Until now our main concern when freezing foods at home was whether they would retain their organoleptic and nutritional properties. We already knew that freezing affects nutrients. Now a recent study from the Institute of Environmental Diagnosis and Water Studies (IDAEA-CSIC) adds another layer of concern: some of the additives that are added to zip bags and household-use containers where we usually place foods before keeping them cold could migrate into our meals. In other words, they release microplastics.
The researchers have paused to analyze what happens to fish during its stay in the home freezer. And the conclusion is that that part of our refrigerator is not like Las Vegas: what happens in the freezer does not always stay in the freezer. It ends up on your plate.
What Can Be Frozen and What Cannot
From the point of view of food hygiene, freezing any food slows down or even stops the biochemical decomposition processes of that product. Well performed, it is a safe process and suitable for a wide range of foods. “Cer meats, fish, vegetables (blanched for one minute in boiling water so they keep their color and texture), fruits for smoothies, bread, cooked rice, pasta, cooked legumes and even soups and stews are foods that tolerate freezing very well,” explains Oihana Monasterio, a dietitian-nutritionist of the SEEDO Therapeutic Diet Group and the Endocrinology and Nutrition Service at Hospital Universitario Basurto.
Those that are outside the list of freezeable products are “those with a high water content, such as some leafy greens, tomatoes or cucumbers. Their structure is very delicate and when frozen that water turns into microcrystals that break the cell walls. That is why, when thawed, they become wilted.”
They also should not be frozen:
- Eggs in shells. “The inner liquid expands and the shells crack.”
- Emulsified sauces. “One example would be mayonnaise, which breaks down when it loses its stability.”
- Soft or fresh cheeses, such as mozzarella or ricotta. “They lose their creamy texture and become grainy and watery, almost like a broken ricotta. By contrast, aged cheeses, like manchego or parmesan, withstand freezing much better, though they can become somewhat brittle.”
For smoothies and baking
Fruit is a highly perishable food. While some, like apples, can last for weeks, others, like strawberries, mangoes, or bananas, have a very short shelf life, as they are very sensitive to ethylene, the plant hormone that accelerates deterioration. Cutting them into pieces, putting them in a zip bag or a container and freezing them is a good way to preserve them for use in smoothies, cakes, fruit creams…
«The enzymes that degrade vitamins and antioxidants remain ‘asleep’ with the cold, so the food stays almost nutritionally intact. In addition, by freezing fruits and vegetables in peak season, we are ensuring their maximum potential of vitamins and minerals, something that does not happen if we buy them out of season and they have traveled halfway around the world» says Oihana.
The Chilly Mystery of Blueberries
Oihana asserts that freezing is one of the methods that best preserves the nutritional value of foods. «Small amounts of water-soluble vitamins, such as vitamin C and some of the B group, may be lost, but these losses usually occur mainly during the pre-blanching (the bath in hot water that we give vegetables before freezing them), not so much from the cold itself».
Other foods gain from this process. “That’s what happens with frozen blueberries, which can have more vitamins than the fresh ones we buy at the fruit shop,” notes. This nuance is important: it only applies to the blueberries that are already bought frozen, not to those that reach that state in our freezer. «Blueberries destined for freezing are harvested at their exact point of ripeness and are frozen within hours, preserving all their vitamins and antioxidants to the maximum. In contrast, the ‘fresh’ blueberries that arrive at our fridge may have spent days or weeks since they were picked, traveling in trucks and exposed to light and oxygen. During all that time, vitamin C (which is very sensitive) degrades little by little», explains the expert.
That liquid under the steak isn’t blood
A little-known case of how freezing affects nutrients is what happens when freezing meat, especially red meat. When we thaw a red meat steak at home it is common to see patches of a reddish-brown liquid on the plate. It seems like blood, but it is myoglobin, the protein responsible for storing oxygen inside muscle fibers. Domestic freezing is slow and gives time for crystals to form inside the muscles. Just like with a tomato, those crystals tear the cell walls. When you thaw your steak, those crystals melt and drag the myoglobin along with them.
This process is known in the food industry jargon as ‘drip loss’ or ‘loss by dripping’ and it is one of the major headaches for distribution companies to preserve the nutritional quality of frozen meat. Because it is not only that the steak ends up drier. Studies reveal that some minerals (especially iron and zinc) and water-soluble vitamins (especially from the B complex, such as niacin, B12 and B6) are also lost. It is important to remember the role of niacin in the formation of NAD+, the enzyme responsible for cellular longevity.
Why supermarket frozen foods stay better than home-frozen ones?
Industrial freezing equipment are capable of reducing a food’s temperature in a matter of minutes. “That is why ultra-frozen foods will maintain physico-chemical characteristics more closely resembling the food prior to the freezing process in an industrial freeze, according to the Spanish Agency for Food Safety and Nutrition (AESAN).
Oihana explains it step by step. “The key is not the ingredients, but the technology used. The food industry uses a process called ultrafreezing, which is not the same as the freezer in our kitchen. Ultracooling reaches temperatures of between -40 °C and -60 °C and freezes the food in just a few minutes. This causes the water inside to turn into ice so rapidly that microscopic crystals form, like sugar powder, that barely damage the cell walls. Therefore, when thawing, the vegetables maintain their firmness and their juice.”
When we freeze in our domestic appliance, which is at around -18 °C, “the cold takes longer to reach the center of the food. Therefore, it takes several hours to freeze completely. During that slow process, water forms huge and sharp ice crystals, blade-like in shape, that perforate and rupture the cell walls. When thawed, those broken cells can no longer retain the water, and all that nutrient- and flavor-packed juice escapes, leaving the food softer and less tasty.”
They don’t help with weight loss
Regarding the eternal fight with the scale, «the simple act of freezing a food does not add or remove any calories. That said, there are foods that, when frozen, definitely do not help us control weight, but not necessarily because of the cold, but because of what they are and how we consume them. For example, frozen ultra-processed items like pizzas, paninis, lasagnas… Or frozen dairy desserts like ice creams and frozen pastries such as brownies, tarts…»
Nevertheless, there is a group of foods that, when frozen cooked, can actually help with weight control: bread, rice and pasta. “When cooled and frozen, part of their starch becomes ‘resistant starch,’ a carbohydrate that is not digested in the intestine, behaves like fiber and provides fewer calories than if it were freshly made,” concludes Oihana.
The Dilemma of Freezing Fish at Home
Freezing is a fabulous household economy tool. For a strict food-hygiene reason, everything we freeze must go into an individual container. The problem arises when the packaging transfers unexpected substances to the food. And this is what the CSIC study, published in Environment International, points out. The culprits are not the plastics proper, but some of the additives added to packaging to improve their properties, especially to provide flexibility, durability, and structural stability. Among them, bisphenols, di(2-ethylhexyl) and adipate (DEHA).
The researchers analyzed samples of salmon, tuna, and hake. The results revealed that the most lipophilic compounds (i.e., fat-soluble) migrated more toward fatty fish such as salmon.
The bispfenols, however, showed greater transfer to those species with more water content. In this case, hake.
The type of container chosen can also increase or reduce that risk. It is higher in compostable plastic trays, while zip bags are the ones that offer the least transfer of potentially dangerous toxins after a month in the home freezer. The researchers emphasize that the risk of plastic contamination is, in any case, much higher than the risk of having consumed the fish fresh as we buy it at the fishmonger.
Is freezing fish dangerous?
The study concludes that, indeed, in almost 50% of the cases the risk threshold established by the EFSA was exceeded, especially for bisphenol A. «Considering only exposure through fish ingestion, the value recommended for bisphenol A is exceeded», warns Ethel Eljarrat, director of IDAEA-CSIC and coauthor of the study. «Exposure levels will be even higher if we also consider intake from the rest of foods, as well as exposure by inhalation and by dermal contact», the scientist notes.
Following this work, the researchers suggest rethinking the composition of household containers suitable for freezing and urge updating the toxicological data of the new additives that are entering the market. It is no longer a matter of measuring how freezing affects the nutritional value of a food, but what other substances are introduced into those foods through the packaging. It should be remembered that the new regulation regarding packaging, the Packaging and Packaging Waste Regulation (PPWR, for its English acronym), already sets stricter limits for ‘eternal plastics,’ such as bisphenol A.
