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3.5 Fermentation and Post-fermentation
3.5.1 The Characteristics of Rice Wine Fermentation
3.5.1.1 Use of Jiu Qu
The characteristics in rice wine brewing are of the use of several types of "Jiu Qu" or "Qu". Wheat Qu is mainly a culture of Aspergillus spp.; Xiao Qu is mainly a mixture culture of Rhizopus spp. and yeasts grown on and within grounded rice meal. Jiu Qu, which is comparable to malt used in beer brewing, is used for liquification and saccharification of the starch contained in the rice grains. Using of Qu (especially wheat Qu) also contributs to flavour substance formation. Without using Qu, the rice wine may lose its typically characteristic flavours and aroma.
3.5.1.2 Parallel Fermentaton
For beer brewing, wort fermentation by yeasts takes place after filtration and boiling of the mash. But for rice wine, the mash which is called "Lao", is composed of geletinized starch contained in steamed rice, Qu and seed mash, so the glucose is gradually liberated from starch in steamed rice by the action of glucoamylase and is fermented successively by yeast. This is called "parallel fermentation" which combines progressive saccharification of starch with slow alcohol fermentation at a low temeprature. This contributes to considerable ethanol production, which can be as high as 20% (v/v) in the final fermented mash.
3.5.1.3 Low Temperature Fermentation for a Long Time
The main mash is fermented without any special sterilization, and the typical feature in rice wine brewing is also characterized by the slow fermentation at a low temperature. The whole fermentation time usually lasts about 3 months or even more than one year.
3.5.1.4 Semi-solid Fermentation
It is seldom seen that rice wine fermentation is conducted in such a dense and mushy state that the rice grain vs water is in proportions of about 1:2 ( for beer brewing, the malt vs water is in proportion of 1:4.3, and for whisky, about 1:5). This situation is called semi-solid state fermentation. The dense fermenting mash is considered beneficial in retaining large numbers of yeast cells in suspended solid or the mash during the fermentation, which is one of the reasons that rice wine contains as much as 20% (v/v) ethanol. For semi-solid fermentation, because the mash easily floats on the surface, it is difficult for the mash to send out the heat generated during the period of main fermentation. The key to properly control of the fermentation temperature is to stir the mash on time.
3.5.1.5 Higher Content of Ethanol Than Other Fermented Drinks
Among the fermented beverages produced around world, the ethanol concentration in original rice wines mash produced by fermentation is the highest, which amounts to above 20%( v/v). It is still difficult to identify how many factors contributing to high concentration ethanol being produced. The reasons often quoted are summerized as follows:(1) combination of progressive saccharification of starch and slow alcohol fermentation
(2) slow fermentation of low concentration of fermentable sugars to ethanol at a low temeprature. Even if the yeasts died, the enzymes are still in action.
(3) the yeasts used in rice wine brewing are characterized by high ethanol concentration tolerance. The high density of yeasts may be protected from damaging caused by high intensity of ethanol by adhering to the semi-solid state of fermenting mash, or by some substances existing in rice and wheat ( Qu is made from wheat) such as specially proteins, vitamine B which can absorb harmful fusel oils excreted by yeasts themselves.
(4) some unclear substnaces may exist in the fermenting mash which is good for yeast growth and fermentation.
3.5.2 Traditional Fermentation Processes
We can roughly divide fermentation processes into two types: traditional brewing process and modern brewing process. In this section, we will first disccuss the rather simple methods of preparing rice wines at home, then go on to discuss about the complete multi- step process used by most commercial makers.The traditional process used Jiu Qu made by natural inoculated cultures of microorganisms. Fermentation was conducted in vats. The fed-batch fermentation, which means adding steamed rice was divided successively into several batches over a period of time was the most popular operating method. For example, in Han Dynasty, adding steamed rice was divided into 9 batches every third day. From Song Dynasty, adding batches of steamed rice was reduced to 3- 4 batches. The traditional brewing process was undertaking under non strict sterile conditions.
According to the methods for rice-cooling and operating procedures, the traditional brewing processes are subdivided into three basic forms given below.
(1) The steamed rice for main mash was cooled by drenching cold water through it,and this procedure was mainly used in traditional seed mash fermentation.
(2) The steamed rice for main fermentation mash was cooled by spreading it on bamboo mats.
(3) The steamed rice was added in successively several batches.
As described previuosly, the rice wines are classified into five types according to their sugar contents remaining in the final products. For making a particular type of rice wine, a process may be composed with several forms of brewing processes, For example, for making the YUAN HONG RICE WINE, a kind of dry type rice wine produced by Shaoxing in Zhejiang Province, The steamed rice for main fermentation mash is cooled by spreading it on bamboo mats, and the steamed rice used for seed mash is cooled by drenching fresh water through it.
3.5.2.1 Temperature Control During Main Fermentation
In ancient times, brewmasters made their rice wines during the cold months when the air and water is relatively free from contaminating microorganisms. Late November is the best time for brewing rice wines. In order to make rice wines during the warmer parts of a year, because of difficulty in controlling the mash's temperature, it was important to set the initial temperature of the main mash as low as possible. Usually the main mash's temperature was set according to the atmospheric temperature. The initial temperature should not exceed above 28 ℃ . Most breweries prefer to start fermentation in a range of 24-26 ℃, in order to control the rate of fermentation, so the maximun temperature of the fermenting mash would be below 30-35℃.Traditionally, the effective control of fermentation temperature was dependent on the stirring operating. It is especially important to choose the right moment for the first stirring. There were two operating procedures handed down from ancient times in Shaoxing. The most popular procedure accepted by the brewmasters in Shaoxing prefered to start the first stirring when the mash's temperature rise to above 35 ℃, and this is called " First Stirring at a High Temperature". This procedure will result in quick fermentation of sugar to ethanol, and the yeasts become old and feeble. More sugar remaining in the main mash. Another operating procedure is characterized by firstly stirring the main mash when the fermentation temperature rises to about 30 ℃, and alwasy the temperature is kept not to exceed 30 ℃. This is called "First stirring at a Low Temperature". This procedure will result in more sugar being converted to ethanol in slow fermentation. Anyhow, after the first stirring, the mash's temperature drops down notably. During the main fermentation, intermittent stirring is done according to the mash's temperature and room temperature. Temperature control pattern is given in Table 5.
| temperature of the mash | interval (h) | |
|---|---|---|
| 1st stirring | 37-38 ℃ | 10 h after |
| mixing the indredients | ||
| 2nd stirring | 31-33 ℃ | 4-6 |
| 3 rd stirring | 29-31 ℃ | 3-4 |
| 4th stirring | 27-30 ℃ | 3-4 |
3.5.2.2 Secondary Fermentation
The main fermentation lasts about 1 week. When the main fermentation is finished which is indicated by settling of the solid parts of the mash, the mash is transferred to jars ( narrow neck jars) for further fermentation ( secondary fermentation). In the narrow neck jars, the mouth of the jar is sealed tightly so that air is kept from contacting with the mash. In the case of big tank fermentation, the main mash is transferred to secondary fermentation tanks. The ethanol content of the main mash has already reached to about 13-14%, and most yeasts are inhibited. Remaining starch and sugar are to be slowly converted to ethanol during secondary fermentation. The mash is often allowed to stand for about 70-80 days at a natural temperature; this final ripening further improves the flavour and aroma. The ethanol content rises by 2~4 % more.
3.5.2.3 Fed-batch Fermentation
Fed-batch fermentation processes have been practiced for at least 2000 years as described previously. This method is still widely used in rice wine breweries. Usually the additions of steamed rice is carried out successively in three batches. The flowchart of fed- batch fermentation process is shown in Fig 4. wheat Xiao rice water rice
Qu Qu 50 kg │ ↓
8-10% 0.4-0.5% ↓ ├───→ steeping
│ │ Washing ←──┤ ↓
│ │ Steeping ├────→ steaming
│ │ ↓ │
│ │ Steaming ←──┤
│ │ ↓ │ ↓
│ │ Cooling by ←─┼────→ Cooling
│ │ Drenching water │ by drenching water
│ │ ↓ │ ↓
│ └──→ Seed mash │ │
│ ↓ │ │
│ Adding water──┘ │
│ ↓ │
│ a half 1st addition of 50 kg │
├───────→ steamed rice ←──────┤
│ ↓ │
│ 1st stirring │
│ ↓ │
│ a falf 2ed addition of 25 kg │
└───────→ steamed rice ←──────┘
↓
Further fermentation
( 70-80 days more)
↓
Squeezing→ spent grains
↓
Natural colorant → New rice wine
↓
Clarification
↓
Pasteurization
↓
Packing
↓
Rice wine
Fig 3.5.2 Fed-batch fermentation process
3.5.3 Post-fermentation Treatments
3.5.3.1 Separation of Spent Grains by Squeezing
3.5.3.1.1 The Characters of the Rice Wine Mash
Since the rice wine mash is very dense and mushy, the specific gravity differences of the mash between the solid part and the liquid part are small. So the situations in squeezing of rice wine mash differ greatly from mash filtration in beer manufacture. It is difficult to obtain the liquid part ( rice wine) by filtration or sedimentation. Combination of filtration and squeezig provides a effective approach for separating the liquid part from the mash.
3.5.3.1.2 Squeezing Equipments
After secondary fermentation, the mash is to be squeezed to divide into filtrate, rice wine, and solids, spent cake. Many types of filter equipment had been used for this operation throughout the history. In ancient times, the new rice wine was collected by making a hollow in the upper surface of the fully- matured rice wine mash in a vat. pressing a bamboo colander with conical bottom into the hollow, and ladling out the liquid part (rice wine) that accumulated in it. In Tang Dynasty, a type of wooden squeezer was invented, as recorded in poems of that times, but the squeezer was not described in detail. A wooden squeezer with more complicated structures was mentioned in < < Bei San Jiu Jin>> published in 12th century. Technological innovations of wooden squeezer continued from Song Dynasty to Qing Dynasty. The large scale squeezer was made of a number of wooden frames where the silk bags containing rice wine mash were held, and a pressing lid was placed on the top surface of the frames. The wooden squeezer had a structural stand with two sets of lever arms. Heavy stones exerted pressure to one of the levers, and the level transfered the pressure to the lip that pushed down the silk bags beneath it. A vat was used for receiving the filtrate: rice wine.After the founding of the People's Republic of china, the wooden squeezers have been substituted by mechanical devices such as screw-type press and plate and frame filter-press.
3.5.3.2 Clarification
The slightly turbid rice wine is pumped to the fining tanks for clarification. A kind of natural colorant called caramel is added to the rice wine which is allowed to stand for 2-3 day's for clarification until the clarified rice wine tastes smooth and sweet. The caramel depens the color of rice wine.
3.5.3.3 Pasteurization
3.5.3.3.1 Batch Pasteurization
As mentioned previously, Chinese firstly invented the heat treatment method for pasteurization of alcoholic beverages in Tang Dynasty ( 618-907 A.D.). The heating equipment used at ancient times was very simple: a ceramic jar containing rice wine was surrounded by chaocoal fire ( popular in Tang Dynasty); or a large cauldron topped with a steamer. The rice wine was filled into a long necked jar which was placed in a steamer. By heating indirectly, the rice wine in the jar boiled ( Popular in Song Dynasty).In Qing Dynasty, a special heating device made of tin was recorded in << Tiao Ding Ji>>. The rice wine was heated by direct fire. The pasteurization conducted in the devices mentioned above was characterized by batchoperation.
3.5.3.3.2 Continuous Pasteurization
Continuous pasteurization of rice wine was realized in 1950's. Various types of heat exchangers have been used, such as the coiled pipe cooler, double-pipe exchanger, shell and tube heat exchanger and plate type exchanger, the latter being most effective.The fresh rice wine is pasteurized at 85 ℃ to 90 ℃ , at this temperature, the rice wine is kept for about 5- 10 minutes, so as to kill the vegetative cells of microorganisms. Enzymes are also denatured, and proteins are coagulated. Heat treatment also has the function of facilitating maturation of rice wine.
3.5.3.4 Aging and Maturation
The pasteurized rice wine is stored in narow necked jars. The jars are tightly sealed and are piled up one above another, and placed outside buildings, letting to experience the rhythmic temperature changes of the four seasons. Most rice wine starts its fermentation during the colder months, undergoes most of its transformation during the warmer ones, and is finally consumed about one year later. Thus the rice wine experiences a rise and fall of temperature.During storage, rice wine is gradually matured, deepens color and diminishes harsh taste. After storage, rice wine is blended and is diluted to the appropriate alcohol content and sugar content. Finally, the rice wine is filtered, and packaged.
3.5.3.5 Packaging
Traditionally, the pasteurized rice wine when it is still hot is packaged in sterilized ceramic jars which are considered good for the further maturation of rice wine. The mouth of the jar is immediately covered with sterilized lotus leaf and other sealing materials and tied with string. the sealed mouth is further sealed with specially treated mud mound that is cylinderly shaped, so as to prevent contaminating microorganism from getting into the jars. The so called "HUA DIAO JIU" made in Shaoxing, Zhejiang Province, is packaged in jars outside of which are beautiful carving and drawings.Packaging machines have been used widely in modern rice wine breweries since 1970's. The three forms of packaging rice wine are bottles, jugs (or pots) and plastic bags.
Modern rice wine brewing process is characterized by fermentation in big fermenting tanks and use of pure cultured yeasts, which will be described later.
