AJFAND

Fermented foods have many advantageous attributes such as improved nutritional value and safety against bacterial pathogens. These foods are also important for weaning purposes and hence play a role in protecting infants against foodborne diseases. However, pathogens have been isolated from some fermented foods and challenge tests have shown the possibility of pathogens to survive and grow in some fermented foods. Post processing contamination is often cited as the major cause of food poisoning. Fermented foods with a pH value below 4 are usually safe as most pathogens are unable to survive under these conditions. However, some pathogens such as Escherichia coli O157:H7 are reported to develop acid tolerance. This is a particular problem for fermented sausages. However, there is very little information on the occurrence and growth of pathogens in African fermented foods. Most work on African fermented foods has focused on the isolation and identification of the desirable microorganisms involved in the fermentation process. Some authors have now started focusing on the possibility of some pathogens to survive and grow in some of the fermented foods. This review highlights some of the cases where pathogens have been detected in fermented foods, or have been shown to survive and grow in such foods. The most commonly encountered pathogens in African fermented foods include Bacillus cereus, E. coli, Salmonella sp., Staphylococcus aureus, Vibrio cholerae, Aeromonas, Klebsiella, Campylobacter and Shigella sp. The approaches that can be used to minimise the risk of foodborne diseases through consumption of fermented foods include improved hygiene, use of starter cultures, use of protective cultures and the use of a combination of factors that inhibit the growth of microorganisms (multiple hurdles). The use of concepts such as Hazard Analysis Critical Control (HACCP) system is still problematic at household level. However, this approach has been suggested for some African fermented foods such as kenkey (Ghana). This is thought to help in channelling resources to steps that provide effective protection.

Foodborne illnesses are a major health problem in many countries, especially developing countries. Many foodborne diseases are caused by the ingestion of food contaminated with pathogenic bacteria. A cardinal symptom of foodborne diseases is diarrhoea. It is estimated that 1400 million episodes of diarrhoea occur annually in children under five years, and up to 70% of these episodes are due to pathogens transmitted through food [1,2].

In many communities in Africa, the beginning of weaning young children off breastmilk is associated with an increase in diarrhoeal episodes [3]. Standards of personal hygiene and public sanitation are low in these communities and it has been shown that indigenous weaning foods are a significant cause of childhood diarrhoea infection[1,4]. Post-cooking contamination and the use of contaminated water in the preparation of foods are often cited as causes of diarrhoea during weaning. Several authors have highlighted the importance of adequate nutritional quality and hygiene during the preparation of weaning foods, and also the link between infection and nutrition [5]. The same authors also note that if it was possible to have weaning foods with reduced contamination, the optimal timing for the introduction of solid food could become less critical. Fermentation is widely believed to be protective against foodborne diseases and is usually recommended as a cheaper way of preparing weaning food. However, the occurrence of bacterial pathogens in fermented foods suggests a need for caution in the use of these foods for infant feeding. This review article aims to highlight some foodborne illness cases associated with fermented foods and to emphasise what can be done to control them.

Fermented foods are estimated to constitute about a quarter of the foods consumed worldwide. In Africa, a majority of the fermented foods are produced at household level and hygiene is a major concern. A wide variety of foods are fermented, including milk, root crops, meat and fish, but the foods of greatest relevance fed to young children are produced by fermentation of cereals, milk and pulses. African fermented foods such as mahewu (Zambia, Zimbabwe, South Africa), togwa (Tanzania), naturally soured milk (for example, amasi - Zimbabwe, ergo - Ethiopia), porridges and beverages (uji - Kenya, ogi - Nigeria, kenkey - Ghana, mawe, koko – Ghana) are used as weaning foods [5,6]. Mahewu, togwa and naturally soured milk are commonly used for these purposes in the countries in which they are produced.

Fermented foods are normally considered to be safe against foodborne diseases because of their low pH. Some of the lactic acid bacteria starter cultures used in fermentation produce antimicrobial compounds such as bacteriocins, hydrogen peroxide, formic acid, acetate and diacetyl [7,8].

For pathogens to grow in fermented foods, which may result in foodborne diseases, the microorganisms must overcome such hurdles as low pH, low water activity (aw, in solid-state fermentation), low redox potential and in some cases, heat treatments and natural antimicrobial compounds. In general, it has been observed that:

Gram-negative bacteria are more susceptible to the low pH in fermented food while the Gram-positive bacteria such as Bacillus cereus may be more resistant because the low pH is favourable to spore formation [9]. The fate of the pathogens during fermentation or storage of the fermented product (such as whether or not the pathogens grow to infectious levels or produce toxins) also depends on the initial levels of contamination of either the raw materials or the final product.
In challenge tests, pathogen survival depends on the species and strain, initial inoculum used, incubation temperature, amount and speed of acid production, resulting pH, and storage temperature and time [10-14].
Some foodborne pathogens grow to high levels during the early stages of fermentation due to the low acid levels. They may develop resistance to acid through a mechanism referred as acid tolerance response (ATR). For example, Escherichia coli O157:H7 and Bacillus cereus are acid resistant and can survive below pH 4. However, less than 10 E. coli cells may be enough to cause illness in humans [15].

Pathogens have been isolated from some fermented foods indicating that they are capable of growing in the food or surviving the fermentation process. Pathogens that are found in fermented foods come from the respective raw materials or from the handlers. Some fermented foods may be contaminated by moulds, which produce mycotoxins. Cereal- and milk-based products used as weaning foods are a major source of diarrhoeal causing microorganisms. For example, Nyatoti et al., [16] reported that out of 12 samples of naturally soured milk used as weaning foods, 2 contained enteropathogenic E. coli. In South Africa, Kunene et al., [3] reported that 40% of the fermented sorghum meal samples contained B. cereus while 8% contained E. coli. On the other hand, Motarjemi et al., [1] reported that unfermented maize porridge samples prepared for infants and children in a Ghanaian village were contaminated with pathogenic bacteria including Aeromonas spp., B. cereus, Salmonella spp., Staphylococcus aureus and Vibiro cholerae. Olasupo et al., [17] reported that among microorganisms of public health concern, S. aureus and Klebsiella spp. were isolated from wara while E. coli, Salmonella spp. and Klebsiella spp. were isolated from nono. Ogi and kunu-zaki contained B. subtilis, E. coli, S. aureus, Klebsiella spp. and Enterococcus faecalis. Table 1 lists some recorded cases where pathogens have been detected in fermented foods.

The inhibitory properties of fermented foods are usually assessed based on their ability to reduce diarrhoea and/or improve microbial quality and antimicrobial activity in vitro. For example, Mbugua and Njenga [18] reported that levels of S. aureus, Salmonella typhimurium, enteropathogenic E. coli and Shigella dysentriae declined during uji fermentation and storage. Simango and Rukure [19] reported that Campylobacter and Aeromonas could not be detected in mahewu and in sour porridge 20 minutes after inoculation (level) Salmonella could not be detected 4 hours later. The same authors, however, reported that pathogenic E. coli and Shigella ssp. strains used were more tolerant to the low pH in mahewu and sour porridge and survived longer under those conditions.

Indirectly, the inhibitory effect of fermented foods towards pathogens can be demonstrated through feeding trials and in monitoring diarrhoeal episodes in children. A group of Tanzanian children fed on fermented cereal gruels had a considerably lower prevalence of diarrhoea [20]. In another study, Kingamkono et al., [21] demonstrated that there was a significant decrease in the proportion of children with faecal pathogens after consumption of togwa.

Control of pathogens in fermented foods, therefore, will rely on understanding the growth properties of the pathogens and methods of transmission. Some strategies to control pathogens in fermented foods are discussed below.

A number of studies have shown that using starter cultures increases the safety of many fermented foods. The major technological importance of role of starter cultures is to produce large amounts of lactic acid from lactose. A biotechnologically essential starter strain should produce a sufficient intensity of acid during initial stages of the industrial fermentation process and favourable low after-acidification conditions during storage. However, the maximum benefit of using starter cultures depends on such factors as the initial level of contamination of the raw materials, levels of hygiene and sanitation and starter culture activity. Hazard Analysis Critical Control Point (HACCP) studies of some fermented products have demonstrated that depending on the process and the hygienic conditions observed during preparation, some fermented foods might still pose a safety risk mainly due to post fermentation contamination [22].

The pH in lactic acid fermented foods is usually reduced to less than 4 and this is usually sufficient to suppress the growth of most foodborne pathogens [23]. Table 2 lists the critical pH limits for growth of some pathogens. However, the extent to which pathogens are inhibited by low pH will depend on the organism concerned, temperature, amount of undissociated acid produced, buffering capacity of the food and presence/absence of any additional hurdles. The undissociated acid will diffuse into the bacterial cell thereby reducing the intracellular pH and slowing down metabolic activities. In order to produce sufficient acid to inhibit bacterial pathogens, relatively large numbers (up to 10⁹cfu/ml) of lactic acid bacteria must be present. Contamination of fermented foods with pathogens, where large numbers of lactic acid bacteria are present and the pH is acidified, poses a lesser health risk compared to pre-fermentation contamination. If a natural fermentation process is involved, the pathogens will probably multiply and cause problems in the final product because lactic acid bacteria will initially be very low. However, if the rate of acid production is fast, pathogen growth may be checked before counts reach infectious levels. Use of starter cultures in the production of fermented foods ensures rapid growth of the lactic acid bacteria with the resultant reduction in pH to below 4, which is critical for controlling pathogens. For example, use of starter cultures was found to render togwa free of B. cereus, Campylobacter jejuni, enterotoxigenic E. coli (ETEC) and Shigella flexineri, after 48hours [24]. In the traditional process, different fermentation procedures are used to make togwa, which leads to variation in the pH of the fermented gruel and therefore the fate of pathogens may be different. Kimaryo et al. [25] reported that no Enterobacteriaceae, yeasts and moulds could be isolated from kivunde (a cassava based fermented product) prepared using Lactobacillus plantarum starter cultures. In challenge tests, E. coli, Listeria monocytogenes and Campylobacter jejuni were also found to grow and survive in Zimbabwean naturally soured milk. However, these same bacterial strains were inhibited in milk fermented with a starter culture [26,27] due to a rapid reduction in pH. These examples highlight the potential for improvement of traditional African fermented foods through the use of starter cultures.

In addition to starter cultures, protective cultures can be used as an additional safety measure of fermented food. Protective cultures are lactic acid bacteria selected for their ability to produce certain antimicrobial compounds. They may not necessarily be very efficient in acid production. These cultures reduce the risks of growth and survival of pathogenic and spoilage organisms [28]. Odunfa et al. [29] described the use of bacteriocin producing organisms as protective cultures that may arrest the occurrence of food borne pathogens in such fermented food products.

Some starter cultures produce antimicrobial substances such as organic acids, bacteriocins and hydrogen peroxide. The organic acids include lactic, acetic and propionic acids. Bacteriocins are peptides or proteins, which are bactericidal in nature produced by members of the lactic acid bacterial family. For example, nisin is bactericidal to many gram-positive bacteria and prevents outgrowth of Bacillus and Clostridium spores [28]. Hydrogen peroxide is oxidative and is an active bactericide.

Lactobacillus starter cultures with antimicrobial activity against diarrhoegenic bacteria were used in Dogik, the commercial version of Nigerian ogi. Salmonella, Shigella, Campylobacter, Aeromonas, Pleisiomonas, enteropathogenic and enterotoxigenic E. coli, Yersinia enterocolitica and Vibrio cholerae could not be detected in Dogik after 6hours, but Salmonella, E. coli and Shigella survived 24hours or more in the traditional ogi during trial studies [30]. Table 3 lists some fermented foods that have been shown to have antimicrobial activity.

Inhibitory activity of Lactobacillus acidophilus against pathogenic species is an important criterion for its use as a dietary adjunct. This microorganism has been reported to produce antibiotic-like compounds such as acidolin, acidophilin and lactocidin [31].

Initial high levels of contamination with pathogens in raw materials such as raw meat for fermented sausages and raw milk for fermented milk are critical for the safety of fermented foods. Minimising contamination of the raw materials is therefore another way of controlling pathogen levels in the final product. Measures should be taken to interrupt the transmission of pathogens to fermented foods at both the household and commercial levels. At the commercial level, improvement of product quality and safety could be achieved by applying Good Manufacturing Practices (GMP), Good Hygienic Practices (GHP) and the Hazard Analysis and Critical Control Point (HACCP) system. Attempts to provide HACCP guidelines for some traditional fermented foods such as Kenkey have been made [32]. However, educating food handlers, particularly mothers and food vendors, on food hygiene is one strategy that can be used in efforts aimed at to preventing foodborne diseases [1].

The hurdle concept is more or less unconsciously used in many traditional fermented foods. The hurdles such as aw, temperature, pH, Eh and preservatives, concertedly control microbial spoilage and food poisoning, leaving desired fermentation processes unaffected [33]. The objective is to inhibit the growth and proliferation of undesired organisms rather than to actually kill them. Adaptation to multiple stresses poses questions in relation to the potential dangers in the inadequate application of multiple hurdle technology. Application of a number of sub-lethal stresses may synergistically combine to stress-harden pathogens in treated foods leading to the emergence of organisms with greater capacity to resist one or more of the stresses applied within current food preservation schemes.

Inspection and monitoring of food processing activities and the enforcement of regulations are necessary actions for food control. This involves taking samples, analysing them and making decisions on proper food handling procedures. A number of developing countries may not have adequate food control structures in place to monitor the quality of food produced at all levels. Of great concern is the food produced at home and sold at local markets. In most African countries there is insufficient legislation that can be used to regulate these activities. In many instances selling of such products is illegal and vendors are regularly raided and moved from the streets. There is also need to continuously update food inspectors on issues of food microbiology and food hygiene so that they can make appropriate decisions on the storage and distribution of food.

Sodium nitrate and/or nitrite can be used in fermented meat products to inhibit some spoilage and food poisoning organisms and to contribute to flavour development. Sodium and potassium lactate can also act as antimicrobial agents in fermented meat products [34]. Sulphur dioxide is used as a meat preservative, where its principle purpose is to delay spoilage.

Consumers need to be educated on how to handle foods including proper storage. Other techniques of control would include use of irradiation which is, however, limited in Africa. Ionising radiation destroys microorganisms without appreciably raising the temperature of food and can be applied for both raw materials and the finished product. Thermal processing and storage of fermented foods under controlled atmosphere can also be used as control measures for pathogens and some companies can consider their use to achieve extended shelf life thereby increasing exports or extending distribution network. However, success of any control method for pathogens, traditional or modern, depends on minimising contamination, which in turn is achieved through application of appropriate cleaning, sanitation, decontamination and hygienic practices.

Fermented foods generally have a very good safety record, even in developing countries where the foods may be prepared under unhygienic environments. However, the incidence of pathogens in fermented foods, as highlighted in this review, suggests that measures that minimise the risk of foodborne illnesses should be taken. The high prevalence of diarrhoeal diseases, particularly among infants is an indication of an underlying safety problem. This is critical to the survival and growth of children that may be HIV positive or malnourished, whose immune system has been compromised. The application of HACCP as advocated by the WHO should be applied to a wider range of fermented foods in addition to other measures such as use of starter cultures, legislation and educating those who prepare such foods. More information on the occurrence of pathogens in African fermented foods and their fate in these foods is still needed in order to carry out a proper risk assessment.

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