A method for evaluating the freezing tolerance of yeast

Abstract

The present invention relates to the technical field of industrial microorganisms, and in particular to a method for evaluating the freezing resistance of yeast. The present invention provides a method for evaluating the freezing resistance of yeast, which comprises the following steps: making dough containing yeast, edible oil, edible salt, a dough improver and flour, then pre-fermenting the dough and freezing it, and finally performing cyclic freezing. Compared with direct freezing, the cyclic freezing method has a more obvious decrease in the yeast activity preservation rate within the same period of time, which greatly shortens the time for evaluating the freezing resistance of yeast, and is conducive to reducing the quality risk of frozen dough that needs to be stored for a long time.

Description

Technical Field

[0001] The invention relates to the technical field of industrial microorganisms, and in particular to a method for evaluating the freezing resistance of yeast. Background Art

[0002] Frozen dough technology is a new technology developed in the 1950s and introduced into China in the mid-1990s. This technology applies freezing principles and techniques to process the dough into several states: unproofed, pre-proofed, and pre-baked. After freezing, the dough is stored in a frozen state, and then thawed, proofed, and baked to make the finished product. The finished products are mainly European bread, puff pastry, and Chinese puff pastry and tart products. In recent years, frozen dough technology has developed rapidly in China, which has the advantages of saving time, labor, materials, and land for suppliers, and can provide consumers with a variety of fresh, flavorful bread. However, frozen dough technology also faces the problem of reduced bread volume and rough bread texture during long-term freezing.

[0003] Among the four factors that affect the quality of frozen dough, namely yeast, flour, additives, and freeze-thaw conditions, yeast is the key factor affecting the quality of frozen dough. The reason is that the yeast in the frozen dough is damaged by ice crystals and oxidation during the preparation and freezing process, and is also affected by osmotic pressure, which leads to a decrease in survival rate and gas production. As a result, as the freezing time of the frozen dough increases, the specific volume of the dough decreases, the texture becomes rough, and the flavor is poor. Although the lack of fermentation power after freezing can be compensated by increasing the amount of yeast, excessive yeast use will produce yeast odor, and yeast will produce reducing glutathione during refrigeration, which will weaken the gluten network and gas retention of the dough, and have an adverse effect on the flavor, volume and organizational structure of the product. At present, in order to improve the freezing resistance of baker's yeast, either glycerol, water hyacinth pulp, γ-polyglutamic acid, etc. are added to the formula to increase the antifreeze protection of yeast, or genetic means are used to overexpress genes encoding lipid transfer proteins, δ-9-fatty acid desaturase genes, trehalose synthase genes or freezing resistance genes, or knock down trehalase-related genes to improve yeast's tolerance to freezing environments, or use hypertonic solution to culture yeast cells to enhance yeast's freezing resistance, or obtain yeast strains with relatively strong freezing resistance through natural selection or ultraviolet mutagenesis.

[0004] In addition, the evaluation of yeast's antifreeze property often examines the fermentation ability or cell survival rate of the frozen dough after 7, 21, 30, and 180 days of frozen storage. Frozen dough includes direct freezing without proofing after forming (freezing at -20°C), pre-proofing (20-38°C, proofing for 30-120 minutes and then freezing at -20°C), etc. Summary of the invention

[0005] The problem with the existing technology is that the freeze resistance of yeast strains is currently mostly determined by directly freezing without proofing or pre-proofing and then freezing, and the yeast activity preservation rate is mainly compared after freezing for 7 days and 21 days, which poses certain quality risks for frozen dough that is frozen for longer periods of time.

[0006] One of the purposes of the present invention is to provide a method for evaluating the freezing tolerance of yeast;

[0007] Specifically, in order to solve the above technical problems, the present invention provides the following technical solutions:

[0008] The present invention provides a method for evaluating the freezing resistance of yeast, comprising the following steps:

[0009] (1) making dough by mixing yeast, edible oil, edible salt, sugar, a dough improver, flour and water to form a dough, wherein the content of the sugar is 0-20% by weight of the flour;

[0010] (2) pre-fermenting the dough from step (1) and then freezing it, wherein the pre-fermentation temperature is 20 to 40° C. and the freezing temperature is -20 to -40° C.;

[0011] (3) placing the frozen dough in step (2) at -20°C for 48 hours;

[0012] (4) placing the frozen dough in step (3) at -7 to -12°C for 12 to 24 hours and at -18 to -22°C for 8 to 12 hours;

[0013] (5) placing the frozen dough in step (4) at -7 to -12°C for 12 to 24 hours and at -18 to -22°C for 16 to 36 hours;

[0014] (6) Repeat steps (4) and (5) to perform cyclic freezing.

[0015] Preferably, the content of the yeast in step (1) is 0.1-5% based on the weight of the flour.

[0016] Preferably, the content of the butter in step (1) is 1-10% based on the weight of the flour.

[0017] Preferably, the content of edible salt in step (1) is 1-5% based on the weight of flour.

[0018] Preferably, the content of the dough improver in step (1) is 0.1-2% based on the weight of flour.

[0019] Preferably, the sugar content in step (1) is 10-20% based on the weight of flour.

[0020] Preferably, the pre-fermentation time in step (2) is 30 to 90 minutes. Preferably, the freezing time in step (2) is 1 to 48 hours.

[0021] Beneficial effects of the present invention:

[0022] Compared with direct freezing, the cyclic freezing method of the present invention has a more obvious decrease in the yeast activity preservation rate within the same period of time. The experimental results show that cyclic freezing for 30 days can achieve the effect of direct freezing for 90 days or pre-fermentation direct freezing for 60 days, which greatly shortens the time for yeast freeze resistance evaluation and helps to reduce the quality risk of frozen dough that needs to be stored for a long time. DETAILED DESCRIPTION

[0023] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0024] The present invention provides a method for evaluating the freezing resistance of yeast, comprising the following steps:

[0025] (1) making dough by mixing yeast, edible oil, edible salt, sugar, a dough improver, flour and water to form a dough, wherein the content of the sugar is 0-20% by weight of the flour;

[0026] (2) pre-fermenting the dough from step (1) and then freezing it, wherein the pre-fermentation temperature is 20 to 40° C. and the freezing temperature is -20 to -40° C.;

[0027] (3) placing the frozen dough in step (2) at -20°C for 48 hours;

[0028] (4) placing the frozen dough in step (3) at -7 to -12°C for 12 to 24 hours and at -18 to -22°C for 8 to 12 hours;

[0029] (5) placing the frozen dough in step (4) at -7 to -12°C for 12 to 24 hours and at -18 to -22°C for 16 to 36 hours;

[0030] (6) Repeat steps (4) and (5) to perform cyclic freezing.

[0031] During the cyclic freezing process, the moisture in the dough is in a cycle of crystallization, thawing, and recrystallization, which is bound to have a negative impact on the yeast cell wall. Therefore, within the same period of time, the activity of the yeast will decrease more significantly after being frozen in this storage method, thereby shortening the evaluation time of the yeast's freeze resistance.

[0032] In step (1), the yeast content is 0.1-5%; the butter content is 1-10% based on the weight of flour; the edible salt content is 1-5%; the dough improver content is 0.1-2%, and the dough improver can be a commercially available product, such as Angel Yeast Company's frozen dough improver F-99; the dough also contains sugar, and the sugar content is 10-20% based on the weight of flour.

[0033] In step (2), the pre-fermentation time is 30 to 90 minutes. Preferably, the freezing time in step (2) is 1 to 48 hours.

[0034] The beneficial effects of the present invention will be further illustrated by means of specific examples below.

[0035] The yeast activity preservation rate of the present invention is obtained by the following method:

[0036] Yeast activity refers to the gas production capacity of yeast. The calculation method of yeast activity preservation rate is as follows:

[0037] Yeast activity preservation rate = gas production of dough after freezing / gas production of dough before freezing * 100%;

[0038] The test method for gas production is as follows: put 60g of dough directly into the Risogragh fermentation tank, cover it with a lid and seal it, place it in a 30℃ water bath, and record the cumulative gas production (mlCO2) of the Risogragh for 3h.

[0039] The sources of the reagents and instruments used in the examples and comparative examples of the present invention are as follows:

[0040] Table 1 Reagents and instruments used in the examples

[0041]

[0042] Example 1

[0043] Evaluation of Yeast Freezing Tolerance by Cyclic Freezing Method

[0044] (1) Weigh 100 g of flour, 2.5 g of commercially available Angel semi-dry yeast, 1.5 g of edible salt, 5 g of butter, 0.1 g of dough improver and 65 g of water, and mix the above ingredients evenly to form a dough;

[0045] (2) pre-fermenting the dough prepared in step (1) at 30° C. for 60 minutes, dividing it into two pieces, one piece being directly subjected to a gas production test, and the other piece being frozen at -30° C. for 24 hours so that the center temperature of the dough is -20° C., and then being placed at -20° C. for 48 hours;

[0046] (3) The dough frozen in step (2) is first placed at -7°C for 12 hours, then at -18°C for 8 hours, then at -7°C for 12 hours, and then at -18°C for 16 hours. The above process is repeated, and the timing is started from the freezing after pre-fermentation. The gas production of the frozen dough is tested at 7 days, 30 days, 60 days and 90 days of freezing, and the yeast activity preservation rate is calculated.

[0047] Example 2

[0048] Evaluation of Yeast Freezing Tolerance by Cyclic Freezing Method

[0049] (1) Weigh 100 g of flour, 5 g of commercially available Angel semi-dry yeast, 1 g of edible salt, 10 g of butter, 1 g of dough improver, 10 g of sugar and 60 g of water, and mix the above ingredients evenly to form a dough;

[0050] (2) pre-fermenting the dough prepared in step (1) at 20° C. for 90 minutes, dividing it into two pieces, one piece being directly subjected to the gas production test, and the other piece being frozen at -40° C. for 1 hour so that the center temperature of the dough is -20° C., and then being placed at -20° C. for 48 hours;

[0051] (3) The frozen dough obtained in step (2) is first placed at -12°C for 12 hours, then at -20°C for 8 hours, then at -12°C for 12 hours, and then at -20°C for 16 hours. The above process is repeated, and the timing is started from the freezing after pre-fermentation. The gas production of the frozen dough is tested after freezing for 7 days, and the yeast activity preservation rate is calculated.

[0052] Example 3

[0053] Evaluation of Yeast Freezing Tolerance by Cyclic Freezing Method

[0054] (1) Weigh 100 g of flour, 0.1 g of commercially available Angel semi-dry yeast, 5 g of edible salt, 1 g of butter, 2 g of dough improver, 20 g of sugar and 60 g of water, and mix the above ingredients evenly to form a dough;

[0055] (2) pre-fermenting the dough prepared in step (1) at 40° C. for 30 minutes, dividing the dough into two pieces, one piece being directly subjected to a gas production test, and the other piece being frozen at -20° C. for 48 hours so that the center temperature of the dough is -20° C., and then being placed at -20° C. for 48 hours;

[0056] (3) The frozen dough of step (2) is first placed at -10°C for 24 hours, then at -22°C for 12 hours, then at -10°C for 24 hours, and then at -22°C for 36 hours. The above process is repeated, and the timing is started from the freezing after the pre-fermentation. When the frozen dough is frozen for 7 days, the gas production of the frozen dough is tested and the yeast activity preservation rate is calculated.

[0057] Comparative Example 1

[0058] Dough was prepared according to the method of step (1) of Example 1, and then tested by direct freezing method: the dough was divided into two pieces, one piece was directly tested for gas production, and the other piece was directly frozen at -20°C. The gas production of the frozen dough was tested and the yeast activity preservation rate was calculated after being directly frozen for 7 days, 30 days, 60 days and 90 days.

[0059] Comparative Example 2

[0060] Doughs were prepared according to the method of step (1) of Example 2, and then tested by direct freezing method: the dough was divided into two pieces, one piece was directly tested for gas production, and the other piece was directly frozen at -20°C. The gas production of the frozen dough was tested and the yeast activity preservation rate was calculated after being directly frozen for 7 days.

[0061] Comparative Example 3

[0062] Dough was prepared according to the method of step (1) of Examples 2 and 3, and then tested by direct freezing method: the dough was divided into two pieces, one piece was directly tested for gas production, and the other piece was directly frozen at -20°C. The gas production of the frozen dough was tested and the yeast activity preservation rate was calculated after being directly frozen for 7 days.

[0063] Comparative Example 4

[0064] Dough was prepared according to the method of step (1) of Example 1, and then pre-fermented at 30° C. for 60 minutes. The dough was then evenly divided into two pieces, one of which was directly subjected to a gas production test, and the other was directly frozen at -20° C. The gas production of the frozen dough was tested and the yeast activity preservation rate was calculated when the dough was directly frozen for 7 days, 30 days, 60 days and 90 days.

[0065] Comparative Example 5

[0066] Dough was prepared according to the method of step (1) of Example 2, and then pre-fermented at 20°C for 90 minutes. The dough was then evenly divided into two pieces, one of which was directly subjected to a gas production test, and the other was directly frozen at -20°C. The gas production of the frozen dough was tested and the yeast activity preservation rate was calculated after being directly frozen for 7 days.

[0067] Example 6

[0068] Dough was prepared according to the method of step (1) of Example 3, and then pre-fermented at 40°C for 30 minutes. The dough was then evenly divided into two pieces, one of which was directly subjected to a gas production test, and the other was directly frozen at -20°C. The gas production of the frozen dough was tested and the yeast activity preservation rate was calculated after being directly frozen for 7 days.

[0069] The experimental results of Examples 1-3 and Comparative Examples 1-6 are shown in Table 2-3:

[0070] Table 2 Experimental results of yeast activity preservation rate after freezing for 7 days

[0071]

[0072] Table 3 Experimental results of different evaluation methods for freezing for 30 days, 60 days and 90 days

[0073]

[0074]

[0075] As shown in Table 2, compared with direct freezing for 7 days, the yeast activity preservation rate decreased by 7.8-12.1% after 7 days of cycle freezing, and compared with direct freezing for 7 days with pre-fermentation, the yeast activity preservation rate decreased by 5.9-9.6% after 7 days of cycle freezing; as shown in Table 3, after 30 days of cycle freezing, the yeast activity preservation rate decreased to 65%, after 60 days of cycle freezing, the yeast activity preservation rate decreased to 52%, and after 90 days of cycle freezing, the yeast activity preservation rate decreased to 35%, while after 30 days of direct freezing, the yeast activity preservation rate was 86%, after 90 days of direct freezing, the yeast activity preservation rate was still 78%, and after 60 days of direct freezing with pre-fermentation, the yeast activity preservation rate was still 68%. Therefore, 30 days of cycle freezing can achieve the effect of direct freezing for 90 days or the effect of direct freezing for 60 days with pre-fermentation, which greatly shortens the time for evaluating the freeze resistance of yeast and is conducive to reducing the quality risk of frozen dough that needs to be stored for a long time.

[0076] The above is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for evaluating the freezing tolerance of yeast, characterized in that: The steps include: (1) preparing dough by mixing yeast, butter, edible salt, sugar, a dough improver, flour and water to prepare the dough, wherein, based on the weight of the flour, the content of the sugar is 0-20%, the content of the yeast is 0.1-5%, the content of the butter is 1-10%, the content of the edible salt is 1-5%, and the content of the dough improver is 0.1-2%; (2) pre-fermenting the dough of step (1) and then freezing it, wherein the pre-fermentation temperature is 20 to 40° C., the pre-fermentation time is 30 to 90 minutes, the freezing temperature is -20 to -40° C., and the freezing time is 1 to 48 hours; (3) placing the frozen dough in step (2) at -20°C for 48 hours; (4) placing the frozen dough in step (3) at -7 to -12°C for 12 to 24 hours and at -18 to -22°C for 8 to 12 hours; (5) placing the dough frozen in step (4) at -7 to -12°C for 12 to 24 hours and at -18 to -22°C for 16 to 36 hours; (6) Repeat steps (4) and (5) to perform cyclic freezing.

2. The method according to claim 1, characterized in that: The sugar content in step (1) is 10-20% based on the weight of flour.

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