Main material changes during glycation - decomposition of protein and β-glucan
1. Breakdown of protein
Like carbohydrates, the breakdown of proteins during glycation is also important. Under the action of protease, protein is decomposed into high molecular nitrogen, medium molecular nitrogen and low molecular nitrogen in turn, and finally decomposed into amino acids. Its decomposition products are not only nutrients for yeast, but also affect the flavor, foam and abiotic stability of beer
1. The main enzymes that decompose protein and their functions
The enzymes in malt that break down proteins are far more complex than those that break down starch. Mainly include: endopeptidase, dipeptidase, aminopeptidase and carboxypeptidase and so on.
2. Factors Affecting Protein Breakdown
During the saccharification process, the factors affecting protein decomposition are: malt quality (solubility and enzyme content), temperature, time, pH value and mash concentration.
(1) The quality of malt is very important. For well-dissolved malt, the macromolecular protein has been decomposed by 60% to 70% in the malting stage, but only 30% to 40% in the saccharification stage. Therefore, the protein solubility of malt affects the nitrogen content of wort.
(2) The saccharification mash is slightly acidic. The enzymes affecting protein breakdown in glycation are mainly endopeptidases and carboxypeptidases. 80% of the free amino nitrogen produced during saccharification is produced by carboxypeptidase. However, if the endopeptidase activity is not high, it is difficult to increase the free amino nitrogen content in wort by relying solely on carboxypeptidase.
(3) Protein decomposition is greatly affected by temperature. Its optimum decomposition temperature is in the range of 45℃~55℃. When resting at 45°C, a large number of low-molecular-weight polypeptides will be formed for yeast reproduction and fermentation; when resting at 55°C, high-molecular protein decomposition products will be formed, with high solubility and increased molecular nitrogen content, which is beneficial to beer foam and wine. body. However, new research shows that a long rest at 50°C can lead to poor head beer, because at this temperature, the substances that make the glutinous substances that form beer head break down.
(4) Low temperature (35°C) cutting. It is beneficial to maintain the activity of endopeptidase and carboxypeptidase and promote the decomposition of protein. Even if the temperature is gradually raised to 50°C or 65°C, it can still maintain high thermal stability. The activity of these two enzymes decreased significantly if the feed was fed at 50°C or 65°C. For poorly dissolved malt, the activity of these two enzymes is low, and the thermal stability is also poor, so low temperature feeding should be adopted. In addition, feeding at low temperature can produce more free amino nitrogen, which is of great significance to the material transformation of yeast metabolism in fermentation.
(5) Thick mash saccharification. Due to the increase of the dissolution of acidic substances, the decrease of pH value and the increase of enzyme concentration, the enzyme can fully contact with the substrate, which is beneficial to the decomposition of protein; at the same time, the thick mash helps the proteolytic enzyme to get colloidal protection and maintain enzyme activity.
3. Methods to control the degree of protein breakdown
In the process of glycation, protein decomposition is very complicated. Methods for evaluating the degree of protein breakdown include:
(1) Longding distinction method. The soluble nitrogen-containing substances contained in the wort were precipitated separately with tannin and ammonium phosphomolybdate, and could be divided into three components: A, B, and C. A component is a high-molecular nitrogen-containing substance, B component is a middle-molecular nitrogen-containing substance, and C component is a low-molecular nitrogen-containing substance. If component A is too high, the abiotic stability of beer is not strong; if component B is too low, the foam performance of beer is poor; if component C is too high, the taste of beer is weak. If the proportion of component B decreases, the proportion of component C will increase; if component B is too low and component C is too high, the protein decomposition is excessive, otherwise it is insufficient decomposition. Based on this analysis, the process method can be adjusted.
Distinguishing criteria: A component, about 25%; B component, about 15%; C component, about 60%.
(2) Kulbach value. Also known as proteolytic strength. That is, under the same concentration of wort, the percentage of the ratio of the nitrogen content of the production wort to the nitrogen content of the laboratory standard agreement method wort (adding the same proportion of hops as the production wort, and boiling for 2 hours).
This value fluctuates between 85% and 120%.
Grading standards:100%~110%, the decomposition is moderate; more than 110%, the protein decomposition intensity is too high; less than 100%, the decomposition is insufficient.
(3) The ratio of formaldehyde nitrogen to soluble nitrogen. That is, the percentage of the ratio of formaldehyde nitrogen to soluble nitrogen in wort.
Grading standards:When the value is 35%~40%, the protein decomposition is moderate; if it is too high, it is excessive decomposition; if it is too low, it is insufficient decomposition.
(4) Content of α-amino nitrogen. The content of α-amino nitrogen in wort is not only related to the nutrition of yeast, but also related to the changes of some yeast metabolites in beer: if the content of α-amino nitrogen is too low, the content of valine is correspondingly low, and the content of valine in beer is also low. The content of diacetyl will increase; if the content of α-amino nitrogen is too high or too low, the content of higher alcohols in beer will be increased.
Grading standards:In the whole malt wort with a concentration of 12°P, the content of α-amino nitrogen should be kept at about 200mg/L; for the 12°P wort with supplementary materials, the content of α-amino nitrogen should be kept at 170~180mg/L, lower than 150mg/L, it will affect the nutrition of yeast.
2. Decomposition of β-glucan
β-glucan is of great significance in beer brewing. On the one hand, the existence of an appropriate amount of glucan is the main component of beer body and foam performance; on the other hand, β-glucan will cause difficulty in filtering wort and beer.
The optimum temperature for β-glucanase decomposition-glucan is 45°C~50°C, and it is inactivated at 52°C~55°C, so it is difficult to decompose β-glucan during the saccharification process. Therefore, the undecomposed dextran in the malting process will bring difficulties to the filtration of wort and beer.
In the mash, if the particles move freely, the shape of the particles will remain unchanged; if there is an external force, such as the rotation speed of the pump or agitator blades is much higher than the rotation speed of the liquid, the shape of the particles will change and rupture. Especially at the edge of the impeller vortex, the bend of the pipeline, the rough surface of the pipeline and the outlet direction of the pump, this kind of external force is easy to form, which is usually called shear force. The intermolecular shear force expands and connects the irregular and irregularly arranged dextran molecules in the mash to form a dextran helical gel through hydrogen bonds, which makes it difficult to filter wort and beer. Therefore, when brewing pure draft beer, the decomposition of dextran should be strengthened in the saccharification stage, so that the low-temperature membrane filtration can go smoothly.
In fact, most of the glucan has been broken down during the malting stage. Because the malt friability instrument value and the conventional wort viscosity are directly proportional to the dextran content of the wort, both analytical values can indicate the filterability of wort and beer. If the malt brittleness value exceeds 80% or the agreed wort viscosity is between 1.51 and 1.63 mPa·s, it indicates that the dextran is decomposed well.
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