Microwaves transfer plastic components to potatoes

Researchers from the University of Almería in Spain have demonstrated that polypropylene is transferred to the vegetable when it’s heated up in a bag

Microondas plástico
Two potatoes that are ready to be cooked in the microwaveAndrew Gardner (Getty Images/iStockphoto)

Supermarkets increasingly offer vegetables that, within just a few minutes of microwaving them in the same plastic bag in which they were purchased, are ready to eat. Potatoes, cabbages, or mixtures of various vegetables come already pre-cooked and — according to the producers’ instructions — it’s not necessary to put them in another container to microwave them. But new research shows that perhaps it would be better to take your veggies out of the bag before heating them up.

Researchers from the University of Almería (Spain) — led by Dr. Francisco José Díaz Galiano — have discovered that this microwave cooking process actually causes a “very pronounced” transfer of plastic components from the bag to the vegetable. The research – published in the journal Food Chemistry — has also identified that the action of the microwave in these cases creates a new compound in the potato, which they call HMPP-maltose. According to Díaz Galiano, this is a “potentially toxic” compound, although new studies are still required to confirm this. Regardless, the transfer of plastic to potatoes remains an issue.

The researchers had the feeling that “in a very energetic situation — like that of the microwave — it was more than possible for certain plastic components to migrate into the food.” They began to do all kinds of tests with potatoes from different brands, dispensed in bags and ready to be cooked or heated up. They were made with all possible variants: “In the microwave in a plastic bag or in a microwave in a glass container… in both cases using the recommended time for the bag, or cooked in water over fire. In this case, we gave [the potatoes] more time, between 10 and 15 minutes,” Díaz Galiano explains. He remembers that the potatoes were also analyzed before cooking them. They did 27 replicates of the experiment and “the same differences always appeared.”

The experimentation gave rise to two results: “one intuited and another totally unexpected,” the head researcher notes. The one that didn’t cause them any surprise was the transfer of polymer from plastic to food. “It has been demonstrated that there’s a very pronounced migration of polypropylene glycol (PPG) polymers from plastic bags to potatoes… [and] only when they’re cooked in the microwave in contact with the plastic. That is, these PPGs — if present in the bags — aren’t transferred to the food unless they’re cooked together, as is done in the microwave,” Díaz Galiano specifies. This transfer, he adds, doesn’t rule out the possibility of “other compounds present in plastic.”

“It’s possible that other [compounds] migrate to the food only by contact,” he acknowledges, “but, in this case, we’ve focused on the differences during the cooking process in contact with the food.” For the researchers, the conclusions are beyond reproach, because “there are chemical compounds that are the exclusive result of the cooking process of the potato in contact with the plastic that aren’t observed either in the raw potato, or in the one boiled in water, or in the cooked in glass in the microwave.”

Along with this more or less expected result, the experience offered an unforeseen one that still has a long way to go until its importance is determined. This is regarding the appearance of a new compound. Plastics, Díaz Galiano explains, have, among their components, “synthetic photoinitiators, [which are] reactive compounds eager to interact and find something to join with to create new plastic molecules, new polymers that will arise from the creation of structures that multiply and multiply. The microwave energy on the bag,” he adds, “seems to trigger a process whose final result is a combination between one of those synthetic photoinitiators used in the synthesis of plastics — HMPP— and maltose, a natural component made of the starch from the potato.” Since this structure —which they have provisionally named HMPP-maltose — “hadn’t been described before,” the research notes, “its properties cannot yet be determined, including its [level of] toxicity or safety.”

In any case, the signs aren’t positive for health. The professor of analytical chemistry affirms that “on the one hand, HMPP — 2-hydroxy-2 methylpropiophenone — in and of itself is toxic. On the other hand, the studies using software models indicate that the HMPP-maltose combination is potentially toxic to living beings.” For now, the team is investigating possible synthetic routes to create said compound. “Then, its properties can be evaluated,” he concludes.

The toxicity (or lack of toxicity) in a plastic for food use has been regulated by the European Union since 2011, specifically in its regulations concerning plastic materials and objects intended to come into contact with food. The suitability of the material is determined from tests that analyze the interaction of foods with so-called “food simulants”— products that, in the tests, simulate being plastic wrappers. There are six simulants, five of them liquid — acetic acid, vegetable oil and three dilutions of ethanol and water — and one solid, called Tenax TA, which is used, for instance, to analyze powdered soup sachets. The interaction between food and these simulants determines their safety based on whether or not they’re transferred to the food.

Díaz Galiano believes that the aforementioned method isn’t totally reliable today, because they reduce all food possibilities to only six groups. He also points out that, after experimenting with potatoes (also with simulants), no transfer of these compounds to the vegetables was determined, hence his research team wasn’t able to attain conclusive results.

“The tests determined that there was no transfer of the food simulant either by mere contact with the plastic, neither before nor after cooking,” he clarifies, which contradicts what happens when cooking the potato in its bag.

“The transfer of plastic polymer components and their additives to food is a well-known and studied issue,” comments Nicolás Olea, upon analyzing the recent study. He’s a medical doctor and professor in the Faculty of Medicine at the University of Almería. An expert in health and the environment, he says that the transfer “places plastic materials for food and kitchen use in continuous doubt [due to safety concerns]. There’s an imperative need to subject any new material to analysis, given the plethora of chemical compounds that plastic transfers to food.”

“Unfortunately,” he continues, “as the researchers of this pioneering work demonstrate, the protocols to investigate this transfer fail in two aspects. On the one hand, because they don’t cover all possible combinations of cooking methods and types of food. On the other hand, what’s most striking in this work is that it has never before been evaluated which new chemical compounds appear in food when plastics are used in its preparation.”

For Dr. Olea, the conclusion is resounding: “Not only is stricter control of any proposed innovation necessary — a task that far exceeds the possibilities of the current food safety and control system — but it is the obligation of the producer and the seller to warn the public about the risks in changes to the usual way of cooking [foodstuffs]. It’s not ethical to wait for the European Food Safety Authority [or the FDA] to give its opinion on the issue; you should simply let the public know that cooking potatoes in the microwave using ready-to-cook plastic containers poses a risk of exposure to toxic contaminants that you would never encounter while boiling them in your typical pot. It’s very straightforward,” the researcher shrugs. He’s recently authored a book — Liberate Yourself From Toxins — that emphasizes his message.

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