Inactivation of bacterial pathogens in yoba mutandabota, a dairy product fermented with the probiotic Lactobacillus rhamnosus yoba

doi:10.1016/j.ijfoodmicro.2015.09.016

International Journal of Food Microbiology

Volume 217, 18 January 2016, Pages 42-48

https://doi.org/10.1016/j.ijfoodmicro.2015.09.016 Get rights and content

Highlights

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L. monocytogenes, E. coli and Salmonella spp. survived in traditional mutandabota.
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None of the tested bacterial pathogens survived in probiotic mutandabota.
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Fermenting mutandabota with L. rhamnosus yoba enhances its microbiological safety.
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L. rhamnosus yoba grew to 9 log cfu/mL in mutandabota inoculated with pathogens.
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Probiotic mutandabota is safer stored than traditional mutandabota.

Abstract

Mutandabota is a dairy product consumed as a major source of proteins and micronutrients in Southern Africa. In this study the microbial safety of traditional and a variant of mutandabota fermented with the probiotic Lactobacillus rhamnosus yoba (yoba mutandabota) was investigated by challenging the products with five important food pathogens: Listeria monocytogenes, Salmonella spp., Campylobacter jejuni, Escherichia coli O157:H7 and Bacillus cereus. Pasteurized full-fat cow's milk was used for producing traditional and yoba mutandabota, and was inoculated with a cocktail of strains of the pathogens at an inoculum level of 5.5 log cfu/mL. Survival of the pathogens was monitored over a potential consumption time of 24 h for traditional mutandabota, and over 24 h of fermentation followed by 24 h of potential consumption time for yoba mutandabota. In traditional mutandabota (pH 3.4 ± 0.1) no viable cells of B. cereus and C. jejuni were detected 3 h after inoculation, while L. monocytogenes, E. coli O157:H7 and Salmonella spp. significantly declined (P < 0.05), but could still be detected (< 3.5 log inactivation) at the end of the potential consumption time. This indicated that consumption of traditional mutandabota exposes consumers to the risk of food-borne microbial infections. In yoba mutandabota, L. rhamnosus yoba grew from 5.5 ± 0.1 log cfu/mL to 9.1 ± 0.4 log cfu/mL in the presence of pathogens. The pH of yoba mutandabota dropped from 4.2 ± 0.1 to 3.3 ± 0.1 after 24 h of fermentation, mainly due to organic acids produced during fermentation. Only Salmonella spp. was able to grow in yoba mutandabota during the first 9 h of fermentation, but then decreased in viable plate count. None of the tested pathogens were detected (> 3.5 log inactivation) after 3 h into potential consumption time of yoba mutandabota. Inactivation of pathogens in mutandabota is of public health significance because food-borne pathogens endanger public health upon consumption of contaminated food, especially in Southern Africa where there are many vulnerable consumers of mutandabota such as children, elderly and immuno-compromised people with HIV/AIDS. The findings of this study demonstrate that mutandabota fermented with L. rhamnosus yoba has antimicrobial properties against the tested pathogens and it is safer compared to the traditional mutandabota.

Introduction

Probiotic bacteria and their health effects are a focus of international food research. Incorporation of selected strains of the genera Bifidobacterium and Lactobacillus in milk products and lately in non-dairy products has been studied in detail (McMaster et al., 2005, Østlie et al., 2003, van Tienen et al., 2011). The beneficial effects of probiotic strains on the host and their mechanism of action have also been demonstrated quite well (Guandalini et al., 2000, Kankainen et al., 2009, von Ossowski et al., 2010). However, little information is available on the survival and growth of pathogens in dairy foods containing probiotic bacteria. Not only good survival of the probiotic bacteria in food products during their specified shelf life is essential, but also the potential antimicrobial action of the probiotic bacteria against contaminating pathogens during the production process and shelf life is relevant.

Mutandabota is a non-fermented, milk-based food consumed daily as a major source of proteins and micronutrients, and it is also sometimes used as a weaning food for infants in Southern Africa (Zimbabwe Ministry of Agriculture, 2001). The product is made by mixing raw cow's or goat's milk 79% (wt/wt), dry baobab (Adansonia digitata L.) fruit pulp 14% (wt/wt) and sugar 7% (wt/wt) (Mpofu et al., 2014a). Mutandabota has a thick, yoghurt-like consistency, a sour taste and a pH of 3.4 ± 0.1. Generally, low pH products are regarded as microbiologically stable and safe to eat (ICMSF, 2002). However, observations on preparation of traditional mutandabota evoked questions about its potential role as a vehicle for food-borne microbial infections. The traditional method utilizes raw milk, which raises a food safety concern since the milk may contain pathogenic bacteria like Salmonella spp., Listeria monocytogenes and Campylobacter jejuni, which can cause illness in humans (Kumbhar et al., 2009, Nanu et al., 2007). Coliforms and enterotoxigenic Escherichia coli have been isolated in raw milk in Zimbabwe and South Africa (Gran et al., 2002, Ibtisam et al., 2008, Mhone et al., 2011). Preparation of mutandabota is carried out at household level in a shaded open space and does not use aseptic techniques. When mutandabota is contaminated by pathogens and then consumed, it might cause microbial infection amongst its consumers.

On the basis of mutandabota, a variant of mutandabota fermented with the probiotic Lactobacillus rhamnosus yoba (referred to as yoba mutandabota) was developed to enable resource-poor populations in Southern Africa to benefit from a functional food (Mpofu et al., 2014b). L. rhamnosus yoba was isolated from a commercially available product, containing L. rhamnosus GG. The identity of the isolate was confirmed by 16S rRNA sequencing and the isolate was deposited at the Belgian Co-ordinated Collections of microorganisms/Laboratorium voor Microbiologie Gent (BCCM/LMG) culture collection under the name of L. rhamnosus yoba (Kort and Sybesma, 2012). There is evidence of beneficial effects of L. rhamnosus GG based on clinical trials with double-blind and placebo-controlled cross-over designs for prevention and treatment of diarrhea and gastrointestinal and upper respiratory tract infections in children (Grandy et al., 2010, Hojsak et al., 2010, Guandalini et al., 2000). For the production of yoba mutandabota, a new process was designed based on traditional mutandabota preparation procedures. Two major steps were incorporated into the traditional procedure, namely the boiling of raw milk and fermentation with L. rhamnosus yoba. Contamination of the product with pathogenic bacteria may occur after the heat treatment; bacterial pathogens have been isolated from pasteurized milk and products from pasteurized milk (Beukes et al., 2001, Gran et al., 2002, Nyatoti et al., 1997). Producing yoba mutandabota through fermentation might enhance its microbiological safety. This study was performed to investigate the survival of bacterial pathogens in traditional and yoba mutandabota.

Section snippets

Preparation of L. rhamnosus yoba inoculum

An isolate of the probiotic bacterium L. rhamnosus GG, under the name L. rhamnosus yoba (Kort and Sybesma, 2012), was used in this study. The bacterium was obtained from Yoba for Life Foundation (http://www.yoba4life.com), Amsterdam, The Netherlands. It was stored at − 80 °C, before being freeze-dried for long-term storage at 4 °C in 50 mL tubes (Greiner Bio-One, BV, Alphen a/d Rijn, The Netherlands). To prepare the inoculum, baobab fruit pulp was added to UHT full-fat cow's milk to a concentration

pH changes in traditional and yoba mutandabota

The time course of acidification in traditional mutandabota was similar in all experiments with the 5 cocktails of bacterial pathogens (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6). The pH was 3.5 (n = 15) immediately after preparation when mutandabota was ready for consumption at time 0 h (t = 0). This pH-value remained rather constant throughout the 24 h potential consumption time, also regarded as the storage period, and at t = 24 the pH was 3.4 ± 0.1. The low pH could be attributed to the acidic nature of

Conclusion

This study focused on the question whether five important pathogens, namely, L. monocytogenes, Salmonella spp., C. jejuni, E. coli O157:H7 and B. cereus, could pose a foodborne risk to consumers of both traditional and yoba mutandabota. In traditional mutandabota (pH 3.5) no viable B. cereus and C. jejuni were detected 3 h after inoculation. However, L. monocytogenes, Salmonella spp., and E. coli O157:H7 significantly declined (P < 0.05), but could still be detected at the end of the storage

Acknowledgments

The authors thank NUFFIC (Grant award number CF6631/2010) for financial assistance, Yoba for Life Foundation for supplying the Lactobacillus rhamnosus yoba strain, and Dr. Eelco Franz for the supply of Escherichia coli O157:H7 strains from Netherlands National Institute for Public Health and the Environment.

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Inactivation of bacterial pathogens in yoba mutandabota, a dairy product fermented with the probiotic Lactobacillus rhamnosus yoba

Highlights

Abstract

Introduction

Section snippets

Preparation of L. rhamnosus yoba inoculum

pH changes in traditional and yoba mutandabota

Conclusion

Acknowledgments

Int. J. Food Microbiol.

Int. Dairy J.

Int. J. Food Microbiol.

Int. J. Food Microbiol.

J. Dairy Sci.

J. Dairy Sci.

Food Control

Clin. Nutr.

Food Microbiol.

Lancet

Trends Biotechnol.

Lancet

Res. Microbiol.

Res. Microbiol.

Int. J. Food Microbiol.

Int. J. Food Microbiol.

J. Dairy Sci.

Int. J. Food Microbiol.

Food Control

Int. J. Food Microbiol.

J. Food Prot.

Trans. R. Soc. Trop. Med. Hyg.

Incidence and characterization of enterotoxigenic Bacillus cereus in some dairy products

Suez Canal Vet. Med. J.

Sugars and organic acids in Adansonia digitata L.

J. Univ. Bombay

Lactic acid permeabilises Gram negative bacteria by disrupting the outer membrane

Appl. Environ. Microbiol.

Characterization of Bacillus cereus isolated from raw milk and some dairy products

Suez Canal Vet. Med. J.

Beneficial lactobacilli in food and feed: long-term use, biodiversity and proposals for specific and realistic safety assessments

FEMS Microbiol. Rev.

Probiotics for treatment of acute diarrhea in children: randomised clinical trial of five different preparations

Br. Med. J.

Ascorbic acid content of baobab fruit

Nature