To Steam or Not to Steam: The Effects of Processing, Storage, and Cooking on Produce + Recipes

Connor Dawedeit Headshot
By: Conner Dawedeit

Modern-day grocery stores can feel overwhelming for many shoppers, and not only due to their size, the crowds, and the weight of the responsibility of providing one’s household with sustenance and nutrition. Since the 1970’s, more and more new food products are available every year in American grocery stores, about 17,000 new products every year, fueling a $32 billion grocery industry1. It is impossible that these new products have come from never-before-tasted foods chanced upon in some remote corner of the globe; in fact, many are likely mere rearrangements and new forms of older, familiar foods which no longer necessarily appear as familiar. While once shoppers could only purchase freshly harvested produce, we can now also choose from a plethora of different brands of canned, frozen, sliced, diced, slivered, seasoned, pre-cooked, and/or otherwise preserved or prepared fruits and vegetables. While these options are convenient for those with less time, energy, or cooking skill, research has demonstrated that processing and cooking foods can negatively impact the nutritional value of what the consumer ends up purchasing and eventually eating2. Despite this, Americans and others living in developed countries where processed crops are sold suffer from nutritional deficiencies extremely rarely, while the World Health Organization lists inadequate fruit and vegetable intake and vitamin A, zinc, and iron deficiencies among the 19 largest global health risk factors3.

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Studies have produced mixed data on food degradation, owing to a large number of different crops, study methods, and sources of food (grocery stores and farms) to study4. Generally, while the processes of pasteurization, canning, blanching, and/or freezing remove nutrients from food, they do serve to slow its degradation significantly. Frozen produce may lose up to 50-80% of some nutrients but stays edible for six months to a year, while canned products may lose up to 90% of some nutrients while remaining shelf stable for years4. Though seemingly destructive, these processes allow for transport and storage practices which produce cheaper and more variable commodities to be consumed around the world. Furthermore, a harvested crop can never be 100% nutritionally efficient unless eaten in the field from which it is picked. Degradation begins at the moment of harvest, as the still-living produce begins to self-cannibalize after being cut off from the stream of nutrients from its mother plant, and nutrients are degraded over time by light, heat, and oxidation; further nutrient loss occurs due to physical removal of part of the plant (skins, stems, etc.) and cooking, where nutrients can leech into cooking water or be heated to degradation and reduced bioavailability4.

According to the Dietary Guidelines for Americans 2015-2020, about 75% of Americans across all ages and sexes eat less fruits and vegetables in a day than are recommended5. This figure seems anomalous, given that the nutritional deficiency diseases of the past are practically absent in the USA as well as the fact that many people may be habitually eating produce which may have lost over half its “usefulness” as a source of micronutrients. It may be tempting to conclude that these considerations disprove the existing body of research on nutritional degradation, and that all forms of fruits and vegetables are eaten equal. However, nutritional deficiency diseases develop under extreme conditions of nutrient scarcity, and their absence in a population or individual only indicates that no essential nutrients are completely absent from the diet. Individuals who do not consume the recommended amount of a vitamin, mineral, or other nutrient are likely to enjoy better health and well-being if they change their dietary habits to include more of that compound. It is also important to note that several nutrients are affected only minimally by processing; for example, only around 10% of minerals such as calcium and potassium tend to be lost during processing, storage, and cooking4.  Additionally, many nutrients, are widespread across many various food sources, so individuals who do not consume these nutrients as fruits or vegetables may be receiving them from grains, meat, or dairy4.

Of the nutrients studied, vitamin C, vitamin A, and phenolic compounds (nonessential, antioxidative molecules) are both affected by processing, storing, and/or cooking and are mainly found in fruits and vegetables.  Mineral and fiber content has not been shown to be significantly different in fresh, canned, and frozen produce, and losses are most significant when a portion of the produce has been removed6. The B vitamins are sensitive to degradation, but are also found in whole grains, meats, and/or enriched foods4, which most Americans habitually consume. Furthermore, relevant studies on vitamin E are rare, as it is not a nutrient typically sourced from plants but from nuts, seeds, and their oils, but current research supports the claim that frozen and canned produce contains essentially the same amount of the vitamin as fresh vegetables.

tomatoes-1280859_640.jpg Vitamin C, or ascorbic acid, is found in all produce, particularly citrus and tomatoes7. It is sensitive to degradation during cooking as well as water-soluble, and so can leach into cooking water and be lost from food easily4. Canned produce tends to contain only 10-50% of the plant’s original vitamin C content, while frozen produce retains about 50% vitamin C. Vitamin A, found in carrots, pumpkins, tomatoes, and green leaves as provitamin A carotenoids7, is lipid soluble and cannot leach into cooking water. However, these unsaturated fatty acid compounds are sensitive to oxidation and heat, and thus degrade during cooking and storage; moreover, cooking and storage can isomerize carotenoids from trans to cis forms, which can lead to a 50% loss of bioactivity for most of these compounds6 (one exception is lycopene, found primarily in tomatoes; bioactivity increases with isomerization8). Phenolic compounds, which have been linked with many putative health benefits, are also oxidized, degraded, and leached during cooking and storage, but data on these compounds is inconsistent4.

Despite the highly variable, hardly-conclusive body of research that exists on nutritional effects of processing and/or cooking food, there are answers emerging for those wondering how best to consume produce for maximum nutrition. Though frozen and canned produce has significantly less of certain nutrients then fresh, the inclusion of these nutrients in one’s diet is still necessary for optimal health, and these options offer less expensive and more convenient ways to eat produce to those who may require such accommodations. Furthermore, somewhat nutritionally depleted produce often still has the advantage of being low-calorie yet high-fiber and rich in minerals. Therefore, these products should not be considered a negative inclusion in one’s diet (particularly if they are purchased because they are more affordable or an individual does not have time to handle preparation of fresh vegetables).

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There are many ways to attempt to maximize one’s intake of nutrients from produce. Cans and frozen vegetables should be used sparingly, but can be beneficial for individuals with less money or without the time or ability to cook fresh produce. Since transportation and storage can significantly impact nutrient content of produce, fresh fruits and vegetables purchased from local sources will likely be more nutrient-dense than factory-farmed produce from several states away. Many grocery stores highlight their locally produced selections, and farmer’s markets and community-supported agriculture (CSA) programs are other good sources. A home garden offers an interesting new hobby as well as the freshest produce possible. Though refrigeration slows degradation, fresh produce should ideally be kept for only a couple of days if possible, so those living near a grocery store may opt to make daily or near-daily trips to buy small batches of produce. Finally, since both cooked and raw produce offer certain nutritional or dietary benefits, individuals should remember to vary their diet and include both raw and cooked vegetables.

The most popular way to eat raw vegetables is in the form of a salad, which can be easily and affordably made in any kitchen. Salads are exceptionally versatile and many ingredients and flavors can be experimented with. The most important thing to remember is the general formula for vinaigrettes: 60% oil, 30% vinegar, 10% anything else you like which will improve flavor and temper the acidity of the vinegar.


Basic Salad (2 servings)

  • 1 head romaine lettuce, shredded
  • 8-10 Kalamata olives, halved
  • 1 chicken breast, baked
  • 3 green onions, chopped
  • 1 large tomato, de-seeded and chopped
  • 4-5 leaves fresh basil
  • 1 large carrot, shredded or cut into quarter-inch thick half-moons
  1. Vinaigrette: 60 mL extra-virgin olive oil, 30 mL red wine vinegar, 1 tbsp. each Italian seasoning and Dijon mustard, and ½ tsp each of onion and garlic powder, shaken until combined
  2. Toss vegetables and cheese with combined dressing until well-mixed; top with chicken breast. With more practice, try experimenting with different vegetables, proteins, cheese, oils, vinegars, herbs, and other flavors.
  3. Coleslaw is another excellent raw-vegetable dish which can serve as a side or as a sandwich component. The following recipe can be found on allrecipes.com9:

California Coleslaw (6 servings)

  • 1 small head cabbage, shredded
  • ½ cup cider vinegar
  • 1 small white onion, chopped
  • 3 tbsp. sugar
  • 1 green bell pepper, chopped
  • ½ tsp salt
  • 1 red bell pepper, chopped
  • ¼ tsp black pepper
  • 1 small carrot, shredded
  • ½ cup vegetable oil
  1. Toss all ingredients to combine.

References:

  1. Pollan, M. (2008). In defence of food: An eater’s manifesto. London: Penguin Books.
  2. United States Department of Agriculture. (December 2007). USDA table of nutrient retention factors release 6. Retrieved from https://www.ars.usda.gov/ARSUserFiles/80400525/Data/retn/retn06.pdf
  3. World Health Organization. (2009). Global health risks: Mortality and burden of disease attributable to selected major health risks. Retrieved from http://www.who.int/healthinfo/global_burden_disease/GlobalHealthRisks_report_full.pdf
  4. Rickman J, Barret D, & Bruhn C. (2007). Nutritional comparison of fresh, frozen and canned fruits and vegetables. Part 1. Vitamins C and B and phenolic compounds. J Sci Food Agric, 87, 930-944.
  5. United States Department of Agriculture and Department of Health and Human Services. (December 2015). Dietary Guidelines for Americans 2015-2020, Eighth Edition. Retrieved from https://health.gov/dietaryguidelines/2015/guidelines/
  6. Rickman J, Barret D, & Bruhn C. (2007). Nutritional comparison of fresh, frozen and canned fruits and vegetables II. Vitamin A and carotenoids, vitamin E, minerals and fiber. J Sci Food Agric, 87, 1185-1197.
  7. Environmental Working Group. (June 19, 2014). How much is too much?: Excess vitamins and minerals in food can harm kids’ health. Retrieved from http://www.ewg.org/research/how-much-is-too-much/appendix-b-vitamin-and-mineral-deficiencies-us
  8.  Dawedeit, C. (December 2016). What’s not to “lyc”?: The health benefits of tomatoes. NutriNews 23, 25-27.
  9. California Coleslaw. Retrieved from http://allrecipes.com/recipe/20762/california-coleslaw/?internalSource=hub%20recipe&referringId=17389&referringContentType=recipe%20hub&clickId=cardslot%2024
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