Degradation of aflatoxin M1 in skim milk using UVC or cold plasma : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology, the School of Food and Advanced Technology, Massey University, Manawatu, New Zealand

Abstract

Contamination of aflatoxin M1 (AFM1) in milk and milk products has been an issue for decades as it is a food safety risk, classified as a Group 1 carcinogen. Cows consuming feed contaminated with fungi (Aspergillus flavus and Aspergillus parasiticus) that produce aflatoxin B1 (AFB1), convert AFB1 to AFM1 that is released into the milk. The best way of controlling AFM1 contamination in milk is to keep the feed dry to prevent the growth of fungi to avoid the production of AFB1. However, this is challenging in some tropical countries where the weather is hot and humid all year around. Treating milk contaminated with AFM1 is an alternative method of control. The aim of this study was to investigate two methods for milk treatment - UVC and cold plasma to reduce AFM1 in milk, investigate the factors influencing these treatments and identify the degradation products after treatment. UVC (254 nm) reduced AFM1 in skim milk to below MRL (0.5 μg/L) from an initial level of 1 μg/L after 20 min treatment. Treatment time (min), depth of samples (mm) and the stirring of the milk sample during treatment were found to significantly (P 0.05) effect the UVC efficacy. A change in milk colour was observed but the pH of the milk samples did not change. The degradant of AFM1 after UVC treatment was identified as an oxidation product which resulted in hydroxylation occurring at the double bond of the furan ring of AFM1 molecules. High voltage atmospheric cold plasma (HVACP) was used to reduce AFM1 in skim milk and explore the effect of treatment times (5, 10 and 20 min), operating gases (air and MA65 - 65% O2, 30% CO2, 5% N2), three voltages (60, 70 and 80 kV), using direct and indirect treatment, AFM1 contamination levels (0.1; 1 and 50 μg/L) and the volume of the sample (10, 20 and 30 mL). A reduction of 64.99 and 78.86% of AFM1 in skim milk after 20 min HVACP treatment using air and MA65, respectively, was achieved with the initial level of 1 μg/L. HVACP did not change the milk colour after 20 min treatment but a slight change in pH was observed. Different treatment times, different operating gases and voltages, direct and indirect treatments were found to have the most effect on AFM1 reduction. While AFM1 contamination levels (0.1; 1 and 50 μg/L) had an insignificant (P > 0.05) effect on AFM1 reduction in milk. A dielectric barrier discharge (DBD) cold plasma set up with small capacity high voltage generator was used to investigate the effects of other operating gases with different mixtures (5, 10 and 20% of air, pure oxygen and nitrogen in helium) and the effect of milk components (casein, lactose and whey protein) on AFM1 reduction. The degradation products of AFM1 after cold plasma treatment were determined. Although this small capability system reduced approximately 70-100% of AFM1 in water after 3 and 10 min treatment by using air/helium (10/90), the reduction of AFM1 in skim milk, whey and casein was much less, although 70% of AFM1 was reduced in lactose. The reduction of AFM1 in water was significantly (P < 0.05) improved by cold plasma with the increase in the concentration of air/pure oxygen in helium but it was unchanged regardless of the ratio of nitrogen in helium. The structure of three degradants of AFM1 after cold plasma treatment was elucidated with the confirmation of two of them resulting from damage to the furan ring of AFM1 molecules. The structure of the third one was proposed but another analysis technique is required to confirm