CHAPTER 4
PRODUCTS OF YEAST FERMENTATATION

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The major products of yeast fermentation are alcoholic drinks and bread. With respect to fruits and vegetables, the most important products are fermented fruit juices and fermented plant saps. Virtually any fruit or sugary plant sap can be processed into an alcoholic beverage. The process is well known being essentially an alcoholic fermentation of sugars to yield alcohol and carbon dioxide.

It should be noted that alcohol production requires special licences or is prohibited in many countries.

4.1 Fermented fruit juices

There are many fermented drinks made from fruit in Africa, Asia and Latin America. These include drinks made from bananas, grapes and other fruit. Grape wine is perhaps the most economically important fruit juice alcohol. It is of major economic importance in Chile, Argentina, South Africa, Georgia, Morocco and Algeria. Because of the commercialisation of the product for industry, the process has received most research attention and is documented in detail. Banana beer is probably the most wide spread alcoholic fruit drink in Africa and is of cultural importance in certain areas. Alcoholic fruit drinks are made from many other fruits including dates in North Africa, pineapples in Latin America and jack fruits in Asia.

4.1.1 Red Grape wine

Location of production

Red grape wines are made in many African, Asian and Latin American countries including Algeria, Morocco and South Africa.

Product description

Red grape wine is an alcoholic fruit drink of between 10 and 14% alcoholic strength. The colour ranges from a light red to a deep dark red. It is made from the fruit of the grape plant (Vitis vinifera). There are many varieties of grape used including Cabernet Sauvignon, Grenache, Nebbiolo, Pinot Noir, and Torrontes (Ranken, Kill and Baker, 1997). The skins of the grape are allowed to be fermented in red wine production, to allow for the extraction of colour and tannins, which contribute to the flavour. The grapes contribute trace elements of many volatile substances, which give the final product the distinctive fruity character. In addition, they contribute non-volatile compounds (tartaric and malic acids) which impact on flavour and tannins, which give bitterness and astringency.

Raw material preparation

Ripe and undamaged grapes should be used. Red grapes are crushed to yield the juice plus skins, which is known as must.

Processing

The crushed grapes are transferred to fermentation vessels. The ethanol formed during this fermentation assists with the extraction of pigments from the skins. This takes between 24 hours and three weeks depending on the colour of the final product required.

The skins are then removed and the partially fermented wine is transferred to a separate tank to complete the fermentation. The fermentation can be from naturally occurring yeasts on the skin of the grape or using a starter culture of Saccharomyces cerevisiae – in which case the juice is inoculated with populations of yeast. This approach produces a wine of generally expected taste and quality. If the fermentation is allowed to proceed naturally, utilising the yeasts present on the surface of the fruits, the end result is less controllable, but produces wines with a range of flavour characteristics (Fleet, 1998), (Rhodes and Fletcher,1966), (Colquichagua, 1994).

Traditionally, fermentation was carried out in large wooden barrels or concrete tanks. Modern wineries now use stainless steel tanks as these are more hygienic and provide better temperature control.

Fermentation stops naturally when all the fermentable sugars have been converted to alcohol or when the alcoholic strength reaches the limit of tolerance of the strain of yeast involved. Fermentation can be stopped artificially by adding alcohol, by sterile filtration or centrifugation (Ranken, Kill and Baker, 1997).

Some wines can be drunk immediately. However most wines develop distinctive favours and aromas by ageing in wooden casks.

Flow diagram

Selection of grapes
Mature and undamaged grapes
Crushing
Traditionally manually, but now by crushers
Pre-fermentation
24 hours to three weeks depending on colour required
Removal of skin
Can add sulphur dioxide to inhibit wild yeasts
Fermentation
 
Maturation
Ageing to develop aromas and flavours

Packaging and storage

Traditionally wine was delivered to the point of sale in casks. The product is traditionally packaged in glass bottles with corks, made from the bark of the cork oak (Quercus suber). The bottles should be kept out of direct sunlight. During storage, wines are prone to non-desirable microbial changes. Yeasts, lactic acid bacteria, acetic acid bacteria and fungi can all spoil or taint wines after the fermentation process is completed.

4.1.2 White Grape wine

Location of production

White grape wines are made in many African, Asian and Latin American countries including Algeria, Morocco and South Africa.

Product description

White grape wine is an alcoholic fruit drink of between 10 and 14% alcoholic strength. It is prepared from the fruit of the grape plant (Vitis vinifera), and is pale yellow in color. There are many varieties used including Airen, Chardonnay, Palomino, Sauvignon Blanc and Ugni Blanc (Ranken, Kill and Baker, 1997). The main difference between red and white wines is the early removal of grape skins in white wine production. The distinctive flavour of grape wine originates from the grapes as raw material and subsequent processing operations. The grapes contribute trace elements of many volatile substances (mainly terpenes) which give the final product the distinctive fruity character.

Preparation of raw materials

Ripe and undamaged grapes should be used. The grapes are crushed to yield the juice and the skins are removed and separated out. Sometimes the juice is clarified by allowing it to stand for 24 to 48 hours at 5 to 10 C, by filtering or centrifugation. Pectolytic enzymes may be added to accelerate the breakdown of cell wall tissue and to improve the clarity of juice. Excessive clarification removes many of the natural yeasts and flora. This is beneficial if a tightly controlled induced fermentation is desired, but less so if the fermentation is a natural one. Long periods of settling out however, encourage the growth of natural flora, which can contribute to the fermentation.

Processing

The clarified juice is transferred to a fermentation tank where fermentation either begins spontaneously or is induced by the addition of a starter culture. Traditionally, fermentation was carried out in large wooden barrels or concrete tanks. Modern wineries now use stainless steel tanks as these are more hygienic and provide better temperature control. White wines are fermented at 10 to 18 C for about seven to fourteen days. The low temperature and slow fermentation favours the retention of volatile compounds (Fleet, 1998).

The fermentation can be from naturally occurring yeasts on the skin of the grape or using a starter culture of Saccharomyces cerevisiae. This approach produces a wine of generally expected taste and quality. If the fermentation is allowed to proceed naturally, utilising the yeasts present on the surface of the fruits, the end result is less controllable, but produces wines having a range of flavour characteristics. It is likely that natural fermentations are practised widely around the world, especially for home production of wine.

During storage, wines are prone to non-desirable microbial changes. Yeasts, lactic acid bacteria, acetic acid bacteria and fungi can all spoil or taint wines after the fermentation process is completed.

Flow diagram

Selection of grapes
Mature and undamaged grapes
Crushing
Traditionally manual but now usually by crushers
Removal of skins
 
Clarification
By standing, filtration or centrifugation
Fermentation
 
Ageing
 

Packaging and storage

Traditionally wine was delivered to the point of sale in casks. The product is traditionally packaged in glass bottles with corks, made from the bark of the cork oak (Quercus suber). The bottles should be kept out of direct sunlight. During storage, wines are prone to non-desirable microbial changes. Yeasts, lactic acid bacteria, acetic acid bacteria and fungi can all spoil or taint wines after the fermentation process is completed.

4.1.3 Banana beer

Location of production

Throughout Africa

Product description

Banana beer is made from bananas, mixed with a cereal flour (often sorghum flour) and fermented to an orange, alcoholic beverage. It is sweet and slightly hazy with a shelf-life of several days under correct storage conditions. There are many variations in how the beer is made. For instance Urwaga banana beer in Kenya is made from bananas and sorghum or millet and Lubisi is made from bananas and sorghum.

Preparation of raw materials

Ripe bananas (Musa spp.) are selected. The bananas should be peeled. If the peels cannot be removed by hand then the bananas are not sufficiently ripe.

Processing

The first step of the process is the extraction of banana juice. Extraction of a high yield of banana juice without excessive browning or contamination by spoilage micro-organisms and proper filtration to produce a clear product is of great importance. Grass is used as an aid in obtaining clarified juice.

One volume of water is added to every three volumes of banana juice. This makes the total soluble solids low enough for the yeast to act. Cereals are ground and roasted and added to improve the colour and flavour of the final product. The mixture is placed in a container, which is covered in polythene to ferment for 18 to 24 hours. The raw materials are not sterilised by boiling and therefore provide an excellent substrate for microbial growth. It is essential that proper hygienic procedures are followed and that all equipment is thoroughly sterilised to prevent contaminating bacteria from competing with the yeast and producing acid instead of alcohol. This can be done by cleaning with boiling water or with chlorine solution. Care is necessary to wash the equipment free of residual chlorine as this would interfere with the actions of the yeast. Strict personal hygiene is also essential (Fellows, 1997).

For many traditional fermented products, the micro-organisms responsible for the fermentation are unknown to scientists. However there has been research to identify the micro-organisms involved in banana beer production. The main micro-organism involved, is Saccharomyces cerevisiae which is the same organism involved in the production of grape wine. However many other micro-organisms associated with the fermentation have been identified. These varied according to the region of production (Davies, 1994).

After fermentation the product is filtered through cotton cloth.

Flow diagram

Raw materials
Ripe bananas
Peel
Peel by hand
Remove residue
Use grass to knead or squeeze out the juice
Mix with water
The water:banana juice ratio should be 1:3
Mix with cereals
Mix with ground and roasted cereals to local taste
Ferment
In plastic container. Leave to ferment for 18 to 24 hours.
Filter
Through cotton cloth
Pack
Store

Packaging and storage

Packaging is usually only required to keep the product for its relatively short shelf-life. Clean glass or plastic bottles are used. The product is kept in a cool place away from direct sunlight.

4.1.4 Cashew wine

Location of Production

Cashew wine is made in many countries in Asia and Latin America.

Product description

Cashew wine is a light yellow alcoholic drink prepared from the fruit of the cashew tree (Ancardium occidentale). It contains an alcohol content of between 6 and 12% alcohol.

Preparation of raw materials

In gathering the fruits and transporting them to the workshop, the prime purpose should be to have the fruit arrive in the very best condition possible. Cashew apples are sorted and only mature undamaged cashew apples should be selected. These should be washed in clean water.

Processing

The cashew apples are cut into slices to ensure a rapid rate of juice extraction when crushed in a juice press. The fruit juice is sterilised in stainless steel pans at a temperature of 85oC in order to eliminate wild yeast. The juice is filtered and treated either sodium or potassium metabisulphite to destroy or inhibit the growth of any undesirable types of micro-organisms - acetic acid bacteria, wild yeasts and moulds.

Wine yeast (Saccharomyees cerevisiae - var ellipsoideus) are added. Once the yeast is added, the contents are stirred well and allowed to ferment for about two weeks.

The wine is separated from the sediment. It is clarified by using fining agents such as gelatin, pectin or casein which are mixed with the wine. Filtration is carried out with filter-aids such as fullers earth. The filtered wine is transferred to wooden vats.

The wine is then pasteurised at 50o - 60oC. Temperature should be controlled, so as not to heat it to about 70oC, since its alcohol content would vaporise at a temperature of 75o-78oC. It is then stored in wooden vats and subjected to ageing. At least six months should be allowed for ageing.

If necessary, wine is again clarified prior to bottling. During ageing, and subsequent maturing in bottles many reactions, including oxidation, occur with the formation of traces of esters and aldehydes., which together with the tannin and acids already present enhance the taste, aroma and preservative properties of the wine (Wimalsiri, Sinnatamby, Samaranayake and Samarsinghe, 1971).

Flow diagram

Selection
Mature, sound cashew apples
Slicing
To increase extraction of juice
Crushing
 
Sterilisation
At 85 C
Filtered
 
Inoculation
With S. cerevisiae
Fermentation

Two weeks

Filtered
 
Pasteurisation
50-60 C
Ageing
In wooden vats for 6 months

Packaging and storage

The product is packaged in glass bottles with corks. The bottles should be kept out of direct sunlight.

4.1.5 Tepache

Tepache is a light, refreshing beverage prepared and consumed throughout Mexico. In the past, tepache was prepared from maize, but nowadays various fruits such as pineapple, apple and orange are used. The pulp and juice of the fruit are allowed to ferment for one or two days in water with some added brown sugar. The mixture is contained in a lidless wooden barrel called a ‘tepachera’, which is covered with cheese cloth. After a day or two, the tepache is a sweet and refreshing beverage. If fermentation is allowed to proceed longer, it turns into an alcoholic beverage and later into vinegar. The microorganisms associated with the product include Bacillus subtilis, B. graveolus and the yeasts, Torulopsis insconspicna, Saccharomyces cerevisiae and Candida queretana (Aidoo, 1986).

4.1.6 Colonche

Colonche is a sweet, fizzy beverage produced in Mexico by fermenting the juice of the fruits of the prickly pear cacti - mainly Opuntia species. The procedure for preparing colonche is essentially the same as has been followed for centuries. The cactus fruits are peeled and crushed to obtain the juice, which is boiled for 2-3 hours. After cooling, the juice is allowed to ferment for a few days. Sometimes old colonche or tibicos may be added as a starter. Tibicos are gelatinous masses of yeasts and bacteria, grown in water with brown sugar. They are also used in the preparation of tepache.

4.1.7 Fortified grape wines

Fortified wines are made in the Republic of South Africa and North Africa. Fortified wines are made by adding spirits to wines, either during or after fermentation, with the result that the alcohol content of the wines is raised to around 20 percent, i.e. approximately double that of table wines (Rose, 1961) .

4.1.8 Date wine

Date wines are popular in Sudan and North Africa. They are made using a variety of methodologies. Dakhai is produced by placing dates in a clean earthenware pot. For every one volume of dates between two and four volumes of boiling water are added. This is allowed to cool and is then sealed for three days. More warm water is then added and the container sealed again for seven to ten days. Many variations of date wine exist: El madfuna is produced by burying the earthenware pots underground. Benti merse is produced from a mixture of sorghum and dates. Nebit is produced from date syrup (Dirar,1992).

4.1.9 Sparkling grape wine

Sparkling grape wines are made in the Republic of South Africa. Sparkling wines can be made in one of three ways. The cheapest method is to carbonate wines under pressure. Unfortunately, the sparkle of these wines quickly disappears, and the product is considered inferior to the sparkling wines produced by the traditional method of secondary fermentation. This involves adding a special strain of wine yeast (S. cerevisiae var. ellipsoideus) - a champagne yeast - to wine that has been artificially sweetened. Carbon dioxide produced by fermentation of the added sugar gives the wine its sparkle. In the original champagne method, which is still widely used today, this secondary fermentation is carried out in strong bottles, capable of withstanding pressure but early in the nineteenth century a method of fermenting the wine in closed tanks was devised, this being considerably cheaper than using bottles (Rose, 1961).

4.1.10 Jack-fruit wine

Jack-fruit wine is an alcoholic beverage made by ethnic groups in the eastern hilly areas of India. As its name suggests, it is produced from the pulp of jack-fruit (Artocarpus heterophyllus). Ripe fruit is peeled and the skin discarded. The seeds are removed and the pulp soaked in water. Using bamboo baskets, the pulp is ground to extract the juice, which is collected in earthenware pots. A little water is added to the pots along with fermented wine inoculum from a previous fermentation. The pots are covered with banana leaves and allowed to ferment at 18 to 30C for about one week. The liquid is then decanted and drunk. During fermentation, the pH of the wine reaches a value of 3.5 to 3.8, suggesting that an acidic fermentation takes place at the same time as the alcoholic fermentation. Final alcohol content is about 7 to 8% within a fortnight (Steinkraus, 1996).

4.2 Fermented plant saps

Virtually any sugary plant sap can be processed into an alcoholic beverage. The process is well known being essentially an alcoholic fermentation of sugars to yield alcohol and carbon dioxide. Many alcoholic drinks are made from the juices of plants including coconut palm, oil palm, wild date palm, nipa palm, raphia palm and kithul palm.

4.2.1 Palm wine

Location of production

Palm ‘wine’ is an important alcoholic beverage in West Africa where it is consumed by more than 10 million people.

Product description

Palm wine can be consumed in a variety of flavours varying from sweet unfermented to sour fermented and vinegary alcoholic drinks. There are many variations and names including emu and ogogoro in Nigeria and nsafufuo in Ghana. It is produced from sugary palm saps. The most frequently tapped palms are raphia palms (Raphia hookeri or R. vinifera) and the oil palm (Elaeis guineense). Palm wine has been found to be nutritious. The fermentation process increases the levels of thiamin, riboflavin, pyridoxin and vitamin B12. Like many African alcoholic beverages, palm wine has a very short shelf-life. The product is not preserved for more than one day. After this time accumulation of an excessive amount of acetic acid makes it unacceptable to consumers. The bark of a tree (Saccoglottis gabonensis) may be added as a preservative. The alkaloid and phenolic compounds which are extracted into the wine have antimicrobial effect (Odunfa, 1985) .

Preparation of raw materials

Sap is collected by tapping the palm. Tapping is achieved by making an incision between the kernels and a gourd is tied around to collect the sap which is collected a day or two later. The fresh palm juice is a sweet, clear, colourless juice containing 10-12 percent sugar and is neutral. The quality of the final wines is determined mostly by the conditions used in the collection of the sap. Often the collecting gourd is not washed between collections and residual yeasts in the gourd quickly begin the fermentation.

Processing

The sap is not heated and the wine is an excellent substrate for microbial growth. It is therefore essential that proper hygienic collection procedures are followed to prevent contaminating bacteria from competing with the yeast and producing acid instead of alcohol (Fellows, 1997) .

Fermentation starts soon after the sap is collected and within an hour or two, the sap becomes reasonably high in alcohol (up to 4%). If allowed to continue to ferment for more than a day, the sap begins turning into vinegar, although the vinegary flavour is preferred by some. Organisms responsible include S. cerevisiae, and Schizosaccharomyces pombe, and the bacteria Lactobacillus plantarum and L. mesenteroides. There are reports that the yeasts and bacteria originate from the gourd, palm tree, and tapping implements. However the high sugar content of the juice would seem to selectively favour the growth of yeasts which might originate from the air. This is supported by the fact that fermentation also takes place in plastic containers. Within 24 hours the initial pH is reduced from 7.4-6.8 to 5.5 and the alcohol content ranges from 1.5 to 2.1 percent. Within 72 hours the alcohol levels increase from 4.5 to 5.2 percent and the pH is 4.0. Organic acids present are lactic acid, acetic acid and tartaric acid (Odunfa, 1985) .

The main control points are extraction of a high yield of palm sap without excessive contamination by spoilage micro-organisms, and proper storage to allow natural fermentation to take place.

Flow diagram

Cut
Cut 10-15 cm from the top of the trunk
Tapping
A gourd is fixed below the cut
Collection
The sap is collected each day
Fermentation
Natural fermentation starts as soon as sap is collected
Filter
Optional
Bottling
Clean bottles should be used

Packaging and storage

Packaging is usually only required to keep the product for its relatively short shelf-life. Clean glass or plastic bottles should be used. The product should be kept in a cool place away from direct sunlight.

4.2.2 Toddy

Location of production

Throughout Asia, particularly India and Sri Lanka.

Product description

Toddy is an alcoholic drink made by the fermentation of the sap from a coconut palm. It is white and sweet with a characteristic flavour. It is between 4 and 6% alcohol and has a shelf life of about 24 hours.

Preparation of raw materials

The sap is collected by slicing off the tip of an unopened flower. The sap oozes out and can be collected in a small pot tied underneath the flower.

Processing

The fermentation starts as soon as the sap collects in the pots on the palms, particularly if a small amount of toddy is left in the pots. The toddy is fully fermented in six to eight hours. The product is usually sold immediately due to its short shelf-life (Fellows, 1997) .

Flow diagram

Tapping
A small pot is fixed below the cut
Collection
The sap is collected each day
Fermentation
Natural fermentation starts as soon as sap is collected
Packaging
Clean bottles should be used

Packaging and storage

Packaging is usually only required to keep the product for its relatively short shelf-life. This is usually clean glass or plastic bottles. The product should be kept in a cool place away from direct sunlight.

4.2.3 Pulque

Location of production

Pulque is the national drink in Mexico, where, it is claimed, it originated with the early Aztecs. Pulque is a traditional beverage that now forms the basis of a national industry, together with the spirits mezcal and tequila that are obtained from it. Pulque plays an important role in the nutrition of low income people in Mexico with B vitamins being present in nutritionally important levels.

Product description

Pulque is a milky, slightly foamy, acidic and somewhat viscous beverage. It is obtained by fermentation of aguamiel, which is the name given to the juices of various cacti, notably Agave atrovirens and A. americana which are often called the "Century plant" in English. The alcohol content on pulque varies between six and seven percent. The beverage obtained upon distilling pulque is called "Mezcal", and if manufactured in the Tequila region from a numbered distillery, it is referred to as "Tequila". The drink is often considered an aphrodisiac. The name Ticyaol is given to a good strain that makes one particularly virile. Pulque is frequently the potion of choice used by women during menstruation.

Preparation of raw materials

The juices are extracted from the plants when they are eight to ten years old and fermentation takes place spontaneously, although occasionally the juices are inoculated with a starter from previous fermentations.

Processing

The juice is allowed to ferment naturally through a mixed fermentation although yeast (Saccharomyces carbajali) is the main actor. Lacto-bacillus plantarum produces lactic acid and the viscosity of pulque is caused by the activity of two species of Leuconostoc which produce dextrans (Wood and Hodge). During fermentation of the juices of the plant, the soluble solids are reduced from between 25-30% to 6%; the pH falls from 7.4 to between 3.5 and 4.0; the sucrose content falls from 15% to 1% and vitamin levels are increased. For instance the vitamin content (milligrams of vitamins per 100g of product) increases from 5 to 29 for thiamine, 54 to 515 for niacin and 18 to 33 for riboflavin (Steinkraus, K.H. (1992) .

Packaging and storage

Packaging is only required to keep the product for its relatively short shelf-life. Clean glass or plastic bottles should be used. The product should be kept in a cool place away from direct sunlight.

4.2.4 Ulanzi (Bamboo Wine)

Location of production

East and Southern Africa.

Product description

Ulanzi is a fermented bamboo sap obtained by tapping young bamboo shoots during the rainy season. It is a clear, whitish drink with a sweet and alcoholic flavour.

Preparation of raw materials

The bamboo shoots should be young in order to obtain a high yield of sap. The growing tip is removed and a container fixed in place to collect the sap. The container should be clean in order to prevent contamination of the fresh sap.

Processing

The raw material is an excellent substrate for microbial growth and fermentation begins immediately after collection. Fermentation takes between five and twelve hours depending on the strength of the final product desired.

Packaging and storage

Packaging is usually only required to keep the product for its relatively short shelf life.

4.2.5 Basi (Sugar cane wine)

Basi is a sugar cane wine made in the Philipppines by fermenting boiled, freshly extracted, sugar cane juice. A dried powdered starter is used to initiate the fermentation. The mixture is allowed to ferment for up to three months, and to age for up to one year. The final product is light brown in colour and has a sweet and a sour flavour. A similar product called shoto sake is made in Japan (Steinkraus, 1996).

4.2.6 Muratina

Muratina is an alcoholic drink made from sugar cane and muratina fruit in Kenya. The fruit is cut in half, sun dried and boiled in water. The water is removed and the fruit is again sun dried. The fruit is added to a small amount of sugar cane juice and incubated in a warm place for 24 hours, after which it is removed and sun dried. The dried fruit is then added to a barrel of sugar cane juice which is allowed to ferment for between one and four days. The final product has a sour alcoholic taste (Steinkraus, 1996).

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