A review of traditional fermented foods and beverages

International Journal of Food Microbiology 53 (1999) 1–11 www.elsevier.nl / locate / ijfoodmicro

Review

A review of traditional fermented foods and beverages of Zimbabwe a a b, c T.H. Gadaga , A.N. Mutukumira , J.A. Narvhus *, S.B. Feresu a

Institute of Food, Nutrition and Family Sciences, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabwe b ˚ , Norway Department of Food Science, Agricultural University of Norway, P.O. Box 5036, N-1432 As c Department of Biological Sciences, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabwe Received 29 July 1999; accepted 6 September 1999

Abstract Several traditional fermented foods and beverages are produced at household level in Zimbabwe. These include fermented maize porridges (mutwiwa and ilambazi lokubilisa) fermented milk products (mukaka wakakora /amasi and hodzeko) non-alcoholic cereal-based beverages (mahewu, tobwa and mangisi) alcoholic beverages from sorghum or millet malt (doro /uthwala and chikokivana) distilled spirits (kachasu) and fermented fruit mashes (makumbi). There are many regional variations to the preparation of each fermented product. Research into the processing technologies of these foods is still in its infancy. It is, therefore, important that the microbiology and biochemistry of these products, as well as their technologies be studied and documented in order to preserve them for future generations. This article reviews the available information regarding traditional fermented foods in Zimbabwe and makes recommendations for potential research areas.  1999 Elsevier Science B.V. All rights reserved. Keywords: Traditional fermented foods; Mutwiwa; Doro; Mahewu; Makumbi; Amasi

1. Introduction Fermented foods make up an important contribution to the human diet in many countries because fermentation is an inexpensive technology which preserves food, improves its nutritional value and enhances its sensory properties (Murty and Kumar, 1995; Steinkraus, 1996). Fermentation may also lead *Corresponding author. Tel.: 1 47-64-948-550; fax: 1 47-64943-789. E-mail address: [email protected] (J.A. Narvhus)

to the detoxification and destruction of undesirable factors present in raw foods such as phytates, tannins and polyphenols (Sharma and Kapoor, 1996). Several traditional fermented products have been documented in different African countries and include non-alcoholic beverages, alcoholic beverages, breads, pancakes, porridges, cheeses and milks (Van der Walt, 1956; Haggblade and Holzapfel, 1989; Ashenafi, 1990; Dirar, 1993; Mwesigye and Okurut, 1995; Steinkraus, 1996). In Zimbabwe, fermented foods are produced at household level from various raw materials such as cereals, milk, fruits, and wood

0168-1605 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0168-1605( 99 )00154-3

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T.H. Gadaga et al. / International Journal of Food Microbiology 53 (1999) 1 – 11

sap. Some of these household fermentation technologies have been converted to an industrial scale in Zimbabwe and South Africa in order to meet the demand for traditional products by the urban population. Routine home preparation of fermented foods is too arduous and time consuming in the urban environment (Okagbue, 1995). Industrialised products include non-alcoholic and alcoholic beverages such as mahewu and chibuku, respectively, and fermented dairy products such as lacto. This review aims to list and summarize the production processes of common Zimbabwean traditional fermented foods and to highlight, where available, some of the microbiological and biochemical properties of the fermented foods and technological improvements which have been achieved on the production of some of the foods.

2. Cereal based fermented products Most cereal-based non-alcoholic and alcoholic beverages in Zimbabwe are prepared using either sorghum (Sorghum bicolor), bulrush millet (Pennisetum typhoideum) or finger millet (Eleusine coracana) malt. The Shona generic name for all these malts is chimera (Gomez, 1989).

2.1. Preparation of malt Sorghum or millet malt (chimera) is prepared by packing the grain in a sack and steeping it in water (pond or river) for about a day (Madovi, 1981; Benhura and Chingombe, 1989). The swollen grains are either spread on the floor in a layer up to 10 cm thick and left to germinate for up to 3 days (Benhura and Chingombe, 1989) or are washed and left to germinate for 2 days in the sack at ambient temperatures (Zvauya et al., 1997). The germinated grains are then spread out on a flat piece of rock or other suitable surface and left to dry in the sun. Traditionally, the malt was coarsely ground using a grinding stone, however today hammer mills are more commonly used (Benhura and Chingombe, 1989).

2.2. Non-alcoholic beverages Mahewu (amahewu) is a beverage, which is prepared from either thin maize porridge or thick

maize porridge (sadza) (Gomez, 1989; Okagbue, 1995). It is an adult-type food, which is commonly used to wean children and is introduced to infants between 4–18 months (Simango, 1997). The maize porridge, if thin, is mixed with water or if thick (sadza) is mashed into small pieces and mixed with water. Sorghum or millet malt or wheat flour is then added to either mix and left to ferment (Simango and Rukure, 1991; Mutasa and Ayebo, 1993; Okagbue, 1995). The fermentation is a spontaneous process in which the natural flora of the malt carries out the fermentation at ambient temperature (Simango and Rukure, 1991; Mutasa and Ayebo, 1993; Okagbue, 1995). The drink is consumed after standing for about 24 h (Okagbue, 1995). This traditional way of producing mahewu requires an extended fermentation time because of the low initial number of desirable lactic acid bacteria (Mutasa and Ayebo, 1993). The use of uncontrolled conditions, especially temperature, may also result in the proliferation of undesirable microorganisms which convert lactic acid to undesirable end products which adversely affect the taste and texture of mahewu (Holzapfel, 1991). Little information has been recorded on the general properties, microbiological processes and safety of traditionally prepared Zimbabwean mahewu. Mutasa and Ayebo (1993) carried out several laboratory experiments in which they simulated the traditional fermentation of mahewu and varied the levels of sorghum malt and type of ingredients added to the porridge; the solid content and the cooking time of the porridge; and the temperature of fermentation. The best mahewu was obtained when 30 g of a mixture of finger millet (1 / 3) and sorghum (2 / 3) malts were added to 500 ml of heated porridge. The porridge contained 14% solids and was a result of mixing 30 g each of maize meal and sorghum malt with 500 ml water followed by boiling the mixture for 10 min. The mix was fermented at 458C for 16 h. The pH and acidity of laboratory produced mahewu were 3.29 and 0.50% (calculated as lactic acid), respectively. This compares well with a pH of about 3.5 and titratable acidity of 0.4–0.5% recorded for the South African traditionally fermented mahewu (Schweigart and Fellingham, 1963; Holzapfel, 1991; Steinkraus, 1996). Very few studies have been done to isolate and characterise the microorganisms responsible for fermenting the traditionally fermented mahewu in


Zimbabwe. The predominant microorganisms in the spontaneous fermentation of the South African mahewu belong to Lactococcus ( Lact.) lactis subsp. lactis (Steinkraus, 1996). Schweigart et al. (1960) used a starter culture of either Lactobacillus ( Lb.) delbrueckii or Lb. bulgaricus to produce mahewu at 458C. This reduced the fermentation time from 36 to 3 h. Simango and Rukure (1991,1992) worked on the microbial safety of traditionally fermented mahewu. They showed that the mahewu produced in Zimbabwean homes had bactericidal and / or bacteriostatic properties against strains belonging to the genera Aeromonas, Salmonella and Shigella as well as strains of Campylobacter jejuni and enteropathogenic Escherichia coli. These observations suggest that traditional fermented mahewu is an unlikely vehicle for the transmission of enteric pathogens (Simango and Rukure, 1991,1992). Industrial production of mahewu has met with some success in South Africa and Zimbabwe (Van Noort and Spence, 1976; Mutasa and Ayebo, 1993). A variety of powdered concentrated forms of mahewu, which can be mixed with water to either produce a beverage, which can be consumed instantly, or be left to ferment for several hours at room temperature, can be purchased from retail outlets in Zimbabwe. These powders are usually fortified with minerals, vitamins and soy protein (Mutasa and Ayebo, 1993). The varieties of commercial mahewu only serve as substitutes as their taste is usually different and perceived to be inferior to that of traditionally fermented mahewu. More work needs to be done to elucidate the microflora responsible for naturally fermenting mahewu and to develop them into starter cultures in order to industrially produce mahewu which more closely resembles the traditional product. Tobwa is another non-alcoholic cereal-based beverage, which is very similar to mahewu but has not been previously documented. Like mahewu, it is made by mashing left over sadza into small pieces and mixing it with water to form a slurry, then leaving the mixture to ferment overnight. The only difference with mahewu is that no malt is added to the fermentation thus making the process a totally lactic acid fermentation. Tobwa is usually drunk by those people whose religion does not allow them to drink beverages to which malt has been added. Mangisi is a sweet-sour beverage made from the

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natural fermentation of millet mash (Zvauya et al., 1997). The preparation varies in the different regions of Zimbabwe. In one variation, finger millet is malted then milled and the flour mixed with water. The mixture is slowly heated for 80 min to almost boiling. The resulting product is the mash (masvusvu) which is cooled, diluted, strained and allowed to stand for some hours during which spontaneous fermentation takes place to give mangisi (Zvauya et al., 1997). The microorganisms responsible for fermentation are thought to come from the utensils, fermentation vessel and to include those organisms from the malt flour that survive cooking. Another variation involves malting the finger millet, milling, mixing the flour with water and boiling the mixture for 1–2 h. The masvusvu is cooled, diluted and allowed to stand overnight. On the second day more malt flour is added and the mixture left to ferment until early on the third day when the coarse solids are strained off and the fermenting mixture returned to the fermentation vessel. The mangisi is ready for consumption later on the same morning of the third day (Benhura and Chingombe, 1989). Although no work has been done to characterize this variety of mangisi, it is likely to contain more alcohol than the variety described by Zvauya et al. (1997) because of the addition of extra malt to the brew on the second day which serves as an additional source of inoculum and also the longer fermentation time. Zvauya et al. (1997) carried out laboratory studies on the microbiological and biochemical changes which occur during the production of manaisi. Counts of aerobic mesophilic bacteria and of lactic acid bacteria as well as those of yeasts and moulds increased during 8 h of fermentation. The four groups of microorganisms produced organic acids, ethanol, carbon dioxide, and other volatile flavour compounds, which gave the characteristic flavour and taste to the final product. The laboratory-prepared mangisi had a final pH of 3.98, titratable acidity of 0.67% and lactic acid concentration of 4.10 g / l. Although the microorganisms found during fermentation were enumerated, no attempt was made to isolate and characterize them or to determine the levels of ethanol and other organic acids which they may produce during the fermentation. Mangisi is a product with potential for scaling-up and industrialization. However, further studies are required to attain this goal.

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2.3. Alcoholic beverages Chikokivana is a 1-day brew produced using a commercial yeast starter culture. Maize meal and millet malt are mixed with water after which yeast is added (Madovi, 1981). The mixture is left to ferment for 24 h at ambient temperature in earthenware pots or metallic drums. The brew is then strained to give the final product. Although the brewing of chikokiyana uses the yeast Saccharomyces cerevisiae as a starter culture, the process has not been studied and the product analysed to ascertain its alcohol content and presence of any other metabolic products of the yeast.

2.3.1. Doro In Zimbabwe, bulrush and finger millet malts are preferred for the traditional brewing process although sorghum malt and sprouted maize are sometimes used for producing alcoholic beverages (Benhura and Chingombe, 1989). The traditional brew is known as doro, hwahwa, mhamba or uthwala in the different regions of the country. These are generic names describing the same or similar products, which vary in the details of the brewing process. Holzapfel (1991) erroneously used the term zezuru for this product. Doro is brewed for important social and cultural gatherings as well as for generating income. Examples of the social gatherings include when neighbouring households come to help with tasks such as weeding and reaping of crops; weddings; celebrations of success; and traditional religious ceremonies such as praying for rain and communicating with ancestors (Madovi, 1981; Benhura and Chingombe, 1989). Madovi (1981) and Benhura and Chingombe (1989) have described different methods of preparing doro. Briefly the brewing process described by Benhura and Chingombe (1989) involves preparing masvusvu in the same way as is done for mangisi. The masvusvu is cooled, diluted in clay pots and left to sour at ambient temperature for about 2 days. On the third day, the soured product (mhanga) is boiled for 3–5 h, reducing the original volume by a quarter in the process. The boiled mhanga is allowed to stand overnight after which more malt flour is added. Typically, the amount of malt added is about half the amount used at the beginning of the brewing process. On the sixth day some masvusvu, two to three times

more than the amount cooked on the first day, is prepared and allowed to cool. Meanwhile, a small portion of the mhanga is strained and kept separately. The strainings (masese), the rest of the unstrained mhanga and the fresh portion of cooled masvusvu are all mixed together with water to give biti. The mixture is left to ferment for about 2 h and the resulting product which is called madirwa is then strained, mixed with the previously strained mhanga and left to ferment overnight. The doro is served in an active fermenting state the following morning. The fermentation process takes 5–7 days depending on ambient temperature. Madovi (1981) described a process where the ground cereal meal is dispersed in water and allowed to simmer to a thin porridge. The porridge is allowed to cool to room temperature, mixed with malt meal and left to ferment for a few days. This is then boiled for approximately 1 h, cooled to room temperature and more malt is added. The brew is again left to ferment for a few days after which a boiled and cooled slurry of coarsely ground malt is added. The mix is left for approximately 1 h then filtered through a coarse cloth filter or wire sieve. The doro is left to mature overnight before consumption. Ethanol is thought to be the main alcohol (about 4% v / v) present although methanol, butanol and other alcohols are also found in doro (Madovi, 1981). Another variation is found in parts of Matebeleland where uthwala (the Ndebele generic name for doro) is usually prepared in 3 days. Sorghum malt and maize meal from germinated maize grains are mixed with boiling water to form a slurry, which is left to ferment overnight. This mixture is boiled on the second day to obtain a thin gruel, more malt is added and the mixture is allowed to ferment overnight. The product is then strained ready to drink on the third day. This variety of doro /uthwala has not been previously documented. Although no work has been done on the biochemical and microbiological properties of doro, it is perceived to be nutritionally superior to watery clear beer. This is because it is colloidal, thick, with some starch, protein, B-group vitamins, minerals, yeast cells and alcohol-tolerant bacteria (Madovi, 1981; Holzapfel, 1991). Because of this perception, employers used to supply sorghum beer as part of rations to indigenous labourers in the mines and towns in South Africa and the then Central African


Federation (now Zimbabwe, Zambia and Malawi) during the 1950s (Schwartz, 1956). In order to meet this demand, the South African Council for Scientific and Industrial Research (CSIR) was tasked with developing the brewing of sorghum beer to an industrial scale in 1954 (Van der Walt, 1956; Haggblade and Holzapfel, 1989). This led to the development of the industrially produced (opaque) sorghum beer that is now well described and industrialised in South Africa, Botswana and Zimbabwe (Schwartz, 1956; Van der Walt, 1956; Holzapfel, 1991; Steinkraus, 1996). The industrial process partly employs the traditional technology of spontaneous lactic acid fermentation and combines it with the use of a yeast (Saccharomyces cerevisiae) starter culture for alcoholic fermentation. The commercialized varieties of sorghum beer have various commercial brands in Zimbabwe such as chibuku, thabani, rufaro, simba, and go-beer. Although these industrial sorghum beers are quite popular, they have not replaced traditionally fermented doro in the rural areas. The microorganisms responsible for the fermentation of doro are poorly studied and thought to include wild yeasts from the malt; and yeasts and bacteria from the beer pots passed from previous brews. Madovi (1981) proposed that if yeast starter cultures were added to the brew, the process could take less time than the current 5–7 days. There is, therefore, a need to isolate, identify and characterize the microorganisms involved in the traditional fermentation to allow for the formulation of starter cultures. That way the fermentation could be carried out under controlled conditions and produce a doro whose quality could be more readily controlled and monitored. It is also necessary to carry out studies to extend the shelf life of doro as it is an actively fermenting product. Kachasu (also known as tototo or nipa) is a traditionally fermented, highly intoxicating distilled alcoholic spirit. It is usually brewed using maize meal but bulrush or finger millet meal, various fruits and banana peels may be used as alternative sources of carbohydrate. The carbohydrate source is added to warm water in a pot with a hole drilled on the side, which is used later during the distillation of the spirit. The mixture is stirred into a slurry and allowed to simmer for a few minutes before the pot is removed from the fire. Sugar and yeast are added after the slurry has been cooled to ambient tempera-

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ture. The hole in the pot is sealed with clay and the mixture allowed to ferment for 4–7 days at ambient temperature (Brett et al., 1992). At the end of the fermentation, the seal on the hole is broken and a narrow pipe connected. The pipe transverses a waterjacket containing cold water, which acts as a condenser. The fermented brew is distilled over a small fire and the clear distillate is collected from the end of the pipe into bottles. The alcohol content of the kachasu can range from 9–41% (Brett et al., 1992). The selling and consumption of kachasu in Zimbabwe has been illegal since 1971 because the spirit is alleged to be toxic as its drinking has been associated with ill health and cases of sudden death (Brett et al., 1992). Toxicity is attributed to several co-generic alcohols such as isoamyl alcohol, isobutanol, and methanol. Other organic compounds, which have been identified in kachasu include acetaldehyde, acetone, ethylacetate, and furfurals (Brett et al., 1992). Kachasu is comparable to waragi, a Ugandan traditionally produced spirit whose production has been commercialised by having small brewers sell their traditionally produced waraai to a large distillery. The distillery then triple distils it to produce a high quality bottled commercial product with up to 40% ethanol (v / v) (Mwesigye and Okurut, 1995). A similar scheme could be introduced to upgrade and control the quality of kachasu and to produce a safer and regulated distilled spirit.

2.4. Porridges Mutwiwa or mudzvurwa is a sour maize meal which can be used to produce sour sadza (Simango, 1997). Dried maize grains are sprinkled with a little water, pounded with a pestle and mortar to remove their husks, then sun-dried and winnowed. The dehulled grains are washed, steeped in clean water and left to ferment until gas production ceases (Simango, 1997; Mawadza et al., 1999). Lactic acid bacteria, aerobic mesophilic bacteria, yeasts and moulds proliferate during the spontaneous fermentation of mutwiwa (Mawadza et al., 1999). The fermentation time can be shortened by back-slopping, which involves retaining a small amount of fermented dehulled maize and adding it to a new batch of dehulled maize. The fermented dehulled maize is then either wet milled by pounding, win-

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nowed and the fine meal sun-dried (Mawadza et al., 1999) or dried and hammer-milled into maize meal (Simango, 1997). The product, mutwiwa, is then used to prepare thin porridge or sadza, which has an improved lactic acid taste. Mawadza et al. (1999) isolated two strains of Pediococcus pentosaceus from traditionally fermented mutwiwa and employed them as starter cultures for preparing mutwiwa. They compared the rates of fermenting maize to mutwiwa when it occurred spontaneously, due to back-slopping and after addition of their starter cultures. Increased rates of fermentation with corresponding faster decreases in pH were observed after back-slopping and with the use of starter cultures than when the maize was allowed to ferment spontaneously. The work was rather limited and needs to be pursued further for it to have a significant impact on improving the process and product. Ilambazi lokubilisa is a fermented thin sour porridge, which is similar to porridge produced from mutwiwa although in this case the maize is fermented after milling. It is consumed widely in Zimbabwe and used as a weaning food (Simango, 1997). Maize meal is thoroughly mixed with a little amount of water and allowed to ferment in a closed vessel for about 2–4 days. The fermented meal is then used to make ilambazi lokubilisa /sour porridge (Simango, 1997). Ilambazi lokubilisa has been shown to be bactericidal to strains of enteric pathogens belonging to the genera Aeromonas, Campylobacter and Salmonella and bacteriostatic to strains of Shigella and E. coli (Simango and Rukure, 1992). Its inhibitory properties were shown to be due to a combination of many substances including lactic and acetic acids (Simango, 1995). These properties make ilambazi lokubilisa a safe readily available weaning food, which should be promoted. Therefore, the microbiology of its fermentation and how it differs from that of mutwiwa need to be studied so that the technology of its production can be improved.

3. Fermented wild fruits products Makumbi is the generic name for beverages made from wild fruits (Gomez, 1989). In general, over 10% of recorded major food plants of Zimbabwe can be fermented into some type of beer or wine

(Okagbue, 1995). Thus the improvement of the technologies for fermenting makumbi could benefit rural communities as they could be involved at several stages during the production of the different wild fruit beverages. Marula wine or beer is traditionally prepared by spontaneously fermenting a mash obtained from the fruits of the marula plant (Scleroecarya caffra, mapfura /umkumbi). The ripe fruits are washed / cleaned and the skins removed. The fleshy stones are soaked in cold water for about 3 days, then removed and the juice allowed to ferment for 2 days. It is then filtered to give a wine which, with slight improvements to the technologies could result in a lovely sparkling wine with a rich flavour (Madovi, 1981). Alternatively, the ripe fruits are left to soak in cold water or pounded to remove the skins and stones. The pulp is mixed with an equal volume of water in a pot, left to ferment overnight and is ready for drinking the next day (Tredgold, 1986). The pot may be sealed and left to ferment for 3 days, after which a gummy scum is skimmed off and the highly intoxicating beer drunk on the fourth day (Tredgold, 1986). Both fermentative and non-fermentative yeasts have been isolated from the marula fruits (Okagbue, 1995). The role of these yeasts in the production of marula wine / beer has however not been elucidated. It is possible that an acceptable product based on this natural flora could be developed. Production of a marula alcoholic beverage on an industrial commercial scale has been achieved in South Africa and a commercial wild fruit cocktail, amarula, is now available on the market in Zimbabwe. It is not clear whether this is the same product as marula liqueur described by Tredgold (1986), which is produced by steeping slashed ripe marula fruits in brandy and adding syrup to taste. Mudetemwa is an alcoholic beverage prepared from the fruits of the sand apple (Parinari curatellifolia; muhacha). The fruits are pounded and the juice extracted by squeezing by hand. The juice is boiled, allowed to stand overnight, boiled again, allowed to cool and then drunk as beer (Tredgold, 1986). Alternatively, the boiled juice is left to ferment further and the container is covered and sealed with damp clay on the fourth day. An opening is then made on the side of the container so that when it is boiled, vapour ensuing from the opening


becomes condensed into a potent spirit, mudetemwa (Tredgold, 1986). Muhacha fruits are abundant in summer and their fermentation has potential for development of different makumbi products on an industrial scale. Murara /ilala wine is another alcoholic beverage prepared from the sap of the murara /ilala tree (Hvphaene benguellensis). The sap is collected by tapping the tree trunk and is fermented into a wine, which tastes like flat ginger beer and is hardly intoxicating (Tredgold, 1986; Okagbue, 1995). The wine can be distilled (10 l being distilled to 1 l) to a very potent spirit, uchema (Tredgold, 1986). The steps in the preparation of these beverages and the microbiology and biochemistry of the processes have not been fully studied and documented. The ripe fruits of the buffalo thorn (Ziziphus mauritiana, musau) are crushed, soaked for some hours in water and allowed to ferment. The fermented liquid called masau may be distilled to make a potent spirit (Tredgold, 1986). Studies are underway at the Zimbabwe Scientific and Industrial Research and Development Centre, to improve the technologies for producing masau at industrial scale. Fruits of Uapaca kirkiana (mushuku /muzhanje) are pounded to break the skins and extract the seeds. The pulp is mixed with cold water and left to ferment until the water turns opaque or grey. The liquid is then used to make a thin porridge by mixing with ground maize (Tredgold, 1986). Alternatively the pulp can be left to ferment into a sweet wine called mutandavira (Tredgold, 1986). This again is a wild fruit, which is seasonally abundant with a potential for producing marketable fermented makumbi products at industrial scale. Mutandabota is a thin fermented slurry made from the juice of monkey orange (Strychnos spinosa, mutamba). The fermentation of this juice has not yet been described. Other wild fruits whose juices are known to be fermented into some alcoholic beverages include donkey berries (Grewia monticola; mutongoro /umtewa; G. flavescens, mubhubhunu /ubhuzu), monkey fingers (Popowia obovata, munyani /umkozombo), torchwood (Balanites aegyptiaca, nyahoko), bird plum (Berchemia discolor, munyii /umcaga), milk plum (Bequaertiodendron magalismontanum, muhorongwa / umhlautshwa), monkey pod (Cassia petersiana, muremberembe), jackal berry (Diospyros

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mespiliformis, mushenje / umdlawuzo), granite garcinia (Garcinia huillensis, mutunduru), wild plum (Harpephyllum caffrum), indaba tree (Pappea capensis, chitununu /uzagogwane), nana berry (Rhus tenuinervis, mudzambuya /umkungu) and water berry (Syzygium cordatum, mukute /umdoni) (Tredgold, 1986). These makumbi products can also be improved through microbiological and other associated research (Okagbue, 1995).

4. Fermented milk Traditional naturally fermented milk is known as mukaka wakakora or zifa (Shona) or amasi (Ndebele) (Mutukumira, 1995; Mutukumira et al., 1995). A soured cottage cheese-like product is called hodzeko, mukaka wakakodzekwa or mukaka wemahwe (Madovi, 1981). The names sometimes vary with region or may be used as generic names, for example, Mutukumira (1995) used mukaka wakakodzekwa for mukaka wakakora. For brevity, the term amasi will be used throughout this review to refer to the traditional plain naturally fermented milk. Most of the rural population in Zimbabwe still spontaneously ferment raw milk to amasi (Feresu and Muzondo, 1989; Mutukumira, 1995). Amasi has however been substituted in urban areas by lacto, an industrially fermented milk. During lacto production, milk is standardised, pasteurised at 928C for 20 min, cooled to 228C and inoculated with 1.2% mesophilic starter cultures similar to those employed to produce filmjolk, a Scandinavian fermented milk. The milk is immediately packaged into sachets, and left to ferment at ambient temperature for 18 h, and then the lacto is stored at refrigeration temperatures until sold (Feresu and Muzondo, 1989). Until recently, lacto had enjoyed a monopoly, but in the past decade other fermented milks such as kumusha and mayo have been introduced onto the market.

4.1. Amasi More work has been carried out on amasi than any other fermented product in Zimbabwe and as a result, there have been several reviews on these studies (Feresu, 1992; Mutukumira et al., 1995; Okagbue, 1995; Tamime and Marshall, 1997). Amasi

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is produced by leaving fresh raw bovine milk to ferment naturally at ambient temperature in earthenware pots or any other suitable containers (Feresu and Muzondo, 1989; Feresu, 1992; Mutukumira et al., 1995). The microorganisms inherent in the milk, the container and the surrounding air are assumed to ferment the milk within 1–3 days depending on the ambient temperature (Feresu and Muzondo, 1989,1990; Mutukumira et al., 1995; Simango, 1997). It is common practice to sometimes speed up the fermentation by back-slopping fresh milk with remains of a previous batch of sour milk (Oliver, 1971). Feresu and Muzondo (1989) investigated the effects of pasteurisation and the container used during natural fermentation of milk at ambient temperature on the total microbial counts, the counts of lactic acid bacteria (LAB), the amount of lactic acid produced and the acceptability of the fermented milk by a sensory panel. They further compared these parameters with those observed for lacto. The total counts, numbers of LAB and lactic acid produced were similar for amasi and lacto but the amasi was significantly more acceptable to a panel than lacto. Earthenware pots were better containers for fermenting amasi than glass containers and may still have a place in milk fermentation in the home (Feresu and Muzondo, 1989). A further study by Feresu and Muzondo (1990) identified the predominant LAB isolated from amasi as belonging to Lactobacillus helveticus, Lb. plantarum, Lb. delbrueckii subsp. lactis, Lb. paracasei subsp. paracasei and Lb. paracasei subsp. pseudoplantarum. The types of LAB isolated in this study were limited as MRS agar, which is known to be selective for the genus Lactobacillus, was the only isolation medium used (Mutukumira, 1996). Despite this observation, however, four different strains of Lactococcus lactis, were isolated from lacto using the same medium. This might indicate that MRS medium may allow growth of strains of Lactococcus when high numbers are present in the inoculum. To assess the risk of the occurrence of pathogens in amasi, and to establish measures for safe manufacture and storage of the product, Feresu and Nyati (1990) and Dalu and Feresu (1996), respectively, determined the behaviour of pathogenic E. coli and Listeria monocytogenes during fermentation of amasi at ambient temperature for 24 h and its storage

at ambient and refrigeration temperatures for 4 days. Lacto was treated similarly for comparison. Lacto was more inhibitory to E. coli than amasi at both ambient and refrigeration temperatures and higher numbers of E. coli survived when the amasi was stored at ambient temperature than refrigeration temperature (Feresu and Nyati, 1990). Although L. monocytogenes grew only marginally during fermentation of both products, . 10 2 cells / ml could still be detected in both products at the end of the fermentation period. In contrast to E. coli however, larger numbers of L. monocytogenes survived at refrigeration temperature than at ambient temperature (Dalu and Feresu, 1996). The challenge studies showed that current practices of naturally fermenting milk are of public health concern because if milk is contaminated with E. coli, and possibly other enteric pathogens during milking or fermentation; or milk from a mastitic cow infected with L. monocytogenes is used for production of amasi the pathogens could multiply to infective doses and / or retain relatively high numbers during storage of the fermented product at both ambient and refrigeration temperatures (Feresu and Nyati, 1990; Dalu and Feresu, 1996). The studies also suggested that the differences in acceptability of amasi and lacto and in their inhibition of pathogens might be due to the types of microorganisms involved in the two fermentations (Feresu and Muzondo, 1989, 1990; Feresu and Nyati, 1990; Feresu, 1992; Dalu and Feresu, 1996). Mutukumira (1996) focused his studies on amasi produced on a larger scale by a small-holder farmer milk collection centre, the Nharira / Lancashire Milk Centre. The centre is part of the Dairy Development Programme initiated in Zimbabwe in 1983, where a group of small-holder dairy farmers are set up in a dairy scheme and sell milk to a central collection centre with refrigeration facilities. At the centre most milk is sold raw, and excess milk is naturally soured at ambient temperatures in churns for periods up to 48 h. The curd is then carefully scooped out using a perforated metallic plate, leaving the whey in the churn. It is then mixed and is ready for sale, or if the firmness is unacceptable or the viscosity too low, cream from another vessel is added, the product mixed and then sold (Mutukumira, 1995; Mutukumira et al., 1996a). Mutukumira (1996) carried out systematic studies in which he looked at how the milk is produced and


handled by the farmers in the Nharira / Lancashire Scheme (Mutukumira et al., 1996a); the chemical and microbiological quality of the raw milk (Mutukumira et al., 1996b); the isolation and characterisation of the LAB predominant in the amasi (Mutukumira, 1996), the potential of some of these isolates for development as starter cultures (Mutukumira, 1996; Narvhus et al. 1998) and the chemical and microbial quality of the amasi (Mutukumira, 1995). The chemical quality of the raw milk was poor and variable with more than half of the ten batches of milk studied not meeting the Zimbabwean regulated minimum standards. The pH values and concentration of titratable acid suggested that some of the raw milk had started to sour before delivery to the milk centre as inefficient transport systems were used to deliver the milk. The total counts in the majority of the raw milk were low but the coliform counts were very high suggesting poor hygiene standards during milk handling (Mutukumira et al., 1996b). The chemical composition of the amasi produced from this poor quality raw milk was inconsistent as the milk was not standardised and since its processing involved whey removal and in some cases addition of cream (Mutukumira, 1995). The microbiological quality of the amasi was also poor and variable as the milk was not pasteurised (Mutukumira, 1995). The product variation, presence of high numbers of coliforms and the observed low product yield due to whey drainage clearly indicated the need for improvement to the current process. One way doing this would be through the development of starter cultures. To this end, Mutukumira (1996) isolated 200 lactic acid bacterial strains from the amasi and identified them as belonging to the genera Lactococcus, Lactobacillus, Leuconostoc and Enterococcus. Twenty one representative strains were further identified to species level as Lact. lactis subsp. lactis, Lact. lactis subsp. lactis biovar diacetylactis, Lb. paracasei subsp. paracasei, Lb. plantarum, Lb. acidophilus, Leuc. mesenteroides subsp. mesenteroides, Ent. faecum and Ent. faecalis. Representative strains for the genera Lactococcus, Lactobacillus and Leuconostoc were further studied for their coagulation of milk, lowering of pH and production of volatile organic compounds. A strain belonging to Lact. lactis subsp. lactis biovar diacetylactis termed

9

strain Cl was found to coagulate milk within 18 h at 258C and to produce adequate levels of diacetyl, acetaldehyde and lactic acid. The amasi produced by this strain was judged the most highly acceptable product although it had a slight malty flavour (Mutukumira, 1996). Further tests were carried out in which the strain Cl was combined with a strain of Lb. plantarum, and with a strain of Leuc. mesenteroides subsp. mesenteroides. The amasi produced by single strain Cl was still the most preferred by a sensory panel. More work has been done to improve the amasi from strain Cl using either enrichment of the milk with 2.5% (w / v) skim milk or by dry matter ultrafiltration then incubating the milk at 22, 30 or 378C (Narvhus et al., 1998). In this study however, a different code name INF-DM1 was used for strain Cl. Both methods markedly improved the viscosity of the amasi, which was judged to be good and acceptable irrespective of incubation temperature. The study however recommended use of skimmed milk powder since milk powder is cheap, readily available, stable and can easily be used in a rural setting (Narvhus et al., 1998). Strain Cl had further desirable characteristics as it was inhibitory to E. coli a common contaminant of dairy products (Mutukumira, 1996). From this review, it appears as though there will be a single starter culture for the production of a fermented milk whose characteristics are nearer to the traditional amasi in the near future. There is however, need for more work. A Norwegian panel tested the acceptability of the amasi and more extensive acceptability tests need to be carried out in Zimbabwe. There is also need to test how scaling-up will affect the properties of the fermented product. The range of microorganisms studied in amasi so far may not be exhaustive as yeasts and coliforms may have a role in the fermentation process. The role of yeasts in traditional fermented milk is therefore currently being studied. In addition, it might be necessary to determine the contribution of coliforms to the sensory properties of amasi, although they are usually undesirable in foods as some of them have been associated with food-borne illnesses. When these studies are completed, there might be need to further test combinations of strain Cl with the other promising microorganisms as single starter cultures are known to be prone to failure due to bac-

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teriophage infection. This might also lead to the production of different varieties of amasi. Little work has been done on the production of hodzeko, a cottage cheese-like product whose fermentation involves pooling fermented milk from three successive days from which whey and cream has been removed (Madovi, 1981). Further development of this product would also increase the types of traditional dairy products on the market.

5. Conclusion Several fermented foods are produced at the household level in Zimbabwe. However, with the exception of amasi, limited work has been done to study these foods with a view to upgrading their traditional technologies. The current industrially produced foods and beverages are only simulations of the traditional products as either different substrates such as sorghum malt are used instead of millet malt (chibuku) or starter cultures developed elsewhere are used for the fermentation (lacto) or unfamiliar ingredients such soy protein (mahewu) are added. These factors may influence the sensory properties of the product. The microbiology and biochemistry of the traditionally fermented products need to be fully understood before the predominant microorganisms in these fermentations are isolated and used in the development of starter cultures. These, together with improved processing and use of good quality raw materials for fermentation could be used in upgrading the traditional technologies. Fermented products prepared in laboratory simulations then need to be compared with those produced in the homes. This way the variations in the traditional fermentation processes could be incorporated in the development of a large variety of novel industrially fermented products.

Acknowledgements The authors would like to thank the Norwegian Universities Committee for Development Research and Education (NUFU), Project 26 / 96, through the Department of Food Science, Agricultural University of Norway and the University of Zimbabwe for financial assistance.

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