REVIEW OF RELATED LITERATURE

Origin and Distribution of Pineapple

Pineapple (Ananas comosus) is the second harvest of importance after bananas, contributing to over 20 % of the world production of tropical fruits (COVECA, 2002). Nearly over 70% of the pineapple is consumed as fresh fruit in producing countries especially in Africa such as Nigeria. Its origin has been traced to Brazil and Paraguay in the Amazonic basin where the fruit was domesticated. It has been defined as the most probable area of origin. The zone comprised of upper Panama and Brazil, Paraguay and Argentina, including the northern Amazonian forest and the semi-arid regions of Brazil, Venezuela and Guyanas (Medina, et al., 2005).

Worldwide distribution and production started by 1500 when pineapple was propagated in Europe and the tropical regions of the world (Medina, et al., 2005). The most wide spread variety is Cayena lisa (smooth cayenne) which was first introduced in Europe from French Guyana. It was until late 14th century when canned pineapple was produced commercially in Hawaii (FAO, 2004).

Thailand, Philippines, Brazil and China are the main pineapple producers in the world supplying nearly 50 % of the total output (FAO, 2004). Other important producers include India, Nigeria, Kenya, Indonesia, México and Costa Rica and these countries provide most of the remaining fruit available (50%) (Medina et al., 2005).

Morphology of Pineapple

Pineapple plant is a short herbaceous perennial with 30-80cm trough-shaped and pointed leaves 30-100 cm long, surrounding a thick stem. This shape of the plant has to drive water onto the stem. This water might be absorbed by axils. The early inflorescences has about 100-200 flowers (Sarah et al., 1997; COVECA, 2002). Flowers of pineapple are spirally placed and each is supported by bracteas. Each flower consists of 3 calyxes, 3 bluish corollas, 6 filaments and a carpel with three parts of stigma. Inflorescence goes to bloom about 3 weeks and it blooms from down to up. Pineapples are auto sterile and fruits developed are parthenocarpic. (Elfic, 2004).

A temperature range between 23 to 24°C is optimal for growing pineapple (FAO, 2002). When ambient temperature drops to 10-16°C, fruit growth is constrained. Plants may stand sub-freezing temperatures for very short periods. Conversely, exposure to temperatures well over 30°C results in heat damage due to increased respiration rate and metabolism and impaired nutrient absorption (Bartholomew and Kadziman, 1977).

Pineapple production regions are usually confined to altitudes below 800m above sea level, although Kenya reports production fields located between 1400 and 1800m, and Malaysia orchards as high as 2400m (Davies, 1994). When pineapple is grown at altitudes greater than 1000m above sea level, smaller fruits are produced; the pulp has less attractive colour, flavour and tartness are elevated. Plant growth occurs within a temperature range of between 21°C and 35°C and an annual rainfall of about up to 1100mm per annum and it should be evenly distributed. The optimal pH for growth is between 5.5 and 6.2 (Sarah et al, 1997; COVECA, 2002).

Variety of Pineapple Cultivated

There are several cultivars with different sugar brix. According to COVECA (2002), Cayena lisa contains 19% sugar brix, Spanish from Singapore (10%-12% sugar brix), Green Selacia (10%-12% sugar brix), Queen (14%-18% sugar brix), Red Spanish (12% sugar brix), Perola (13%-16% sugar brix), Perolera (12% sugar brix). However, several new varieties have been introduced to improve the quality of the fruit for the international markets such as MD2 (Golden ripe, Extra sweet and Maya gold). These varieties are hybrids that were developed in Hawaii from Cayena lisa with an average weight ranging from 1.3 to 2.5 kg. It has an intense orange to yellow-orange colour and a high sugar content of 15 to17° Brix. The fruit are sweet, compact and fibrous. Main differences found with respect to the Cayena lisa variety are: better resistance to internal darkening, lesser ascorbic acid content more prone to rotting and sensitive to Phytophthora (COVECA, 2002; FAO, 2002).

The La Josefina variety was released in 1996 for the fresh fruit market (FAO, 2002). It is a hybrid developed from other two clones. Its production cycle is annual with a generation of 2 to 3 suckers per plant. Average fruit weight is 1.1 to 1.3 kg and contains an elevated sugar concentration (17 to 22°Brix). Differences with respect to the Cayena lisa variety are: longer shelf life, greater sugar content and resistance to black heart disorder and shorter production cycles. Finally, variety RL41 is a hybrid obtained from cultivars Cayena lisa and “Manzana” with an average weight of 1.4 to 2 kg and a high sugar content, 15 to 18° Brix. Compared to Cayena lisa, this variety has a greater ascorbic acid content and shorter production cycles, as well as lesser resistance to rotting but more resistant to flower induction (FAO, 2002).

Fermentation

Concept of Fermentation

According to Garrison (1993), the process of fermenting is basically feeding sugars and nutrients in solution to yeast, which return the favour by producing carbon dioxide gas and alcohol. This process goes on until either all the sugar is gone or the yeast can no longer tolerate the alcoholic percentage of the beverage. Different yeasts produce different results, and have different tolerance levels (Anon, 2005).

Fermentation is a process of deriving energy from the oxidation of organic compounds, such as carbohydrates, and using an endogenous electron acceptor, which is usually an organic compound (Klien et al., 2005), as opposed to respiration where electrons are donated to an exogenous electron acceptor, such as oxygen, via an electron transport chain.

The risk of stuck fermentation and the development of several wine faults can also occur during this stage which can last from 5 to 14 days for primary fermentation and potentially another 5 to 10 days for a secondary fermentation. Fermentation may be done in stainless steel tanks, which is common with many white wines like Riesling, in an open wooden vat, inside a wine barrel and inside the wine bottle itself as in the production of many sparkling wines (Wikipedia, 2010; Robinson, 2006; Kunze, 2004).

Fermentation is a cheap and energy efficient means of preserving perishable raw materials such as pineapple juice (FAO, 2002). Harvested fruits may undergo rapid deterioration if proper processing and storage facilities are not provided, especially in the humid tropics where the prevailing environmental conditions accelerate the process of decomposition (FAO, 2010). Although there are several options for preserving fresh fruits, which may include drying, freezing, canning and pickling, many of these are inappropriate for the produce and for use on small-scale in developing countries. For instance the canning of fruits at the small-scale has serious food safety implications and contamination especially botulism (FAO, 1998).

Fermentation requires very little sophisticated equipment, either to carry out the fermentation or for subsequent storage of the fermented product. It is a technique that has been employed for generations to preserve fruits in the form of drinks and other food for consumption at a later date and to improve food security. Basically most fruits can be fermented; if not all provided they are well prepared (Garrison, 1993).

History of Fermentation

Fermentation is one of the oldest forms of food preservation technologies in the world. Indigenous fermented foods such as bread, cheese and wine, have been prepared and consumed for thousands of years and are strongly linked to culture and tradition, especially in rural households and village communities. The development of fermentation technologies is lost in the midst of history (Yokotsuka, 1985). Anthropologists have postulated that it was the production of alcohol that motivated primitive people to settle down and become agriculturists. Some even think the consumption of fermented food is pre-human (Stanton, 1985).

The first fermented foods consumed probably were fermented fruits. Hunter-gatherers would have consumed fresh fruits but in times of scarcity would have eaten rotten and fermented fruits. Repeated consumption would have led to the development of the taste for fermented fruits. There is reliable information that fermented drinks were being produced over 7,000 years ago in Babylon (now Iraq), 5,000 years ago in Egypt, 4,000 years ago in Mexico and 3,500 years ago in Sudan (Dirar, 1993; Pedersen, 1979).

There is also evidence of fermented meat products being produced for King Nebuchadnezer of Babylon. China is thought to be the birth-place of fermented vegetables and the use of Aspergillus and Rhizopus moulds to make food. The book called "Shu-Ching" written in the Chou dynasty in China (1121-256 BC) refers to the use of "chu" a fermented grain product (Yokotsuka, 1985).

Knowledge about traditional fermentation technologies has been handed down from parent to child, for centuries. These fermented products have been adapted over generations; some products and practices no doubt fell by the wayside. Those that remain today have not only survived the test of time but also more importantly are appropriate to the technical, social and economic conditions of the region (FAO, 1998). In Ghana, corn dough and cassava dough are fermented and cereal grains are locally brewed into local drinks such as „pito,‟ and „toosi‟.

According to Robinson (2006), natural occurrence of fermentation means it was probably first observed long ago by humans. The earliest uses of the word "Fermentation" in relation to winemaking was in reference to the apparent "boiling" within the must that came from the anaerobic reaction of the yeast to the sugars in the grape juice and the release of carbon dioxide. In the mid-19th century, Louis Pasteur noted the connection between yeast and the process of the fermentation in which the yeast act as catalyst through a series of a reaction that convert sugar into alcohol. The discovery of the Embden–Meyerhof–Parnas pathway by Gustav Embden, Otto Fritz Meyerhof and Jakub Karol Parnas in the early 20th century contributed more to the understanding of the complex chemical processes involved the conversion of sugar to alcohol (FAO, 2010).

Fermentation in Fruit Juice

According to Robinson (2006), the process of fermentation in wine is the catalyst function that turns fruit juice into an alcoholic beverage. To Walker (1988), this organic process is the "slow decomposition process of organic substances induced by micro-organisms, or by complex nitrogenous substances (enzymes) of plant or animal origin. During fermentation yeast interacts with sugars in the juice to create ethanol, commonly known as ethyl alcohol, and carbon dioxide (as a by-product). In winemaking the temperature and speed of fermentation is an important consideration as well as the levels of oxygen present in the must at the start of the fermentation (Keller, 2010; Van Rooyen and Tromp, 1982). Fermentation does not necessarily have to be carried out in an anaerobic environment. For example, even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to oxidative phosphorylation, as long as sugars are readily available for consumption (Dickinson, 1999).

Sugars are the most common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, and hydrogen (Garrison, 1993). However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone (Au Du, 2010). Yeast carries out fermentation in the production of ethanol in beers, wines and other alcoholic drinks, along with the production of large quantities of carbon dioxide (Keller, 2010). Fermentation products contain chemical energy (they are not fully oxidized), but are considered waste products, since they cannot be metabolised further without the use of oxygen or other more highly-oxidized electron acceptors. The consequence is that the production of adenosine triphosphate (ATP) by fermentation is less efficient than oxidative phosphorylation, whereby pyruvate is fully oxidized to carbon dioxide. Juice temperature must be warm for fermentation. However, yeast cells will die if temperature is too hot (Robinson, 2006).

Ethanol fermentation performed by yeast and some types of bacteria breaks the pyruvate down into ethanol and carbon dioxide. It is important in bread-making, brewing, and wine-making. Usually only one of the products is desired; in bread-making, the alcohol is baked out, and in alcohol production, the carbon dioxide is released into the atmosphere or used for carbonating the beverage. When the ferment has a high concentration of pectin, minute quantities of methanol can be produced (Au Du, 2010).

Hydrogen gas can be produced in many types of fermentation (mixed acid fermentation, butyric acid fermentation, caproate fermentation, butanol fermentation, glyoxylate fermentation), as a way to regenerate NAD+ from NADH. Electrons are transferred to ferredoxin, which in turn is oxidized by hydrogenase, producing H2. Hydrogen gas is a substrate for methanogens and sulphate reducers, which keep the concentration of hydrogen sufficiently low to allow the production of such an energy-rich compound (Madigan and Martinko, 2005). However, in the case of some fruit juice a risk factor involved with fermentation is the development of chemical residue and spoilage, which can be corrected with the addition of sulphur dioxide (SO2), although excess SO2 can lead to a wine fault (Robinson, 2006).

Fruit Fermented into Wine

There are many fermented drinks made from fruit in Africa, Asia and Latin America. These include drinks made from bananas, grapes, pineapples and other fruit. Grape wine is perhaps the most economically important fruit juice alcohol (FAO, 2010). 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 palm in North Africa, pineapples in Latin America and jack fruits in Asia (FAO, 2010).

White grape wine is an alcoholic fruit drink of between 10 and 14% alcoholic strength. This prepared from the fruit of the grape plant (Vitis vinifera), and is pale yellow in colour (Ranken, et al., 1997). There are many varieties used including Airen, Chardonnay, Palomino, Sauvignon Blanc and Ugni Blanc. 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. In the case of cashew, the 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 850C in order to eliminate wild yeast (Wimalsiri et al., 1971). The juice is filtered and treated with 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) is added. Once the yeast is added, the contents are stirred well and allowed to ferment for about two weeks (Au Du, 2010).

After fermentation is completed, the wine is separated from the sediment by racking. It can also be clarified further by using fining agents such as gelatine, pectin or casein which are mixed with the wine. Filtration can be carried out with filter-aids such as fullers earth after racking. The wine is then pasteurised at 50oC – 60oC. Temperature should be controlled, so as not to heat it to about 70oC, since its alcohol content would vaporise at a temperature of 75-78oC (Au Du, 2010). 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 et al , 1971).

Factors Influencing Fermentation

Effect of Temperature on Fermentation

To avoid contamination and unpleasant odours in wine, everything that comes in contact with the wine must be very clean. This is especially critical when cleaning the fermenting vessel. Just as there are weeds in the garden, so there are weeds in wines. There are micro organisms that feed on alcohol and cause a poor flavour (Anon, 2005). Vinegar bacilli will change sugar to vinegar. Moulds give a stale flavour. To prevent these unwelcome intruders, cleanliness is the only answer. An effective agent is Sal soda (sodium carbonate). Baking soda is fairly effective if given time to work. Either of these agents will remove odours and flavours from the containers (Van Rooyen, et al., 1982; Riley, 1978). All these chemicals may reduce the wine quality if the right quantities are not added. To avoid this situation, fruit juice for fermentation can be sterilised in stainless steel pans at a temperature of 85oC in order to eliminate wild yeast after extraction. The juice is filtered and treated with 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 (Van Rooyen, et al., 1982; Wimalsiri et al, 1971). Also, increasing temperatures above 60oC may kill wild yeast and other micro organisms (FAO, 2010; Robinson, 2006).

During fermentation there are several factors that winemakers take into consideration. The most notable is that of the internal temperature of the must (Keller, 2010). The biochemical process of fermentation itself creates a lot of residual heat which can take the must out of the ideal temperature range for the wine (Keller, 2010). Thus fermentation is an exothermic process (it releases heat). But in wine-making, the temperature must not exceed 29.4oC for red wines or 15.3oC for (white wines), otherwise the growth of yeast cells will stop. Therefore a lower temperature is desirable because it increases the production of esters, other aromatic compounds and alcohol itself. This makes the wine easier to clear and less susceptible to bacterial infection (Anon, 2005). In general, temperature control during alcoholic fermentation is necessary to facilitate yeast growth, extract flavours and colours from the skins, permit accumulation of desirable by-products, and prevent undue rise in temperature, that might kill the yeast cells. The low temperature and slow fermentation favours the retention of volatile compounds (Fleet, 1998).

Typically, white wine is fermented between 64-68 °F (18-20 °C) though a wine maker may choose to use a higher temperature to bring out some of the complexity of the wine (Battcock and Sue, 1998). Red wine is typically fermented at higher temperatures up to 85 °F (29 °C). In most cases, fermentation at higher temperatures may have adverse effect on the wine in stunning the yeast to inactivity and even "boiling off" some of the flavours of the wines. Some winemakers may ferment their red wines at cooler temperatures more typical of white wines in order to bring out more fruit flavours (Robinson, 2006).

Yeasts are active in a very broad temperature range - from 0 to 50 oC, with an optimum temperature range of 20oC to 30oC (Mountney and Gould, 1988). The temperature of fermentation is usually from 25 to 30oC this makes yeast an important microorganism for fermentation. White wines are fermented at 10 to 18º C for about seven to fourteen days. The low temperature and slow fermentation favours the retention of volatile compounds. Red wines are fermented at 20 to 30ºC for about seven days to fourteen days. This higher temperature is necessary to extract the pigment from the grape skins (Keller, 2010). With reference to other organisms, different bacteria can tolerate different temperature which provides enormous scope for a range of fermentations. While most bacteria have a temperature optimum of between 20 to 30ºC, there are some (the thermophiles) which prefer higher temperatures (50 to 55ºC) and those with colder temperature optima (15 to 20ºC). Most lactic acid bacteria work best at temperatures of 18 to 22ºC. The Leuconostoc species which initiate fermentation have an optimum temperature of 18 to 22ºC. Temperatures above 22ºC, favour the lactobacillus species (FAO, 2010; Anon, 2005).

As soon as the desired degree of sugar disappearance and alcohol production has been attained, the microbiological phase of wine making is over (Stanier et al, 1972). 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 (Au Du, 2010).

Effect of pH on Fermentation and Wine Quality

According to Fleet (1998), pH directly affects wine stability. This may be as a result of the fact that at a pH close to neutral (7.0), most micro organisms like bacterial, moulds including some yeasts become more active for fermentation and subsequent spoilage of wine, whilst pH below 3.5 eliminate most of the microbes, and favours only few of the micro organisms for fermentation. Specifically, the optimum pH for most micro-organisms is near the neutral point (pH 7.0). Moulds and yeasts are usually low pH tolerant and are therefore associated with the spoilage of foods with low pH. Yeasts can grow in a pH range of 4 to 4.5 and moulds can grow from pH 2 to 8.5, but favour low pH (Mountney and Gould, 1988). A solution s pH is the measure of hydrogen ions (H+) concentration of an acid solution such as pineapple and grape juice or wine or conversely the concentration of hydroxyl ions (OH-) in alkaline solution such as lye. Because the numerical value of the hydrogen ions (H+) concentration is often extremely small fraction (1× 10-7) the pH unit is used to express this concentration (Encyclopedia Britannica. 2000). A pH unit has been expressed as the negative logarithm of the hydrogen ion (H+) concentration and it is determined by a pH meter (Lacroux et al., 2008; Encyclopedia Britannica. 2000; Gallander, et. al., 1987).

From the pH scale, the lower the pH value, the higher the concentration of H+ ions, the higher the degree of acidity, thus there is an inverse relationship between decreasing pH value and increasing H+ ions concentration. For example a wine at a pH of 3.0 is 10 times more acidic than a wine at a pH of 4.0, thus there is a ten fold change in acidity (Encyclopedia Britannica, 2000).

The traditional process of fermentation involves extracting fruits juice and adjusting the pH to 4.0 using sodium bicarbonate and adding yeast nutrient (ammonium phosphate) at 0.14g per litre (Steinkraus, 1996). For example, during fermentation of fruit juice, reductions of soluble solids are possible from pH between 7.4 to 3.5 and 4.0 in worm fermentation (Steinkraus, 1992). A pH level of 4.0 may be conducive for the development of unwanted microbes like L. oneos, and this can be prevented by controlling the pH by reducing the wine pH to below 3.2 (Fleet, 1998). According to Rotter (2008), most fining and clearing agents such as Earths: bentonite, kaolin, Proteins: gelatine, isinglass, casein, pasteurised milk, albumen, yeast, Polysaccharides: alginate (agar), gum arabic (acacia), Carbons, Synthetic polymers: PVPP, silica gel, Tannins, Others: metal chelators, blue fining, enzymes are more effective in clearing the wine when pH is below 3.5.

pH plays an important role in aging, clarifying or fining. As the strength of the relative charge of suspended particles decreases in the wine, the pH of the wine increases. At high pH, organic protein fining agents may possess a positive charge insufficient to bind to the negatively charged particulates, thus potentially increasing the turbidity of the wine. This phenomenon is called "overfining" (Rotter, 2008).

Effects of Sugar Content on Fermentation

Sugar is the main substrate for fermentation of fruits juice into alcohol (Keller, 2010); although, other food nutrients such as protein and fats can be broken down by some micro organism in some cases where sugar is limited, but as long as sugar is present yeast cells will continue the process of fermentation until other factors that affect the growth of yeast become unfavourable (Dickinson, 1999). According to Garrison (1993), sugars are the most common substrate of fermentation to produce ethanol, lactic acid, hydrogen and carbon dioxide.

Although sugar is an important substrate of fermentation, higher sugar concentration inhibits the growth of micro-organisms (FAO, 2010). For example, during fermentation of the juices of the plant (Agave Americana), the soluble solids should be at the optimum, and should be reduced from between 25-30% to 6%; the sucrose content falls from 15% to 1% (Steinkraus, 1992). However, yeasts are fairly tolerant of high concentrations of sugar and grow well in solutions containing 40% sugar. At concentrations higher than this, only a certain group of yeasts – the osmophilic type – can survive. There are only a few yeasts that can tolerate sugar concentrations of 65-70% and these grow very slowly in these conditions (Board, 1983). A winemaker who wishes to make a wine with high levels of residual sugar (like a dessert wine) may stop fermentation early either by dropping the temperature of the must to stun the yeast or by adding a high level of alcohol (like brandy) to the must to kill off the yeast and create a fortified wine (Robinson, 2006).

Effect of Micro Organisms on Fermentation

For many traditional fermented products, the micro-organisms responsible for the fermentation are unknown to scientists. However there have been several researches to identify the micro-organisms involved in fruits fermentation. For example, the microorganism responsible for banana beer production is Saccharomyces cerevisiae, which is the same organism involved in the production of grape and other fruit wine like pineapple wine. However many other micro-organisms that are associated with the fermentation have been identified. These organisms vary according to the region of production (Davis and Noble, 1995).

Yeast is a unicellular fungus which reproduces asexually by budding or division, especially the genus Saccharomyces which is important in food fermentations has the ability to reproduce much faster (Walker, 1988). Yeasts and yeast-like fungi are widely distributed in nature. They are present in orchards and vineyards, in the air, the soil and the intestinal tract of animals. Like bacteria and moulds, they can have beneficial and non-beneficial effects in foods. Most Yeast strains are larger than most bacteria. The most well known examples of yeast fermentation are in the production of alcoholic drinks and the leavening of bread. For their participation in these two processes, yeasts are of major importance in the food industry. Some Yeast strains are chromogenic and produce a variety of pigments, including green, yellow and black. Others are capable of synthesizing essential B group vitamins (Kawo and Abdulmumin, 2009; Adams and Moss, 1995; Walker, 1988).

Although there is a large diversity of yeasts and yeast-like fungi, (about 500 species), only a few are commonly associated with the production of fermented foods. They are all either ascomycetous yeasts or members of the genus Candida. Varieties of the Saccharomyces cervisiae genus are the most common yeasts in fermented foods and beverages based on fruit and vegetables. All strains of this genus ferment glucose and many ferment other plant derived carbohydrates such as sucrose, maltose and raffinose. In the tropics, Saccharomyces pombe is the dominant yeast in the production of traditional fermented beverages, especially those derived from maize and millet (Adams and Moss, 1995).

Brewer's yeast, Saccharomyces cerevisiae var ellipsoideus, and Saccharomyces uvarum are very common in the brewery and the wine industry. These yeasts are the microorganisms that are responsible for fermentation in beer and wine (Keller, 2010). Yeast metabolises the sugars extracted from grains and fruits, which produces alcohol and carbon dioxide, and thereby turns wort into beer and fruits into wine respectively. In addition to fermenting the beer and wine, yeasts influence the character and flavour (Ostergaard et al., 2000). The dominant types of yeast used in fermenting alcoholic beverages are the Saccharomyces species. For example, to make beer the ale yeast (Saccharomyces cerevisiae) and lager yeast (Saccharomyces uvarum) are used (Dittmer and Desmond, 2005), whilst in wine (Saccharomyces cerevisiae var ellipsoideus and (Saccharomyces cerevisiae) may be used (Keller, 2010). Other microorganisms used in fermentation wine and beer may include: Brettanomyces species for lambics (Hornsey, 1999), Torulaspora delbrueckii for Bavarian Weiss bier (Horwitz, 1999). Before the role of yeast in fermentation was understood, fermentation involved wild or airborne yeasts. A few styles such as lambics rely on this method today, but most modern fermentation adds pure yeast cultures (Hui and Khachatourians 1994).

The most common genera of wild yeasts found in winemaking include Candida, Klöckera/Hanseniaspora, Metschnikowiaceae, Pichia and Zygosaccharomyces (Wikipedia, 2010). Wild yeasts can produce high-quality, unique-flavoured wines; however, they are often unpredictable and may introduce less desirable traits to the wine, and can even contribute to spoilage (Keller, 2010). Traditional wine makers, particularly in Europe, advocate use of ambient yeast as a characteristic of the region's terroir; nevertheless, many winemakers prefer to control fermentation with predictable cultured yeast. The cultured yeasts most commonly used in winemaking belong to the Saccharomyces cerevisiae (also known as "sugar yeast") species (Wikipedia, 2010). Within this species are several hundred different strains of yeast that can be used during fermentation to affect the heat or vigour of the process and enhance or suppress certain flavour characteristics of the wine. The uses of different strains of yeasts are a major contributor to the diversity of wine, even among the same grape variety (Robinson, 2006). According to Panjai, et al., (2009), mixture of yeast thus dual culture (T. delbrueckii species and S. cerevisiae) can be used to produce a complex fruit wine from pineapple.

Yeast in general has a natural protein removal effect during fining or clearing. It is also sometimes used, in the dried (and dead) form, to remove copper sulphate, ethyl acetate, browning, oxidation and excess oak that may be associated with cloudy wine (Rotter, 2008). Doses commonly recommended are 240-1000 mg/l. It is important to rack the wine soon after yeast fining in order to avoid reductive aromas (Rotter, 2008).

According to Madigan and Martinko (2005), homolactic fermentation can occur in some kinds of bacteria (such as lactobacilli) and some fungi. It is this type of bacteria that converts lactose into lactic acid in yoghurt, giving it, its sour taste. These lactic acid bacteria can be classed as homofermentative, where the end product is mostly lactate, or heterofermentative, where some lactate is further metabolized and results in carbon dioxide, acetate or other metabolic products (Axelsson, 1998; Dickinson, 1999).

Bacteria may not always be bad in fermentation; this is because to clarify the wine, the fermented juice maybe transferred into a settling vat, or if made on a smaller scale, into a demijohn (FAO, 2008). In these, suspended yeast cells, cream of tartar and particles of skin and pulp settle to the bottom of the container. As the yeast cells break down within the precipitate, they stimulate the growth of Lactobacillus bacteria that convert the wine's malic acid into lactic acid. This process is especially important in wines made from highly acidic grapes because lactic acid is a weaker acid than malic acid. (Bacteria decarboxylate malic acid, thus removing the acidic carboxyl group), therefore it mellows the wine's taste (Anon, 2005).

Effect of Acid on Fermentation and Wine Quality

Acid is said to directly affect wine quality, but wine owes its acid composition to citric acid, tartaric acid, and some traces of other acids like lactic acid which replaces malic acid during malolactic fermentation (Fleet, 1998). These acids in fruits juice or wine can be determined by titration (; Laroux et al., 2008; Iland, et al., 2000). Fruit acids are weak acids, compared to strong mineral acids such as sulphuric and hydrochloric. In solution, strong acids tends to yield their hydrogen ion (H+) component nearly completely; weak acids dissociate only about one percent of their hydrogen ion. Thus such acid solutions like fruit wine have more hydrogen ions (H+) than hydroxyl ions (OH-). As hydrogen ion concentration increases, the solution becomes more unfavourable for most micro organisms associated with spoilage of wine and acidic foods. However some moulds and yeasts which are needed in the fermentation of fruit juice into wine are usually acid tolerant therefore they are very important in the production of dry wine (wine with a very low or no sugar), (Mountney and Gould, 1988).

Wines produced from grapes grown in colder climates tend to have a higher concentration of malic acid and a lower pH (3.0 to 3.5) and the taste benefits from this slight decrease in acidity. Wines produced from grapes in warmer climates tend to be less acidic (pH > 3.5) and a further reduction in acidity may have adverse effects on the quality of the wine. Decreasing the acidity also increases the pH to values which can allow spoilage organisms like L. oenos to multiply to embark on malo-lactic fermentation (Fleet, 1998). During fermentation of palm sap, within 24 hours pH can be 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 level 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).

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). Fruit juices often have all that yeast needs all by themselves. Notably grape juice is a favourite, as it has the acids, tannins and sugars needed. Apple juice stands on its own quite well too. Other juices may need acids (not just for the yeast, but for flavour), and many commonly need tannins to be added. Yeasts are very hardy microorganisms that will get by with most fruits sugar and juices in fermentation. They can even work on plain white sugar so far as the right acid and nutrient blend is available, although this is difficult to do by most microorganisms (Garrison, 1993). Acids present in wine enhance the taste, aroma and preservative properties of the wine (Van Rooyen and Tromp, 1982; Wimalsiri et al., 1971).

Relationship Between Sugar Content And Alcohol Production

Sugar is essential for making wine, as without it yeast may not be able to produce alcohol. Natural sugars in some fruits are often insufficient to produce anything stronger above 8% by volume of alcohol. This means that to produce an alcoholic fruit wine of stronger alcoholic strength above 8% by volume of alcohol a white granulated cane or beet sugar must be added (Dull, 1971). Corn sugar can be used in direct proportions to granulated sugar. Fructose (fruit sugar) is sweeter than other kinds and should be used only when a sweet wine is desired or in sweetening a wine after fermentation (Keller, 2010).

Duration and pH variances also affect the sugar composition of the resulting must during fermentation (Kunze, 2004). As fermentation time increases, more sugars are digested, more antioxidants will be produced and the pH will probably settle around pH 3.5 making a drier acidic drink. There will also be a greater yeast activity producing more scum and sediment from dead bodies of yeasts and lactobacilli (Sulz, 2011). 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 et al., 1997). Any wine that is absolutely bone dry (little or no sugar) will stabilize itself within a few days to weeks, as no food remains to keep the yeast alive. For bone dry wines (specific gravity of 0.990 or lower), allow them to sit for 30 days before bottling (Van Rooyen and Tromp., 1982; Anon, 2005: Davis and Noble, 1995).A wine given a hydrometer reading of a specific gravity of 0.990 and a sugar ᵒBrix of below 3.5 is a dry wine; therefore, addition of sugar would be required if the wine is to be sweetened. On the other hand, a wine with a specific gravity of 1.020 and a sugar ᵒBrix of 5.08 is a sweet wine (Wine World FDW, 2002; Ed Kasper, 2007).

Microorganisms Present In Pineapple Juice And Wine

The main bacteria contaminants in fresh pineapple juice included Lactobacillus sp., Gluconobacta sp., Streptococcus sp. and Leuconostoc sp. Fungal or wild yeast contaminants also isolated included Sacharomyces sp. Candidda sp. and Aspergillus sp. After sterilization of the juice, the microorganisms present will all destroyed be killed through sterilization process. Au Du (2010), Robinson (2006) and Fleet (1998) reported that increasing the must temperature above 60oC and the addition of Potassium Meta-bisulphate may kill or destroy or inhibit the growth of wild yeast, acetic acid bacteria, wild yeast and moulds present in wine.