Detoxification of aflatoxin B1 and T-2 by probiotic yoghurt bacteria : a thesis submitted in partial fulfilment of the requirements for the Degree of Master of Applied Science (Food Science)

Abstract

Certain strains of lactic acid bacteria (LAB) have been shown to be able to detoxify some mycotoxins. Different LAB species such as Lactobacillus casei and L. acidophilus are used as probiotics in several products including yoghurt, cheese, buttermilk, and frozen desserts. Limited work has been done to isolate LAB from foods such as probiotic-enriched yoghurts or fermented dairy foods to assess their ability to detoxify aflatoxin B₁ (AFB₁) and trichothecene-2 toxin (T-2). The objectives of this project were to (1) isolate L. casei shirota from a fermented milk drink (FMD) and L. acidophilus, L. bulgaricus and Streptococcus thermophilus from a conventional yoghurt (CY), (2) determine the viability of these LAB species when incubated with AFB₁ (5, 10, 20, 50 and 100 µg/L) and T-2 (20, 50, 100, 200 and 250 µg/L), (3) determine the capacity of these LAB species (10⁹ cells/mL) to detoxify AFB₁ and T-2, and (4) compare the detoxification rate between live (non-heat-treated) and denatured (heat-treated) cells of L. casei shirota. A pure commercial L. acidophilus AS1.3342 culture was used as a positive control. L. acidophilus AS1.3342 and the food-derived L. casei shirota were used singly whereas L. acidophilus, L. bulgaricus and Streptococcus thermophilus isolated from CY were used as a mixture. All bacteria were able to detoxify AFB₁ and T-2 but there were no significant differences between the bacteria in their capacity to detoxify the toxins. The toxin removed [both in absolute quantity (µg/L) and as a percentage] by live cells significantly increased as the toxin concentration increased in spite of the bacterial cell viability declining with exposure to increase in toxin concentrations. Live cells of L. casei shirota detoxified 11–43% of AFB₁, 19–38% of T-2; L. acidophilus AS1.3342, 10–46% AFB₁ and 15–45% T-2; and the mixture of live CY strains, 14–43% AFB₁ and 15–45% T-2. The absolute quantity of toxin removed by denatured cells also significantly increased with increase in toxin concentration. Expressed as a percentage, denatured cells detoxified more of the toxins (L. casei shirota detoxified 48–62% AFB1 and 42–53% T-2) compared with exposure to live bacterial cells, lending support to the hypothesis that denatured bacterial cells are able to remove more toxins than live cells possibly due to the higher binding of toxins to the cell membrane of non-viable cells. These findings show that bacteria in probiotic milk foods, irrespective of whether live or denatured, can bind significant amounts of AFB₁ and T-2, especially when exposed to higher toxin concentrations, and thereby markedly reduce exposure of consumers to these mycotoxins.