There’s something intuitively appealing about fresh produce. A freshly made salad, with freshly picked veggies — so refreshing and light!
On the flip side, canned and frozen versions have typically been the unwanted stepchildren of the produce family. But what exactly happens to nutrients when fruits and vegetables are canned or frozen?
Before we take a look at fresh, frozen, and canned produce, let’s have a word about bioavailability.
Some micronutrients are bound in our food so tightly that our bodies can’t shake them loose for digestion. Others are just hard for our bodies to absorb. The ease with which a nutrient comes loose and gives up its goodies to our bodies is known as its bioavailability.
Cooking can increase the bioavailability of some nutrients and decrease that of others, and so can meal composition. For instance, it’s easier for our bodies to absorb fat-soluble vitamins (A, D, E, and K) if we eat them with fats. In other words, the compounds in and around fruits and vegetables can affect, positively or negatively, the bioavailability of their nutrients.
We can describe a fruit or vegetable as fresh if it’s either “postharvest ripened” (i.e., if it ripens during transport) or “vine ripened” (i.e., if it is picked and sold ripe, as at a farm’s fresh market or a farmer’s roadside fruit stand).
You might expect vine-ripened produce to be more nutritious, since it has more time to absorb nutrients from the soil — and in some cases, it is. But plants absorb a large percentage of the most crucial minerals during the early stages of growth, and fruits and vegetables can still synthesize macronutrients and micronutrients during postharvest ripening.
Several studies suggest that postharvest-ripened produce is nutritionally equivalent to vine-ripened produce — and in some cases, even better. The nutritional content of either kind of produce will depend on a host of other factors, including soil, season, weather, farming method, and storage conditions and duration.
Vine-ripened produce isn’t necessarily more nutritious than postharvest-ripened produce. Much of a produce’s nutritional content depends on other factors, starting with soil quality.
In general, frozen produce is fully vine ripened and undergoes only minimal processing. Most vegetables and some fruits undergo blanching in hot water for a few minutes before freezing, in order to inactivate enzymes that may cause unfavorable changes in color, smell, flavor, and nutritional value.
Although blanching can leach out minerals and break down biomolecules like vitamins, postharvest-ripened produce and blanched frozen produce have very similar overall nutritional content. The main impact of blanching seems to be on taste: certain fruits and vegetables have that unique just-picked flavor, which the blanching-freezing process greatly reduces.
Frozen fruits and vegetables are minimally processed.
Canned fruits and vegetables are usually vine ripened, like frozen produce, but they tend to undergo a lot more processing. Blanching is common, as well as placement in syrup, the addition of salt, and the introduction of additives that carry health risks of their own.
Moreover, some cans are lined with bisphenol-A (BPA), a chemical associated with heightened risk of cancer, and both the acidity of the produce (e.g., tomato sauce) and the heat from sterilization can help BPA leak into what you eat. (You can read more about BPA in this article from NERD 6).
Fruits and vegetables reach your store’s shelves through a variety of methods, each of which has its pros and cons. Furthermore, the method that works best for one type of produce may not work best for another. And at the end of the day, different ways of cooking can have a much bigger impact on the produce’s nutrient content and bioavailability.
Whether they’re fresh, frozen, or canned, fruits and vegetables are likely to lead to nutritional benefits.
Everyone knows that cooking kills pathogenic bacteria and other microorganisms. But while cooking effectively destroys them, the damage from contamination may already be done long before it’s time to eat. Pathogens produce toxins that can result in food poisoning, so it’s essential to limit their growth prior to cooking in order to prevent poisoning.
The following are observed minimum temperatures for growth for common microbes. In °C (°F):
Bacillus cereus: 4 (39.2)
Campylobacter jejuni: 30 (86), 31 (87.8)
Carnobacterium (spp): -1.2 (29.84)
Clostridium botulinum: 3.3 (37.94)
Clostridium perfringens: 10 (50)
Enterococci (spp): 0 (32)
Escherichia coli (pathogenic): 6.5 (43.7)
Escherichia coli (spp): -2 (28.4)
Klebsiella aerogenes: 0 (32)
Listeria monocytogenes: -0.4 (31.28), 0.5 (32.9), 1 (33.8)
Salmonella (spp): 5.2 (41.36), 6.7 (44.06)
Shigella (spp): 6.1 (42.98)
Staphylococcus aureus : 6.7 (44.06), 7 (44.6)
Vibrio cholerae: 10 (50)
Vibrio parahaemolyticus: 5 (41)
Vibrio vulnificus: 8 (46.4)
Yersinia enterocolitica: -1.3 (29.66)
Yersinia (spp): -1.2 (29.84)
This suggests that a typical freezer (-18°C, -0°F) will be sufficient to prevent growth from the major pathogenic and spoilage-causing food-borne bacteria, while fresh food will see some growth while in the refrigerator. This doesn’t mean that food kept in the refrigerator (typically 4 °C, 39.2°F) will necessarily reach the infective dose before being eaten, but freezing is a safer bet. If your freezer temperature dial has accidentally been set at a warmer temperature, or your freezer isn't working correctly, pathogen growth may have occured.
We don’t yet have solid evidence on SARS-CoV-2, the virus that causes COVID-19, but other coronavirus strains appear to be stable at low and freezing temperatures for an indefinite period. One study found that coronavirus 229E was stable after 25 cycles of freezing at -70°C for 2 hours followed by thawing at 37°C in a water bath.  This suggests a lack of vulnerability to freezing itself. However, -70°C avoids the formation of ice crystals, which damage viruses, so this may not apply to freezing at lower temperatures.
Populations of various pathogenic bacteria are able to grow in refrigerators, albeit more slowly than at room temperature. Standard freezing will halt growth entirely.
Common freezer burn is simply a loss of moisture on the surface of food, which is unrelated to the safety or microbial composition of food, and as such, freezer-burnt parts of food can simply be cut off safely.
Freezer burn occurs due to sublimation (ice converting directly to vapor without becoming liquid first). This happens when the vapor pressure of ice on a food’s surface is greater than that of the vapor in the air. Self-defrosting freezers increase the rate of freezer burn since frost in a freezer increases the average water vapor pressure of the air, which decreases sublimation.
Individually quick-frozen products (when the parts of the product, like pieces of meat or piece of fruit, are separate from each other in the packaging, as opposed to being one homogenous block) can also increase freezer burn because they expose more surface area and tend to have less ice build-up on the outside, which is sublimated in place of ice directly on the food. Protecting food from air, and having lower temperatures and less air circulation in a freezer can prevent freezer burn.
Freezer burned portions of food can be cut off and discarded. To avoid freezer burn, wrap food tightly to limit air exposure, and don't leave foods in the freezer for too long.
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