time ago. Although Kobo has contributed to human diet for several thousand years, the term itself has been used for a hundred and several tens of years only
1) Bread and yeast
Figure1 shows yeast’s function in bread making. When baker’s yeast starts fermenting in dough such as flour, carbon dioxide is released and the dough starts to rise. Without punching the dough down, the finished bread will have big holes. In order to make fine breads, careful gas release is needed. Bread-making technology is ever-improving.
Baker’s yeast that survives refrigeration was explored to be applied in dough making (1). Frozen storage technology has been developed, and now can be gas-released at a factory and kept refrigerated. This has enabled small bread shops at stations or supermarkets to serve fresh breads easily by baking the frozen doughs with an electric oven on the site. Dough-making used to require a great deal of skill, labor and time, but now it can be finished at factories using the refrigeration-resisting yeast.
It is believed that people already ate bread around 4000B.C. In those days, they baked unfermented dough. In the course of time, people started to bake fermented dough too. I guess fermented dough and its product, delicious bread, was brought about incidentally owing to wild yeast. Yeast is said to be discovered by Leeuwenhoek, inventor of microscope, when he observed dough with his microscope in 1683.
It is Pasteur (featured in my column #8) who discovered the yeast’s role in dough. In 1857, he revealed that yeast dissolves sugar into alcohol and carbon dioxide. Yeast is a key player for delicious bread making, isn’t it?
2) The use of refrigeration-resisting yeast
Baker’s yeast surviving refrigeration is used to make an indicator warning food temperature rise caused by such accidents as improper handling of refrigeration-needed foods or stoppage of refrigerating devices due to power outage (2). In the experiment, bubbles of carbon-dioxide are generated and then the sachets become deformed over time as shown in Figure2. As shown in Figure3, gas is generated rapidly in the intermediate temperature range. Gas generation at 4°C suggests the possibility that this indicator can be used for low temperature control too. As shown in Figure2, which is an experiment at 10°C, gas-generation amount can be controlled in accordance with sugar (glucose) concentration. The amount of ATP in sachets shows an increasing trend similar to gas-generation trend.
As shown in Figure4, another type of baker’s yeast indicator is available using red cabagge pigment with two indexs of color change and bubble generation. If baker’s yeast generates considerable amount of carbon dioxide and pH of the medium decreases, the red cabagge pigment gives a warning with its color change from blue to red along with generation of carbon-dioxide bubbles.
Some microorganisms, such as listeria covered in my column #6, can proliferate even at low temperature and cause food poisoning. Baker’s yeast indicator can be kept refrigerated and will be helpful in warning food poisoning or preventing quality loss when attached to foods. By the recommendation of Codex Alimentarius Commission, Japan is standardizing the distribution/storage temperature of food with possibility of listeria proliferation under 6°C. Baker’s yeast is expected to play an active role in this field too.
1) Hino, A., “Refrigeration-Resisting Yeast”, Kagaku to Seibutsu, 28(11), 736-739(1990)
2)H.Kogure,et.al., Development of a novel microbial sensor with baker's yeast cells for monitoring temperature control during cold food chain,J.Food Protection, 68, 182-186 (2005)