At present, researches related to the improvement of yeast species mainly focus on the mechanism of resistance to high sugar, utilization of maltose, adaptability to low temperature, utilization of maltose and lactose. Among them, the mechanism of high sugar tolerance and maltose utilization are closely related to the fermentation capacity of bread yeast.
Bread yeast is sensitive to high osmotic pressure caused by sugar or salt in dough, or both. Bread dough contains more sugar, salt and other components, all produce osmotic pressure. Yeast resistant to high osmotic pressure must be resistant to high sugar, so according to observation, high sugar resistant yeast must have one of the following three conditions.
(1) have the ability to rapidly accumulate high glycerine content and high osmotic pressure environment phase balance in the cell;
(2) high trehalose content, can maintain the integrity of cell membrane;
(3) when sucrose is used as a carbon source, the saccharase activity of yeast should be adapted to the sugar utilization rate of yeast, so as to maintain a low osmotic pressure environment.
In general, maltose is used later in dough fermentation than glucose. For bread yeasts with poor maltose adaptability, glucose is used first when maltose coexists with glucose, and maltose is not used until the later stage of fermentation. For bread yeasts with good maltose adaptability, maltose can be used quickly in the presence of both maltose and glucose, so the dough will start to grow faster under the sugar-free dough.
After the dough is prepared, it can be frozen to keep it from rancidity for a period of time. After thawing, the dough can still be baked after waking up, which requires the yeast to have good adaptability to low temperature. The genetic mechanism of low temperature tolerance of bread yeast is not well understood. It has been speculated that there may be a relationship between the low temperature resistance and osmotic pressure resistance of yeast. There is 0.5%~2.0% gossypol in beet molasses. Generally, the sucrase of bread yeast can decompose raffinose into fructose and mellitose, but the lack of melliase cannot decompose raffinose into glucose and galactose, so only 1/3 of raffinose can be utilized. If molasses can be utilized by bread yeast, it will not only increase the yeast yield per molasses, but also reduce the BOD content of waste liquid. The main sugar in whey is lactose, which bread yeast cannot assimilate. Only a few yeasts can make use of lactose, but they are not very gas-producing. The utilization of lactose by bread yeast is also a direction for the improvement of bread yeast species.
At present, there are four basic methods to improve bread yeast species: mutagenesis by physical and chemical factors, hybridization and protoplast fusion, genetic engineering and so on.
Ultraviolet light is usually used as mutagenesis agent in the mutagenesis of bread yeast by physical and chemical factors. The problem in mutagenesis breeding is that yeast diploid cells are very stable and not easy to show gene changes. Haploid cells or spores are usually used for mutagenesis.
Hybridization is one of the important methods of bread yeast breeding. Commercial bread yeast is usually polyploid and aneuploid, which makes it difficult to produce spores. Although some bread yeasts can produce seed decay spores, the spore conjugation ability is poor and the survival rate is low. In particular, the haploid of bread yeast often no longer has good fermentation characteristics of bread yeast, which makes cross breeding difficult to succeed.
Protoplast fusion has become the main method for the selection and breeding of bread yeast. It has the following advantages: high fusion frequency; Less restricted by conjugation or sterility; The transmission of genetic material is more complete. Compared with chemically induced protoplast fusion, electroinduced protoplast fusion has the following characteristics: the complete fusion process can be monitored and observed under a microscope; Electric fusion is a controlled process that synchronizes space and time. Electrofusion has less damage to cells. Electrical induction is directly related to the understanding of the molecular level of membrane and can better explain the mechanism of membrane fusion. Fusion frequency is high.
At present, genetic engineering has been successfully used to overcome the obstacle of glucose to maltose utilization. The engineered bacteria constructed by Gistbrocades of the Netherlands can absorb maltose more quickly than ordinary bread yeasts, so they have higher fermentation activity and are suitable for dough fermentation with sucrose content of 0~20%. However, the potential safety problems of genetically engineered bread yeast have attracted close attention from governments and international organizations.