Study on the Malting Process of IPA-type Beer

Firstly, high-quality barley and wheat that meet the brewing standards of IPA-type beer are obtained through strict screening. The selected barley and wheat are germinated to produce barley malt and wheat malt suitable for IPA-type beer brewing, which can be used for IPA-type beer production after germination. The malt after malting is crushed by a grinder according to certain standards—generally, the malt husk is broken but not crushed—and the crushed barley malt and wheat malt can be used for feeding; the crushed barley malt and wheat malt are put into a mash tun for saccharification; the wort after saccharification undergoes a series of steps such as wort filtration and hot water sparging; the filtered wort is pumped into a boiling kettle for wort boiling, during which hops are added according to a certain type and amount of hops; the boiled wort is pumped into a whirlpool tank for whirlpool sedimentation for a certain period of time to remove impurities generated after boiling; the wort after whirlpool sedimentation needs to be cooled to the temperature suitable for fermentation (according to process requirements) through sterilized pipelines and plate heat exchangers, and then enters the fermentation tank to wait for inoculation. After all the wort enters the fermentation tank, we use an oxygenation device to supplement an appropriate amount of oxygen to the wort in the fermentation tank, and add a certain amount of hops during the primary fermentation. This is a general description of the entire wort preparation process.

Barley malt: The germination process of barley refers to the process in which barley grows plumules and radicles. The purposes of barley germination are: first, to release and activate the enzymes contained in barley through germination; second, to generate new enzymes through the germination process; third, to dissolve and decompose the internal substances of the grains and change the endosperm structure through the germination process[6]. During this process, barley will accumulate various hydrolases in the grains through germination. The core of barley germinating into barley malt is the formation of hydrolases, among which many hydrolases are of great significance for beer: α-amylase, which is produced in the aleurone layer after barley germination; β-amylase is also a very abundant enzyme in barley, generally located in the embryo of barley; pullulanase, whose main function is to decompose pullulan, is an important member of amylases; proteases are a class of enzymes that can decompose proteins, mainly including endopeptidases and exopeptidases. Generally speaking, the proteases we refer to are endopeptidases, which play a crucial role in wort. Hemicellulases are the first enzymes to act in malt decomposition.

The three key factors in the barley germination process are moisture, temperature, and air[7]. Moisture can be maintained by spraying water to keep the relative humidity of the germination workshop relatively high, and humid air is introduced into the malt layer to provide moisture. The requirements for controlling humidity are different in different germination processes and stages, and the moisture and humidity to be controlled should be mutually appropriate and adjusted in a timely manner. Temperature control requires maintaining balanced heat; barley germination and growth require a certain temperature, and excessively high or low temperatures will have an important impact on the growth process[8]. In winter, the temperature of the malt layer can be adjusted by introducing hot air, and in summer, it can be adjusted by introducing cold air. Turning the malt layer is also a necessary heat dissipation method. Air is mainly provided by ventilation, including malt layer ventilation and workshop environment ventilation. Ventilation in the germination workshop is particularly important for floor germination. The temperature of the ventilation air should also be controlled according to the malt layer temperature in different seasons. Ventilation can not only control the temperature but also loosen the malt layer, maintain air circulation between the grains, and prevent the entanglement of malt roots[9].

From the analysis of the data given in Table 1.1, it can be seen that this variety of barley can be defined as first-class barley in terms of appearance attributes, but in terms of physical and chemical properties, it should be classified as second-class barley. Since the analysis result of this variety of barley is not very satisfactory and its overall performance does not fully meet the requirements of IPA beer brewing, under the condition that this variety of barley is used as the raw material and cannot be changed, we will try to make it meet the requirements of our IPA-type beer brewing by adopting an appropriate steeping method and changing the germination process of this barley.

Table 1.1 Physicochemical Properties of Barley Raw Materials Used for IPA-type Beer

Indicators	Results
Appearance	Light golden color, delicate luster, intact grains, no putrid odor
Impurities (%)	0.6
Moisture content (%)	9.8
Dry thousand-grain weight (g)	36.9
Germination rate (%)	96
Germination capacity (%)	92
Protein (%)	11.9
Water sensitivity (%)	<14

Wheat malt: For the production of ordinary beer, barley malt is the main raw material. For wheat beer, adding an appropriate amount of wheat malt formed by wheat germination is the key to producing this type of beer. Compared with barley malt formed by barley germination used in ordinary beer production, as another main raw material for wheat beer production, it has its own characteristics, which are extremely important for adding wheat malt to produce wheat beer. Secondly, for beer, the formation of foam is greatly related to the protein content in the raw materials, and the protein content in wheat malt is significantly higher than that in barley malt, which is of great significance for beer foam stability and foam quality[10]. Thirdly, the starch rich in wheat is more likely to gelatinize compared with barley starch, and this characteristic reflected in beer brewing has the advantage of solving the problem of insufficient wort saccharification. Fourthly, wheat malt has no hard outer husk, which makes its extract yield relatively large—about 5% higher than that of barley malt. Fifthly, compared with barley malt, wheat malt contains more soluble nitrogen, which has a positive effect on controlling the production of unfavorable substances such as diacetyl during beer fermentation. Sixthly, compared with barley malt, wheat malt contains relatively less tannin, only 30% to 50% of the former, which is positive for the stability of beer taste and the coordination of mouthfeel. Seventhly, compared with high-quality barley malt, wheat malt is cheaper and easier to purchase, especially when used together with barley malt for beer brewing, which is beneficial to saving production costs and raw material consumption[11]. Although using wheat malt as the main raw material for beer has quite obvious advantages, it also has defects that are not conducive to beer production: the glumes and grains of wheat malt are separated. During wort filtration, wort clarification is mainly carried out through the mash bed formed by mash. Due to the separation of wheat grain husks and grains, it is very difficult to form a filter layer. In addition, the high starch and protein content of wheat malt will lead to high wort viscosity, which is very easy to block the filter layer, resulting in extremely slow filtration efficiency. Since wheat is rich in proteins, which are usually macromolecular and have excellent solubility, this leads to a large amount of macromolecular proteins in wort that are not easy to precipitate and decompose, which is very unfavorable for beer colloidal stability[12]. Therefore, we must fully consider these characteristics and select wheat malt suitable for beer production according to these characteristics, or carry out malting through an optimized malting process to meet our brewing needs[12].

The purpose of wheat steeping is to provide sufficient moisture for the wheat germination process, while maintaining good ventilation conditions to create an oxygen-rich environment conducive to wheat germination. The determination of steeping degree is very important for the quality of malt, and it is one of the key steps in the preparation of wheat malt[13]. As mentioned earlier, in the barley steeping process, we chose a steeping degree of 40%. The reason for determining a 40% barley steeping degree is that barley, unlike wheat, is a hulled grain with a husk on the outside. During barley steeping, the water absorption rate is relatively slow due to the husk, resulting in a relatively long germination time. In addition, the protein content of barley malt is not very high, about 10%. Therefore, according to the physicochemical properties of barley, a steeping degree of more than 40% can be selected for barley steeping, which can improve the efficiency of barley germination[14]. Compared with barley, wheat grains are not surrounded by husks. Without the barrier of husks, the water absorption rate of wheat is relatively faster than that of barley, and it is easier to absorb moisture, resulting in a shorter germination time for wheat than for barley. At the same time, since wheat contains relatively higher protein than barley, about 13%, we select a slightly lower steeping degree (about 37%) according to these properties of wheat. The purpose of this is to give wheat sufficient germination time to allow various substances in the malt to fully react[15].

We can meet the needs of brewing IPA beer by selecting high-quality wheat varieties, but it is sometimes very difficult to select wheat varieties that are very suitable. To solve this problem, for wheat varieties whose overall performance does not fully meet the requirements of IPA beer brewing, and under the condition that this variety of wheat is used as the raw material and cannot be changed, we will try to make it meet the requirements of our IPA-type beer brewing by adopting an appropriate steeping method and changing the germination process of this wheat.

2.2 Experimental Materials and Methods

2.2.1 Main Raw Materials

Australian imported barley (provided by micetcraft)
White-skinned low-gluten wheat produced in northern Henan Province ,china

2.2.2 Main Experimental Equipment and Instruments

Steeping tank, constant temperature drying oven, malt grinder, electronic balance, KN520 Kjeldahl nitrogen analyzer, constant temperature water bath, constant temperature incubator, visible spectrophotometer.

2.2.3 Main Experimental Reagents

Anhydrous ethanol; citric acid; disodium hydrogen phosphate; potassium dihydrogen phosphate; sodium hydroxide; ninhydrin; fructose; potassium iodate; glycine; sodium chloride; mercaptoethanol; bovine hemoglobin.

2.2.4 Experimental Methods

2.2.4.1 Barley Steeping Experiment

(1) Barley Steeping Experiment

Wash the barley with distilled water, adopt three different steeping schemes (soaking for 4 hours and interrupting for 4 hours, soaking for 4 hours and interrupting for 8 hours, soaking for 6 hours and interrupting for 6 hours), soak the barley in several temperature gradients in turn, and carry out barley germination when the final steeping degree reaches more than 40%.

Large Petri dish test: Put 200g of barley into a large Petri dish with a diameter of about 10cm, soak the barley using three different steeping schemes (soaking for 4 hours and interrupting for 4 hours, soaking for 4 hours and interrupting for 8 hours, soaking for 6 hours and interrupting for 6 hours). While soaking the barley with these three different schemes, control the temperature at four gradients of 16℃, 21℃, 26℃, and 31℃. Carry out steeping according to the method given in the barley steeping experiment. When the steeping degree >40%, end the steeping process, then carry out continuous germination for one week in an environment with a temperature of 21℃ and a relative air humidity of about 75%. After complete dehydration, investigate the correlation between the three methods under different temperature gradients, the change of steeping degree with time, and the correlation between steeping temperature and time (see Table 1.2 for barley soaking methods).

Table 1.2 Design of Barley Steeping Schemes

Steeping Temperature	16℃	21℃	26℃
Soaking for 4h and interrupting for 6h	Saccharification time/min, Extract/% (mg/100g), α-AN/%, Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/% (mg/100g), α-AN/%, Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/% (mg/100g), α-AN/%, Kolbach index/%, Diastatic power/WK
Soaking for 6h and interrupting for 6h	Saccharification time/min, Extract/% (mg/100g), α-AN/%, Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/% (mg/100g), α-AN/%, Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/% (mg/100g), α-AN/%, Kolbach index/%, Diastatic power/WK
Soaking for 4h and interrupting for 8h	Saccharification time/min, Extract/% (mg/100g), α-AN/%, Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/% (mg/100g), α-AN/%, Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/% (mg/100g), α-AN/%, Kolbach index/%, Diastatic power/WK

Detect the external morphology, total thousand-grain weight of barley, germination capacity, and germination rate of barley malt samples according to the methods specified in QB/T1686-2008 Beer Malt; analyze the saccharification time of barley malt, the activity of starch saccharifying enzyme (diastatic power) of barley malt, the content of extracted substances in barley malt, the content of α-AN in barley malt, and the Kolbach index according to the methods specified in QB/T1686-2008.

2.2.4.2 Barley Germination Experiment

Barley soaking method: First, wash the barley with distilled water, then soak the barley (soak for 4 hours and interrupt for 8 hours, the soaking temperature of barley is about 22℃). The final soaking of barley malt is carried out until the steeping degree is more than 40%, and then the barley germinates.

Barley germination test: Obtain the optimal barley steeping process through the 2.2.4.1 steeping experiment, use this result to explore the optimal conditions for barley germination, and determine the optimal scheme for barley germination.

The barley used in this experiment is Australian imported barley, and its main quality characteristics are shown in Table 1.1 above:

(1) Experiment on exploring the optimal germination temperature

When the steeping degree of malt reaches 40%, carry out constant temperature germination experiments at germination temperatures of 14℃, 17℃, 20℃, 23℃, and 26℃ respectively. Dry the malt after germination experiments at various temperatures to remove moisture, then detect its physicochemical properties and indicators. The experimental method is shown in Table 1.3:

Table 1.3 Design of Barley Germination Temperature Gradients

Germination Temperature	14℃	17℃	20℃	23℃	26℃
Soaking for 4h and interrupting for 8h, 22℃	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK

(2) Experiment on exploring the optimal germination method of barley malt

Barley malt with a steeping degree of about 40% is germinated by three methods: increasing temperature, decreasing temperature, and first decreasing temperature then increasing temperature. The barley malt formed after the three germination methods is completely dehydrated, and then its physicochemical properties are identified. The experimental method is listed in Table 1.4.

Table 1.4 Design of Barley Germination Processes

Germination Process	Gradually decreasing from 26℃ to 14℃	Gradually decreasing from 23℃ to 17℃ then to 14℃	Gradually increasing from 14℃ to 20℃ then to 26℃
Soaking for 4h and interrupting for 8h, 22℃	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK

Large Petri dish germination test: Put 200g of barley into a large Petri dish with a diameter of about 10cm, and soak the malt according to the following method: first, clean the barley; after soaking the barley for 4 hours, filter the barley out of the water, and soak the barley in water again after 8 hours. The selected soaking temperature of barley malt should be controlled at about 22℃; the final soaking degree of barley malt must be more than 40%; when the steeping degree of barley reaches more than 40%, stop steeping; after steeping, let the barley germinate for one week in air with the same relative air humidity according to the germination temperature and germination method given above; finally, dry the germinated barley malt and determine its corresponding malt physicochemical values.

Detect the external morphology, total thousand-grain weight of barley, germination capacity, and germination rate of barley malt samples according to the methods specified in QB/T1686-2008; analyze the saccharification time of barley malt, the activity of starch saccharifying enzyme of barley malt, the content of extracted substances in barley malt, and the content of α-AN in barley malt, and the Kolbach index according to the methods specified in QB/T1686-2008.

Analyze the experimental data using Office Excel 2007, SPSS13.0, and DPS7.05 as statistical software.

2.2.4.3 Wheat Steeping Experiment

Wheat steeping experiment scheme process: Wash the wheat with distilled water, adopt two different steeping schemes (soaking for 2 hours and interrupting for 4 hours, soaking for 3 hours and interrupting for 3 hours), soak the wheat in several temperature gradients in turn, and carry out wheat germination when the final steeping degree reaches 38%.

Large Petri dish test: Put 300g of wheat into a large Petri dish with a diameter of about 10cm, soak the wheat using two different steeping schemes (soaking for 2 hours and interrupting for 4 hours, soaking for 3 hours and interrupting for 3 hours) respectively. While soaking the wheat with these two different schemes, control the temperature at three gradients of 12℃, 14℃, and 16℃. Carry out steeping according to the method given in the wheat steeping experiment. When the steeping degree >38%, end the steeping process, then carry out continuous germination for one week in an environment with a temperature of 15℃ and a relative air humidity of about 75%. After complete dehydration, investigate the correlation between the two methods under different temperature gradients, the change of steeping degree with time, and the correlation between steeping temperature and time.

Analyze the saccharification time of wheat malt, the activity of starch saccharifying enzyme of wheat malt, the content of extracted substances in wheat malt, the content of α-AN in wheat malt, and the Kolbach index according to the methods specified in QB/T1686-2008, as shown in Table 1.5.

Table 1.5 Design of Wheat Steeping Schemes

Steeping Temperature	12℃	14℃	16℃
Soaking for 2h and interrupting for 4h	Saccharification time/min, Extract/%, α-AN/(mg/100g), Diastatic power/WK, Kolbach index/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Diastatic power/WK, Kolbach index/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Diastatic power/WK, Kolbach index/%
Soaking for 3h and interrupting for 3h	Saccharification time/min, Extract/%, α-AN/(mg/100g), Diastatic power/WK, Kolbach index/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Diastatic power/WK, Kolbach index/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Diastatic power/WK, Kolbach index/%

2.2.4.4 Wheat Germination Experiment

Obtain the optimal wheat steeping process through the 2.2.4.3 wheat steeping experiment, use this result to explore the optimal conditions for wheat germination, and determine the optimal scheme for wheat germination.

Wheat soaking method process:

First, wash the wheat with distilled water, then soak the wheat (soak for 2 hours and interrupt for 8 hours, the soaking temperature of wheat is about 14℃). When the steeping degree reaches more than 38%, end the wheat soaking process, and the wheat germinates.

(1) Exploration of the optimal temperature for wheat germination

When the steeping degree of wheat reaches 38%, carry out constant temperature germination experiments at germination temperatures of 13℃, 15℃, and 17℃ respectively. Dry the wheat malt after germination experiments at various temperatures to remove moisture, then detect its physicochemical properties and indicators. The experimental method is shown in Table 1.6.

Table 1.6 Design of Wheat Germination Temperature Gradients

Germination Temperature	13℃	15℃	17℃
Soaking for 2h and interrupting for 4h, 14℃	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%

(2) Exploration of the optimal germination method of wheat malt

Wheat malt with a steeping degree of about 38% is germinated by three methods: increasing temperature, decreasing temperature, and first decreasing temperature then increasing temperature. The wheat malt formed after the three germination methods is completely dehydrated, and then its physicochemical properties are identified. The experimental method is listed in Table 1.7.

Table 1.7 Three Different Wheat Germination Processes

Germination Process	Gradually increasing from 13℃ to 17℃	Gradually decreasing from 17℃ to 13℃	First decreasing from 15℃ to 13℃ then increasing to 17℃
Soaking for 2h and interrupting for 4h, 14℃	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%

(3) Exploration of the optimal germination time of wheat malt

When the steeping degree of wheat reaches 38%, carry out constant temperature (15℃) germination experiments with wheat germination times of 4d, 5d, 6d, and 7d respectively. Dry the wheat malt after germination experiments at various temperatures to remove moisture, then detect its physicochemical properties and indicators. The experimental method is shown in Table 1.8.

Table 1.8 Design of Wheat Germination Time Gradients

Germination Time	4d	5d	6d	7d
Soaking for 2h and interrupting for 4h, 14℃	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%	Saccharification time/min, Extract/%, α-AN/(mg/100g), Kolbach index/%, Diastatic power/WK, Viscosity/mPas, Protein/%

Large Petri dish germination test: Put 200g of wheat into a large Petri dish with a diameter of about 10cm, and soak the wheat according to the following method: first, clean the wheat; after soaking the wheat for 2 hours, filter the wheat out of the water, and soak the wheat in water again after 4 hours. The selected soaking temperature of barley malt should be controlled at about 14℃; the final soaking degree of wheat malt must be more than 38%; when the steeping degree of wheat reaches more than 38%, stop steeping; after steeping, let the wheat germinate for 5d in air with the same relative air humidity according to the germination temperature and germination method given above, set the temperature at 15℃, carry out germination experiments for 4d, 5d, 6d, and 7d respectively by the method of first decreasing temperature then increasing temperature, finally dry the germinated wheat malt and determine its corresponding malt physicochemical values.

Analyze the saccharification time of wheat malt, the activity of starch saccharifying enzyme of wheat malt, the content of extracted substances in wheat malt, the content of α-AN in wheat malt, the Kolbach index, viscosity, and protein content according to the methods specified in QB/T1686-2008.

Analyze the experimental data using Office Excel 2007, SPSS13.0, and DPS7.05 as statistical software.

2.3 Results and Discussion

2.3.1 Selection of the Optimal Barley Steeping Scheme

Detect the external morphology, total thousand-grain weight of barley, germination capacity, and germination rate of barley malt samples according to the methods specified in QB/T1686-2008; analyze the saccharification time of barley malt, the diastatic power of barley malt, the content of extracted substances in barley malt, the content of α-AN in barley malt, and the Kolbach index according to the methods specified in QB/T1686-2008.

2.3.1.1 Correlation between Steeping Degree and Different Temperatures and Time under the Scheme of Soaking for 4h and Interrupting for 4h

Adopt the scheme of soaking for 4 hours and interrupting for 4 hours to explore the experimental results of the correlation between steeping degree and different temperatures and time.

(Figure 1.2 Change of Barley Steeping Degree with Steeping Time at Different Temperatures)

From Figure 1.2, it can be concluded that when using the scheme of soaking for 4 hours and interrupting for 4 hours for steeping, when the final steeping degree is controlled above 40%, the steeping time can be controlled at about 20h at temperatures of 31℃ and 26℃, which takes less time. However, when completing a 40% steeping degree at 21℃, it takes 24h to meet the requirement, and the time consumed for the steeping process at 16℃ is as long as 28h; it can also be seen that the water absorption rate of barley decreases sequentially from 31℃ to 16℃.

2.3.1.2 Correlation between Steeping Degree and Different Temperatures and Time under the Scheme of Soaking for 4h and Interrupting for 8h

Adopt the scheme of soaking for 4 hours and interrupting for 8 hours to explore the experimental results of the correlation between steeping degree and different temperatures and time.

(Figure 1.3 Change of Barley Steeping Degree with Steeping Time at Different Temperatures)

From Figure 1.3, it can be concluded that when using the scheme of soaking for 4 hours and interrupting for 8 hours for steeping, when the final steeping degree is controlled above 40%, the steeping time can be controlled at about 20h at temperatures of 31℃ and 26℃, which takes less time. However, when completing a 40% steeping degree at 21℃, it takes 24h to meet the requirement, and the time consumed for the steeping process at 16℃ is as long as 28h; when selecting the scheme of soaking for 4 hours and interrupting for 8 hours, the water absorption rates of barley at the four temperature gradients are not much different, which is conducive to barley germination.

2.3.1.3 Correlation between Steeping Degree and Different Temperatures and Time under the Scheme of Soaking for 6h and Interrupting for 6h

Adopt the scheme of soaking for 6 hours and interrupting for 6 hours to explore the experimental results of the correlation between steeping degree and different temperatures and time.

(Figure 1.4 Change of Barley Steeping Degree with Steeping Time at Different Temperatures)

From Figure 1.4, it can be concluded that the experimental results of the correlation between steeping degree and different temperatures and time obtained by adopting the scheme of soaking for 6 hours and interrupting for 6 hours are basically consistent with those obtained by adopting the scheme of soaking for 4 hours and interrupting for 8 hours and the scheme of soaking for 4 hours and interrupting for 4 hours. However, in the actual production process, the longer the barley soaking time, the greater the water requirement, which is not conducive to the environmental protection principle of energy conservation and emission reduction. Therefore, compared with the scheme of soaking for 4 hours and interrupting for 4 hours and the scheme of soaking for 6 hours and interrupting for 6 hours, the scheme of soaking for 4 hours and interrupting for 8 hours is the most suitable process scheme.

2.3.2 Selection of the Optimal Barley Steeping Temperature

We have selected the scheme of soaking for 4 hours and interrupting for 8 hours through experiments, and under this scheme, we need to find the ideal steeping temperature.

(Figure 1.5 Influence of Different Steeping Temperatures on 5 Physicochemical Properties of Barley Malt)

We can draw the following conclusions from Figure 1.5: When carrying out the barley soaking process using the scheme of soaking for 4 hours and interrupting for 8 hours, the saccharification time, the activity of starch saccharifying enzyme of barley malt, the content of extracted substances in barley malt, the content of α-AN in barley malt, and the Kolbach index of barley malt at the temperature gradients of 16℃, 21℃, 26℃, and 31℃ show significant differences. The shortest saccharification time is at the temperature gradient of 26℃, and the longest saccharification time is at the temperature gradient of 31℃. Although the extracts at the temperature gradients of 16℃, 21℃, 26℃, and 31℃ show significant differences, the difference in their contents is not obvious. At the temperature gradient of 26℃, the activity of starch saccharifying enzyme is the strongest; there is no significant difference between barley soaked at 26℃ and 31℃, and there is also no significant difference in the Kolbach index between barley soaked at 16℃ and 21℃, but the value is relatively high, and there is a relatively significant difference compared with barley soaked at 26℃ and 31℃; in terms of α-AN content, there is no significant difference between barley soaked at 16℃ and 21℃, but there is a significant difference compared with 26℃ and 31℃. At temperatures of 26℃ and 31℃, there is also a significant difference in α-AN content, and it is significantly higher at 31℃ than at 26℃.

2.3.3 Selection of the Optimal Barley Germination Temperature

The germination temperature of barley has a significant impact on the quality of barley malt, so it is very necessary to explore the optimal germination temperature for the quality of barley malt. The physicochemical indicators of barley malt at different temperatures are shown in Figure 1.6 below.

(Figure 1.6 Influence of Different Germination Temperatures on 5 Physicochemical Properties of Barley Malt)

From Figure 1.6, it can be concluded that for barley malt soaked using the scheme of soaking for 4 hours and interrupting for 8 hours, the α-AN, starch saccharifying enzyme activity, and Kolbach index of the malt after germination show significant differences at temperatures of 14℃, 17℃, 20℃, 23℃, and 26℃. The barley germination temperature at which the Kolbach index is the largest and the starch saccharifying enzyme activity is the strongest is 17℃, but the α-AN content in the malt at 17℃ is relatively lower than that at 14℃; in terms of the time used for saccharification, there is not much difference in the germination temperatures at 14℃ and 16℃, and the values are very close; the same is true for 23℃ and 26℃, but there is a significant difference between these two temperatures and the other three temperatures. In addition, the time used for saccharification is the shortest when the germination temperature is 17℃; in terms of the extract of barley malt at the 5 germination temperatures, 17℃ is very different from 14℃, 20℃, 23℃, and 26℃, while there is no very significant difference in the extract between the three temperatures of 14℃, 23℃, and 26℃. The temperature at which the extract value of barley during germination is the largest is 17℃. In summary, through experiments, we found that 17℃ is the most favorable and suitable temperature for barley germination.

2.3.4 Selection of the Optimal Barley Germination Scheme

Different barley germination schemes also have an important effect on the quality of barley malt, so the most suitable germination scheme plays an important role in the quality of barley malt. The influence of different germination schemes on several physicochemical properties of barley malt is shown in Figure 1.7 below.

(Figure 1.7 Influence of Different Steeping Schemes on 5 Physicochemical Properties of Barley Malt)

According to the data in Figure 1.7, for barley malt soaked using the scheme of soaking for 4 hours and interrupting for 8 hours, the α-AN, starch saccharifying enzyme activity, and extract of the malt after germination show significant differences when the temperature is from 14℃ to 26℃, from 26℃ to 14℃, and from 23℃ to 26℃ (first decreasing then increasing). The barley germination scheme with the largest α-AN content, the strongest starch saccharifying enzyme activity, and the largest extract content is from 26℃ to 14℃. There is no significant difference in the saccharification time between the two schemes of from 14℃ to 26℃ and from 26℃ to 14℃, but there is a significant difference between the scheme of first decreasing then increasing from 23℃ to 26℃ and the other two schemes. The saccharification time is the shortest in the scheme of from 26℃ to 14℃ among the three germination schemes; the scheme of continuous heating from 14℃ to 26℃ has a significant difference in the Kolbach index compared with the other two schemes, while there is basically no difference between the scheme of continuous cooling from 26℃ to 14℃ and the scheme of first decreasing then increasing from 23℃ to 26℃, and the Kolbach index obtained by the scheme of first decreasing then increasing from 23℃ to 26℃ is the largest. In summary, it can be concluded that the scheme of continuous cooling from 26℃ to 14℃ is the optimal scheme.

2.3.5 Selection of the Optimal Wheat Steeping Temperature

2.3.5.1 Correlation between Wheat Steeping Degree and Different Temperatures and Time under the Scheme of Soaking for 2h and Interrupting for 4h

Adopt the scheme of soaking for 2 hours and interrupting for 4 hours to explore the experimental results of the correlation between steeping degree and different temperatures and time.

(Figure 1.8 Change of Wheat Steeping Degree with Steeping Time at Different Temperatures)

From Figure 1.8, it can be seen that for the wheat steeping scheme of soaking for 2 hours and interrupting for 4 hours, the water absorption rates at the three temperature gradients are relatively fast, and the time consumed when the steeping degree reaches more than 38% is generally about 20h. Among them, the water absorption rate at 16℃ is relatively faster than that at the other two temperatures during the entire steeping process. The water absorption rate of wheat steeping at 14℃ is relatively average compared with the other two temperature gradients, which is more conducive to the occurrence of various metabolic reactions during the later germination process of wheat.

2.3.5.2 Correlation between Wheat Steeping Degree and Different Temperatures and Time under the Scheme of Soaking for 3h and Interrupting for 3h

Adopt the scheme of soaking for 3 hours and interrupting for 3 hours to explore the experimental results of the correlation between steeping degree and different temperatures and time.

(Figure 1.9 Change of Wheat Steeping Degree with Steeping Time at Different Temperatures)

From Figure 1.9, it can be seen that for the wheat steeping scheme of soaking for 3 hours and interrupting for 3 hours, the water absorption rates at the three temperatures are longer than those of the scheme of soaking for 2 hours and interrupting for 4 hours. When the steeping degree reaches about 38%, it generally takes more than 20h. By investigating the relationship between steeping degree and steeping time at the three temperature gradients, it is found that the relationship between steeping degree and steeping time at each temperature gradient under the two steeping schemes is not much different.

2.3.6 Selection of the Optimal Wheat Steeping Temperature

We have selected the scheme of soaking for 2 hours and interrupting for 4 hours through experiments, and under this scheme, we need to find the ideal steeping temperature.

Set the temperature gradients at 12℃, 14℃, and 16℃ respectively, and under a certain atmospheric humidity, when carrying out steeping using the scheme of soaking for 2 hours and interrupting for 4 hours, determine the saccharification time of wheat malt, the amylase activity of wheat malt, the content of extracted substances in wheat malt, the content of α-AN in wheat malt, and the Kolbach index at the three temperature gradients for experimental analysis.

(Figure 2.0 Influence of Different Steeping Temperatures on 5 Physicochemical Properties of Barley Malt)

Through the analysis of Figure 2.0, it can be seen that when carrying out the wheat soaking process using the scheme of soaking for 2 hours and interrupting for 4 hours, the saccharification time of wheat malt, the starch saccharifying enzyme activity of wheat malt, the content of extracted substances in barley malt, the content of α-AN in wheat malt, and the Kolbach index at the temperature gradients of 12℃, 14℃, and 16℃ show a certain gap. In terms of saccharification time, the saccharification time increases with the increase of temperature, but the difference in saccharification time at the three temperature gradients is not very large, basically maintaining below 15min; in terms of extract content, the extract content at 14℃ is significantly higher than that at the other two temperatures, and the extract content at the three temperatures is maintained at around 80%, and there is no significant positive correlation between extract content and temperature; in terms of Kolbach index, the Kolbach index increases with the increase of temperature; in terms of starch saccharifying enzyme activity, i.e., diastatic power, the activity of starch saccharifying enzyme at 14℃ reaches 310WK, which is higher than that at 12℃ and 16℃. A high amylase activity value is very beneficial for later germination; in terms of α-AN content, the values at the three temperature gradients are not much different, basically maintaining around 140(mg/100g), which meets the brewing requirements.

2.3.7 Selection of the Optimal Wheat Germination Temperature

Maintain 14℃, carry out the wheat steeping process according to the method of soaking for 2 hours and interrupting for 4 hours. The influence of different germination temperatures on the saccharification time, extract, Kolbach index, starch saccharifying enzyme activity, α-AN, viscosity, and protein content of wheat malt is shown in Table 1.9:

Table 1.9 Physicochemical Properties of Wheat Malt at Different Germination Temperatures

Items	Germination Temperature
	13℃	15℃	17℃
Saccharification time/min	14	12	10
Extract/%	80.56	83.06	81.23
α-AN/(mg/100g)	130	137	143
Kolbach index/%	38.5	41.6	43.1
Diastatic power/WK	306	315	298
Viscosity/mPas	1.9	1.5	2.2
Protein/%	13.4	12.2	13.8

Through the detection of the physicochemical properties after germination, we can see that when the germination temperature of wheat malt is 15℃, compared with the other two temperature gradients, its saccharification time is moderate, the extract content is the largest, the activity of starch saccharifying enzyme is the highest, and due to the relatively low protein content, the viscosity of wort is smaller than that at the other two temperatures, and the values of Kolbach index and α-AN are also relatively moderate. In summary, the germination temperature of 15℃ is the most favorable for wheat germination, and the physicochemical properties after germination are the most in line with our requirements for brewing IPA beer.

2.3.8 Selection of the Optimal Wheat Germination Scheme

Maintain 14℃, carry out the wheat steeping process according to the method of soaking for 2 hours and interrupting for 4 hours. The influence of different germination schemes on the saccharification time, extract, Kolbach index, starch saccharifying enzyme activity, α-AN, viscosity, and protein content of wheat malt is shown in Table 2.0:

Table 2.0 Physicochemical Properties of Wheat Malt under Different Germination Schemes

Items	Germination Scheme
	Gradually increasing from 13℃ to 17℃	Gradually decreasing from 17℃ to 13℃	First decreasing from 15℃ to 13℃ then increasing to 17℃
Saccharification time/min	12	15	11
Extract/%	82.92	81.06	80.75
α-AN/(mg/100g)	134	128	140
Kolbach index/%	42.0	39.6	40.3
Diastatic power/WK	317	310	303
Viscosity/mPas	1.7	2.0	2.5
Protein/%	12.4	12.9	13.3

Through the detection of the physicochemical properties after germination, we can see that when the germination scheme of wheat malt is gradually increasing from 13℃ to 17℃, compared with the other two schemes, its saccharification time is moderate, the extract content is the largest, the activity of starch saccharifying enzyme is the highest, and due to the relatively low protein content, the viscosity of wort is smaller than that of the other two germination schemes, and the values of Kolbach index and α-AN are also relatively moderate. In summary, the germination scheme of gradually increasing from 13℃ to 17℃ is the most favorable for wheat germination, and the physicochemical properties after germination are the most in line with our requirements for brewing IPA beer.

2.3.9 Selection of the Optimal Wheat Germination Days

Maintain 14℃, carry out the wheat steeping process according to the method of soaking for 2 hours and interrupting for 4 hours. The influence of different germination days on the saccharification time, extract, Kolbach index, starch saccharifying enzyme activity, α-AN, viscosity, and protein content of wheat malt is shown in Table 2.1:

Table 2.1 Physicochemical Properties of Wheat under Different Germination Days

Items	Germination Days
	4d	5d	6d	7d
Saccharification time/min	13	12	11	10
Extract/%	81.54	82.86	82.90	83.28
α-AN/(mg/100g)	128	132	139	129
Kolbach index/%	42.23	42.85	43.02	43.84
Diastatic power/WK	312	320	306	310
Viscosity/mPas	1.7	2.0	2.2	2.0
Protein/%	11.7	12.2	13.5	12.5

Through the detection of the physicochemical properties after germination, we can draw the following conclusions: The saccharification time, extract content, and Kolbach index of wheat malt are positively correlated with the germination days respectively. The amylase activity is considered to be the highest when the germination days are 5d, and the protein content and viscosity value are more moderate compared with the other three schemes. Based on the above experimental results, we select the 5d germination scheme, which is considered to be more in line with the brewing requirements of IPA beer.

2.4 Summary of This Chapter

Through the above experiments, we finally draw the following conclusions: In terms of the barley steeping scheme, the scheme of soaking for 4 hours and interrupting for 8 hours is determined for barley soaking; in terms of the temperature used for steeping, the steeping process is determined to be carried out at 26℃. In terms of the barley germination scheme, the scheme of continuous cooling from 26℃ to 14℃ is determined as the optimal scheme. In terms of wheat steeping, the scheme of soaking for 2 hours and interrupting for 4 hours is selected as the best wheat steeping method, and 14℃ is selected for wheat soaking, which is the optimal temperature for wheat steeping. In summary, the comprehensive effect of steeping by the method of soaking for 2 hours and interrupting for 4 hours at 14℃ is the best. In terms of the germination temperature of wheat, the germination temperature of 15℃ is the most favorable for wheat germination. In terms of the germination scheme of wheat, the germination scheme of gradually increasing from 13℃ to 17℃ is the most favorable for wheat germination. In terms of the germination days of wheat, the 5d germination scheme is selected. Carry out the malting process according to the above experimental results, and the barley malt and wheat malt after malting meet the brewing requirements of IPA beer

References

[1] Cai, Y. (2014). Studies on IPA Beer Brewing. Master’s Thesis, Qilu University of Technology.

Author Profile: Alex Chen, Lead Brewery Process Engineer

— Alex Chen

“Lead Brewing Process Engineer at Micetcraft”

My mission is simple: to empower brewers with the tools and knowledge they need to turn their vision into exceptional beer. Every detail in our equipment is engineered with the brewer’s success in mind. Because when you thrive, the entire craft community thrives.”

[1] LI Yan, XU Subo. A Century of the British Beer Industry (1904-2004)[J]. China Brewing, 2005, (3): 16-19.
[2] ZHANG Youdong. Analysis on the Development Prospect of Micro-brewed Beer in Chinese Market[J]. Fujian Light Industry & Textile, 2001, (5): 1-3.
[3] SUN Liguo, GAO Zedong. The Micro Beer Industry in the United States[J]. Brewing Science & Technology, 2001, (11): 67-68.
[4] WANG Ailin. Discussion on the Current Situation and Development Ideas of China’s Beer Industry[J]. Theoretical Investigation, 2009, (12): 144-145.
[5] Barth S, Meier S, Georgensgmuend. Market Leaders and Their Challengers in the Top 40 Countries[M]. Hersbruck: Germany COS Druck & Verlag GmbH, 2008.
[6] QIN Yaozong. Beer Technology[M]. Beijing: China Light Industry Press, 1999.
[7] Picking G J. Low-and Reduced Alcohol Wine[J]. Journal of Wine Research, 2000, 11(2): 129-144.
[8] GU Yaochen. Minor Grain Processing (10) – Barley Malt Preparation[J]. Cereal & Feed Industry, 2000, (1): 22.
[9] LU Jian. Additives in the Barley Germination Process[J]. Jiangsu Food and Fermentation, 1996, (1): 12-14.
[10] CHENG Dianlin. Beer Production Technology[M]. Beijing: Chemical Industry Press, 2005.
[11] WEI Wangen, ZOU Deqing. Technology and Prospect of Brewing Beer with Wheat Malt[J]. Brewing Science & Technology, 2003, (9): 21-23.
[12] CHENG Dianlin. Study on Main Steeping Technologies of Wheat[J]. Journal of Qingdao University, 2001, 14(1): 71.
[13] JIN Yuhong. Effect of Malting Technology on the Quality of SN1391 Wheat Malt[J]. Science and Technology of Food Industry, 2005, (10): 99.
[14] DONG Xiaolei, ZHOU Guangtian, CHI Yongqing, et al. Production and Research of Wheat Beer[J]. Brewing Science & Technology, 2004, (5): 82-84.
[15] Thidrval-brignon. On-line Estimation and Prediction of Density and Ethanol Evolution in the Brew[J]. Technical Quarterly of the Master Brewer’s Association of the Americas, 2000, 37(2): 156-158.
[16] MENG Zhaolei, MA Meifan. A Brief Discussion on the Brewing of Pure Raw Wheat Beer[J]. Brewing, 2001, (3): 92-93.
[17] XIAO Liandong. α-Amino Nitrogen in Wort and Beer Flavor[J]. Brewing Science & Technology, 2001, (5): 71-73.
[18] Lewis M J. Dye-binding Method for Measurement of Protein in Wort and Beer[J]. ASBC Journal, 1980, (2): 37-41.
[19] JU Yundong, ZHANG Kaili, JIANG Shufen, et al. Study on the Effect of Mashing Temperature on Arabinoxylan and β-Glucan in 7°P Wort[J]. Brewing, 2008, (04): 45-47.
[20] WANG Zhijian. Discussion on Brewing Beer with Wheat Malt Replacing Part of Barley Malt[J]. Food Industry, 2002, (4): 4-6.
[21] XIA Zhenjiang, GUO Zhongsen. The Effect of β-Glucan on Beer Production and Its Control in the Malting Process[J]. Brewing, 2004, (5): 52-55.
[22] WU Beili, XU Baoquan, WANG Aizhong, et al. Feasibility Test Report on Increasing Wheat Malt Dosage for Beer Brewing[J]. Brewing Science & Technology, 2006, (1): 65-67.
[23] TANG Peng, FANG Yaye, FU Wubing. Application Test of Enzymes in Beer Production with Wheat Malt Partially Replacing Barley Malt[J]. Brewing Science & Technology, 2007, (1): 45-46.
[24] Leach A A. Nitrogenous Components of Worts and Beers Brewed from All-malt and Malt plus Wheat Flours[J]. Journal of the Institute of Brewing, 1988, (4): 183-192.
[25] CAO Limin, HE Guoqing. Research Progress of Wheat Application in Beer Brewing[J]. Beer Industry, 2002, (4): 8-10.
[26] Debyser W, Devaux F, Delcour J A. Activity of Arabinoxylan Hydrolyzing Enzymes during Mashing with Barley Malt and Unmalted Wheat[J]. Journal of Agricultural and Food Chemistry, 1998, 46(1): 125-126.
[27] Faridi H, Gaines C, Finney D. Soft Wheat Quality in Production of Cookies and Cracks[J]. Cereal Chemistry, 1990, (6): 72-75.
[28] GENG Jianhua, HUANG Lijuan, MA Chi. Modern Wheat Beer Production Technology[J]. Brewing, 2002, (6): 52-54.
[29] Sofie A Depraetere, Filip Delvaux, et al. Wheat Variety and Barley Malt Properties: Influence on Haze Intensity and Foam Stability of Wheat Beer[J]. Journal of the Institute of Brewing, 2004, 110(3): 200-206.
[30] WANG Haiming, WANG zhi. Discussion on Brewing Beer with Wheat[J]. Brewing Science & Technology, 2004, (1): 52-56.
[31] WANG Lan, LI Jiabiao, XIAO Dongguang. Research Progress of Wheat Beer[J]. Brewing Science & Technology, 2000, (6): 53-55, 65-69.
[32] HUANG Xiangbin. Enzyme Changes During Malting[J]. Barley Science, 2003, (3): 43-45.
[33] YIN Shenghua. Application of Wheat Malt in Beer Production[J]. Brewing, 2004, (4): 46-47.
[34] YU Xiaohong. Study on the Optimal Content of α-Amino Nitrogen in Wort[J]. Brewing Science & Technology, 2003, (5): 66-68.
[35] XU Gaohong. Influencing Factors and Optimization of Wort Filtration[J]. Brewing Science & Technology, 2009, (12): 41-45.
[36] LI Fuzhong. Main Factors Affecting Wort Filtration and Their Control[J]. Brewing Science & Technology, 2010, (10): 51-53.
[37] WANG Weimin. Discussion on the Filterability of Mashed Wort[J]. Brewing Science & Technology, 2007, (4): 27-29.
[38] WANG Yong. A Brief Discussion on the Optimization of Wort Filtration Process[J]. Brewing Science & Technology, 2012, (1): 58-60.
[39] LIU Hui. Discussion on the Mashing Technology of Triticale Beer[J]. Science and Technology of Food Industry, 2000, (5): 46-48.
[40] SHAO Fadu, WANG Zhao. Brewing Characteristics of Wheat Malt and Its Application in Beer Brewing[J]. Brewing, 2000, (6): 52-53.
[41] MA Xizhuang, DU Jinhua, LIU Zhaoxi, et al. Study on the Effect of Wheat Malt Dosage on Wort Quality[J]. Brewing Science & Technology, 2007, (3): 20-22.
[42] YIN Shenghua. Application of Wheat Malt in Beer Production[J]. Brewing, 2004, (4): 46-47.
[43] SHEN Yaofei. Study on the Brewing Technology of Whole Wheat Beer[J]. Brewing Science & Technology, 2005, (6): 26-28.
[44] WANG Fengsong. Quality Parameters and Production Practice of Wheat Beer[J]. Brewing Science & Technology, 2007, (5): 35-38.
[45] CHEN Xiaomei, ZHANG Xiaofeng. Brewing Pueraria Beer with Whole Wheat Beer Technology and Its Nutritional Evaluation[J]. China Brewing, 2009, (4): 173-175.
[46] Berne L J, Laurie A M, Debora F. Quantitative Study of the Formation of Endoproteolytic Activities during Malting and Their Stabilities to Killing[J]. Journal of Agricultural and Food Chemistry, 2000, (9): 3898-3905.
[47] GUO Zefeng. Optimization of Wort Boiling Process Conditions[J]. Brewing Science & Technology, 2011, (6): 63-66.
[48] ZHENG Jincheng. Wort Boiling and Beer Stability[J]. Fujian Light Industry & Textile, 1998, (8): 1-5.
[49] ZHAO Yuxiang. Optimization Test of Wort Boiling Process[J]. Brewing Science & Technology, 2007, (5): 20-21.
[50] LIU Wenyu, YANG Dayi. Composition of Wort and Changes of Substances During Boiling[J]. Brewing, 2012, (1): 104-106.
[51] HAN Long. A Brief Discussion on the Technical Optimization of Wort Boiling[J]. Brewing Science & Technology, 2007, (4): 24-26.
[52] LIU Xinzhong. Effect of Residence Time in Whirlpool on Wort Quality[J]. China Brewing, 2004, (1): 35-37.
[53] ZHOU Guangtian, NIE Cong, CUI Yunqian, et al. Beer Brewing Technology[M]. Jinan: Shandong University Press, 2004.
[54] DONG Xiaolei, ZHOU Guangtian, et al. Production and Research of Wheat Beer[J]. Brewing Science & Technology, 2004, (5): 82-84.
[55] GUAN Dunyi. Beer Industry Handbook (Revised Edition)[M]. Beijing: China Light Industry Press, 1986.
[56] GU Guoxian. Brewing Technology (2nd Edition)[M]. Beijing: China Light Industry Press, 1990.
[57] ZHOU Guangtian. Modern Beer Technology[M]. Beijing: Chemical Industry Press, 2007.
[58] ZHOU Deqing. Microbiology Tutorial[M]. Beijing: Higher Education Press, 2005.
[59] TAN Cuihui, TAO Wenyi, ZHANG Xingyuan, et al. Microbiology[M]. Beijing: China Light Industry Press, 1990.
[60] GU Guoxian. Discussion on the Relationship Between Brewing Conditions and Diacetyl Content in Beer[J]. Brewing, 1998, (5): 11-12.
[61] CAI Dingyu. Brewing Industry Analysis Handbook[M]. Beijing: China Light Industry Press, 1998.
[62] Fritch H, Brauerei B, Kaltner D, et al. Unlocking the Secret Behind Hop Aroma in Beer[J]. Brauwelt International, 2005, (1): 22-30.
[63] QUAN Qiaoling, JIANG Wei, WANG Deliang, et al. Detection of Hop Aroma Components and Experimental Research on Beer Rich in Typical Hop Aroma[J]. Brewing Science & Technology, 2013, (2): 28-36.
[64] WANG Zhijian. Effect of Hop Composition on Beer Quality[J]. Brewing Science & Technology, 2005, (3): 41-42.
[65] Irwin A. Varietal Dependence of Hop Flavour Volatiles in Lager[J]. Journal of the Institute of Brewing, 1989, 95(3): 185-194.
[66] King A, Dickinson J. Biotransformation of Hop Aroma Terpenoids by Ale and Lager Yeasts[J]. FEMS Yeast Research, 2003, (3): 53-62.
[67] Havig V. Maximizing Hop Aroma and Flavor Through Process Variables[J]. Master Brewers Association of America, 2010. doi:10.1094/TQ-47-2-0623-01

2.1Overview of Wort Preparation for IPA-type Beer