Aflatoxin B1 (AFB1) is a potent carcinogen that has been identified by the UN Cancer Council as a class 1 carcinogens. Food and feed in warm and humid climates are highly susceptible to contamination of AFB1. Especially cereals, cottonseed, and peanuts are susceptible to contamination of AFB1 before and after harvesting. After the animal ingests AFB1, it is converted into aflatoxin M1 (AFM1) by microsomal cytochrome P-450 in the liver. AFM1 is also a strong carcinogen, which enters the milk through blood circulation in animal and endangers food safety. Therefore, it is very necessary to study the secretion and conversion rate of AFM1 in the feed through the metabolism of an animal body and provide basic data for effectively controlling the impact of AFB1 pollution to ensure the quality and safety of dairy products.
Formation and transformation rules of AFM1 in milk
After cows eat AFB1 in the diet, they can quickly convert AFB1 into AFM1 and absorbed AFM1 in blood. AFM1 in secrete transfer into milk through the breast, causing contamination of AFM1 in milk. Previous studies have reported the transformation rules of AFB1 in milk.
Frobish et al. (1986) found that after 24 hours of feeding food with AFB1 for the cow, the content of AFM1 in milk can reach a stable level. And when stopped feeding for 3-4 days, the content of AFM1 in milk can be removed completely.
Diaz et al. (2004) found that 3 weeks after the intake of 55ng/kg AFB1 by a cow, the content of AFM1 in milk reaches a stable concentration. And when stopped for 4 days, and the content of AFM1 in milk is completely removed.
Moschini et al. (2006) reported that after 15 minutes of ingestion of AFB1 by the cows, the content of AFM1 appeared in the blood and after 12 hours the content of AFM1 appeared in the milk. And after 24 hours, the content of AFM1 in milk reached the highest concentration.
Masoero et al. (2007) reported that after 4 days the intake of AFB1 by a cow(98 ng/cow.day), the content of AFM1 in milk reaches a stable concentration. And when stopped feeding for 3 days and the content of AFM1 in the milk can be removed.
Wang Lifang et al. (2015) reported that after feeding AFB1 (1mg) in the food for in dairy goats, the content of AFM1 in goat milk is peaked at 4-8 h, and the content of AFM1 decreased rapidly after the peak. The maximum average conversion rate between AFB1 and AFM1 at each time point was 0.19%, which occurred at 4 h after ingestion. The average conversion rate of AFM1 within 24h after feeding AFB1 in the feed is accounted for 81% of the total conversion rate of AFM1. It can be seen that the cows fed the AFB1 contaminated diet for 2-4 days, the content of AFM1 in the milk can reach a stable level, and the content of AFB1 in the milk can be removed by stopping the feeding for 3-4 days.
Many researchers have conducted a study on the association between AFB1 intake and AFM1 in milk after ingestion. For example, Battacone et al. (2003) studied the relationship between the daily intake of AFB1 and the AFM1 concentration in using lactating ewes. AFM1 (ng/kg) )=-0.0043+0.00136 AFB1 (ng/d).
Most trials have shown that the conversion rate of AFM1 in milk is between 0.1% and 6% with an average of 1.7%. According to the conversion rate of 1.7%, when the content of AFB1 in the diet exceeds 30μg/kg, the content of AFM1 in the milk will reach the safety limit of 0.5μg/kg in the United States and other countries. Similarly, when the diet contains more than 3μg/kg AFB1, the content of AFM1 in the milk will reach the safety limit of 0.05μg/kg in the EU.
Influencing factors on AFM1 conversion
Many factors affect the conversion rate of AFs, such as nutritional or physiological factors health status, milk production, and individual differences in sensitivity to mycotoxins. Besides, there are large differences between different dates and different stages of milk production.
Veldmand et al. (1992) reported that the rate of toxin transfer in high-yield dairy cows during lactation reached 6.2%.
Masoero et al. (2007) found that in high-yield array milk cows with high somatic cell(milk yield >30 kg/head/d, somatic cell count >350000) the conversion rate of AFM1 was 2.32%, and in a high-yield array, with low somatic cell the conversion rate of AFM1 was 2.7. %. In the meantime, the conversion rate of AFM1 was 1.48% in the low-yield group with high somatic cell and the conversion rate of AFM1was 1.29% in the low-yield array with low somatic cell. It is considered that the milk yield is the main factor affecting the conversion efficiency of AFM1, and the number of somatic cells as the index for evaluating mastitis is not related to the conversion rate of AFM1.
Masoero et al. (2007) established a regression equation based on the test results between milk yield and conversion rate of AFM1. Conversion rate (%) = -0.326 + 0.077 ╳ milk yield (kg) (RSEM = 0.692; R2 = 0.58).
Besides, the conversion rate of AFM1 can also be reduced by the addition of a mycotoxin adsorbent. Kutz et al. (2009) reported that when the AFB1 concentration in the TMR was 112 μg/kg, the conversion rate of AFM1 was 2.7% DM. And when the inorganic mycotoxin hydrated sodium aluminum silicate (HSCAS) adsorbent was used, the conversion rate of AFM1 1.48%. When using the yeast cell wall, the conversion rate of the organic mycotoxin was 2.5%. It is indicated that the addition of mycotoxin adsorbent to a dairy cow in the feed can reduce the conversion rate of AFM1, and the adsorption effect of different types of mycotoxin in adsorbents is quite different.