Chapter 7 Mycotoxins by Dr. William Riley在线视频

Chapter 7 Mycotoxins by William

Welcome to this lecture. I would like to focus on one of the three hazards of HACCP, and that is chemical hazards. In particular, I’m going to talk about one that we don’t often think of a as chemical hazard, and that is mycotoxins.

Mycotoxins are what we call a secondary metabolite, and they come from plants, and they come from the fungi that grow on plants. We often think of them as perhaps a biological hazard, and the fungi themselves are a biological compound, and I say that because it is important that we know and think of the mycotoxins as a chemical hazard. And I say that because in fact a chemical cannot be destroyed by heat. The fungi can be killed by heat, but the resultant secondary metabolite – the mycotoxin - can’t be destroyed by heat. And it’s important then that we understand that because with mycotoxins occur in the plant product or the food product; we cannot any longer destroy that by heat. So, mycotoxins and their corollary, the aflatoxins, are important chemical hazards in particular food products. And that’s what I would like to talk about today.

What the actual function of mycotoxins is in the plant is still not completely known but what we think they do is to help the fungi to invade the plant. And they probably decrease the competition for the fungi to actually come in and habituate the plant. So, it allows them to come in and set up home in what is a natural habitat for them, in particular, here in China, what would be the corn crop and the peanut crop.

So, we call these toxigenic fungi, and the main type of mycotoxins present is produced by species of Aspergillus, Fusarium, Penicillium, Stachybotyrs and Myrothecium. And I’m going to talk mainly about Aspergillus and Fusarium.

So, what I would like to do now is show you some pictures of them and what they do to the corn crop, and some other crops in particular. You can see here an ear of corn that has been invaded by Aspergillus, which are the main pathogens – along with Fusarium - that produce the aflatoxins. No, you can’t consume a crop that looks like this, but quite often, in producing a corn crop and then processing it for feed, for animal feed, they won’t look at individual corn cobs, they will in fact they will cut the whole crop down and then break it down into the feed corn. So, the aflatoxins end up being in the crop that’s fed to the animals.

Here you see Fusarium and the corn cob that’s been infested by it. You also see the pumpkin on the right, or Nangua in Chinese. You can see the damage done by the fusarium and the production then of the mycotoxins in it. Again, you wouldn’t consume that directly, but if it’s in a whole crop that may have been processed together, then it could infest that crop.

So, the types and amounts of toxins produced would depend on fungal strains, growing conditions, presence or absence of other organisms in them. They tend to produce fungal spores, fragments of the crop, substrates and also how many nutrient sources are available on them.

The toxic enzymes or toxic exposure usually occurs from indoor fungi growth and excessive moisture, but it also depends also on the conditions at the time that the crop is grown, and it is very different at the beginning of the season and also in the post-harvest exposure. So, the tendency is for the conditions at the beginning of the year to be when there is stress on crops, and I will talk about that in just a moment.

Mycotoxins themselves don’t tend to be directly toxic to humans; they are more toxic to animals that would be consuming the mycotoxins. Our real risk in the human population tends to come from aflatoxins. Aflatoxin poisoning would result from our ingestion of crops that are contaminated directly with aflatoxins. And two crops that are of particular concern are corn and peanuts. And they come, as I said, from Aspergillus. Specifically, Aspergillus flavus and Aspergillus parasiticus.

The most pronounced contamination is again the peanut and the corn, but also in tree nuts and some of the oilseeds crops as well. There are different types of aflatoxins: B1, B2, G1 and G2. But the one that’s most toxic is the aflatoxin B1.）

As I alluded to earlier, the conditions of both pre-harvest and post-harvest really determine when a crop is going to be exposed to the fungi that produce the aflatoxins. And here in Asia, unfortunately the conditions are usually ripe all year long. So, most Asian countries are infested with mycotoxins and aflatoxins. At the beginning of the season, generally water, a lack of water, or drought conditions, high-temperature stress, and insect damage to the host plants, are what would predispose the crop to the fungi that produce aflatoxins and mycotoxins. And generally, in China that is always the condition. In North America, for instance, that isn’t always the condition. But here in Asia, most countries that produce corn and peanuts, including China, always have those conditions that predispose the crop to mycotoxin and aflatoxin production.

So, that’s the pre-harvest situation that would produce the toxins. In the post-harvest, that is after actually you’ve actually harvested the corn and the peanuts, the conditions in storage tend to be high temperatures and high humidity. So, particularly when there is high moisture content in the atmosphere or the crop was harvested with a high moisture content in it that leads to more mycotoxin and aflatoxin production. And again, that generally is the case here in China and in the rest of the Asia, that there would be high humidity post-harvest.

What are the repercussions of high aflatoxins in the diet? And this would be true in animals, and we generally consider that what we would see in other animals, other species, also is the situation in humans. And that is acute necrosis, or cell death, cirrhosis or death of the cells in the liver and carcinoma of the liver. Carcinomas have been seen to occur in rodent models and most other species where it’s been directly tested. Of course, we can’t test for carcinoma in human species, because that would be predisposing the human subject to cancer. But we assume that what we see in other species also occurs in the humans. There’s a wide variation in the LD₅₀ for the dosage of the aflatoxin that would kill 50% of the population.

But in species tested, they tend to be at very low dosage. And that would be 0.5 to 10 mg/kg, which means parts per million in the crops tested. So, fairly low doses of aflatoxins lead to these negative consequences, including cancer formation. And again, the most toxic compound we have seen tends to be the aflatoxin B1. So, the assumption is that because it’s a well known carcinogen in other species, we assume it’s a carcinogen in the human population. And it happens to be one of the most toxic, one of the most potent, carcinogens to animal species.

Adverse effects tend to come in two forms from aflatoxins and also other mycotoxins to whatever species it exposed to. It’s the acute effect, meaning high levels of the toxin in the diet at fairly substantial levels. And acute effects tend to be focus on both the liver and the kidney. And that could be hemorrhaging, damage to the cells, death to the cells, edema or pooling of fluid in the body, extracellular fluid, diarrhea, poor digestion and even death in that species if it’s at high enough level.

Chronic effects and we don’t know really what those chronic effects are in human populations. Those studies really have not been conclusive. But in animals, we know what the effects are chronically. And those are very poor feed conversion in the species tested, and it would be especially in swine, where they grow over a longer period of time than, say, chickens do. Poor feed conversion, poor daily gain, so a very poor use of the crops that are fed to them. And in particular, that would be corn, because corn tends to be the largest proportion of feeds that are fed to both chickens and to pigs, and in the case of aflatoxins, the real risk in feeding it to a dairy cow is that the aflatoxins are transferred from the feed into the milk and that aflatoxins go from the milk and the dairy cow into the human population. And that is actually a food safety risk that occurred right here in China, not so many years ago, where one of the large dairy firms was feeding low grade corn that contained aflatoxins in it and it actually got into the human population through the milk. That company was in fact fined by the regulatory agencies in China, and they paid that penalty for using the aflatoxin containing corn in their dairy feed rations.

What are the economic impacts of aflatoxins in the food supply? It’s determined or assumed that about 25% of all the world’s food crops actually contain high levels of aflatoxins. So, the main economic impact is on the regulatory agencies that have to be funded to monitor that, but especially on mainly the animal feeds that pay the price for both monitoring that feed containing the toxins, and also the compensatory feed additives that have to be put in the crops that bind the toxins so they don’t get absorbed into animals’ diet. And that’s a cost to the feed producers, that’s passed on to the consumers that eat those meat products and consume the milk that come from that production chain. So, the consumer ends up paying that price and it’s quite a large price that is paid for that compensation.

So again, here in China, mycotoxins, aflatoxins, are found throughout the food chain. But in particular, in the corn crop, and by the way, China is the second largest corn producer in the world, second only to the United States. And also in the peanut crop. China is a large producer of peanuts, with Shandong province being the largest producer of peanuts within China. And peanuts are particularly high source of aflatoxins. And peanut meal that is produced when peanuts are used as an oil crop, which peanut oil is actually quite well used across China, in particular, in Guangdong province. Peanut meal is then produced from that, it tends to be a high source of aflatoxins, and that can be detrimental to the animals that are fed with it. So again, toxins produced from the fungi that infest those crop plants in China are a very real chemical toxin in the food chain, and we need be aware of that as food safety advocates.

Thank you for your attention.