We have all heard about the benefits of eating more fruits and vegetables. Gardeners have the advantage of freshly grown and harvested food. However, we can't always eat our produce raw directly from the garden, especially when we have a bountiful harvest, nor can we grow everything we eat.
Cooking, freezing, and preserving are ways to make use of more fruits and vegetables, but have you ever wondered how these methods affect the nutrients in your food? Or how your home-grown freshly-picked food differs in nutrition from the same food purchased at a supermarket?
Fresh from the Garden Versus a Commercial Farm
Let’s begin by addressing how locally-grown produce differs from store-bought. By the time you purchase produce in a market, it may be anywhere from one to four weeks old. Refrigerated shipping helps fruits and vegetables last longer, but during this extended post-harvest period, significant decreases in moisture and nutrient content will occur. (Ref 1)
In addition to the effects of transport and storage on fruit and vegetables, the variety/cultivar and stage of ripeness all have an impact on the levels of vitamins, minerals, and phytochemicals within fruit and vegetables. Most of the hybrid fruits and vegetables that come from large commercial farms are not chosen for their nutritional content, but instead, selected for their appearance, yield and their ability to withstand long-distance transport. (Ref 2 – 3). Therefore we may be starting with a nutritionally-inferior product that will continue to degrade as it is shipped and handled.
The stage of ripeness has a significant impact on the nutritional quality of fruit and vegetables. (Ref 3). Raw food contains the highest content of nutrients at the moment it is harvested and begins to decline as it is exposed to light and air during handling. Additional nutrient loss occurs when the food undergoes preparation (peeling, slicing, chopping), and declines further once exposed to heat and air in cooking, preservation, and storage. Most produce from commercial farms is harvested before it reaches full ripeness, to ensure it can survive the processing and travel time necessary to go from the farm through distribution centers to your local market.
Fruits such as tomatoes, apples, melons, and peaches will continue to ripen and reach their peak color after being detached from the mother plant. However, there are considerable losses of vitamin C compared to that found when the product had been freshly picked at its peak of maturity. (Ref 2 – 4).
If you do not grow a particular food yourself, consider buying it locally-grown. Farmer’s market produce and local foods are likely fresher, contain more nutrients if picked at full ripeness, and are considered by many consumers to be better tasting than foods that have endured under-ripe harvesting and many miles of transportation. Consuming local produce helps our communities by stimulating the local economy and protecting the environment (less shipping means less fuel use). (Ref 5)
To determine what fruits and vegetables are ripening at any given time in my home state of Arizona I can refer to the calendar in Publication AZ1665: Local Foods in Arizona, a Cooperative Extension Publication of the University of Arizona. The calendar from the publication is shown below. You may find similar publications from your local Extension Office or other state entity.
Several of our Arizona supermarket chains are now highlighting foods grown locally, and the farmers who produce them. The photos below are from a Fry's Food store.
Preservation of fruits and vegetables is a long-standing method of dealing with perishability. Humankind has been drying meats, fruits, and vegetables for millennia. Ice/snow, sun, wind, and heat from the fire were used by our ancestors to preserve food obtained from bountiful hunts and harvests to make it last through lean times. Evidence shows that the Middle East and Asian cultures actively dried foods in the hot sun as early as 12,000 B.C. The Romans were particularly fond of any dried fruit they could make. (Ref 6). Fermentation has also been used for centuries as a method of food preservation.
Thermal processing, which includes canning, is one of our more 'recent' discoveries in food processing. Canning was first pioneered in the 1790s by Frenchman Nicolas Appert, who won a challenge by Napoleon for creating a method of food preservation for traveling armies.
Nutrient Loss in Canning and Freezing
How does the nutritional quality of canned and frozen fruits and vegetables stack up, as compared to their fresh counterparts? Storage and cooking can lead to overall losses of up to half of the original nutrient content. However, with some produce, freezing and canning processes may preserve nutrient value. While the initial thermal treatment of canned foods results in a nutrient loss, nutrients are relatively stable during subsequent storage due to the lack of oxygen. Frozen products lose few nutrients in freezing, but they lose more nutrients during storage due to oxidation. (Ref 7 – 8)
Desert hackberries and cactus fruit picked for canning
Fresh vegetables are not necessarily better than frozen since frozen foods are picked riper and usually processed soon after picking, whereas fresh foods may spend weeks in processing. If you cannot obtain foods you know are fresh (usually either locally-produced or home-grown), frozen produce may be a nutritious option.
Nutrient Loss in Dehydration
The nutrient loss for dried foods varies between 30 – 80% for vitamin C and 10 – 50% for vitamin A. Thiamin can be significantly reduced as well. Fruits like apples, apricots, peaches, and prunes lose about 6 percent of their vitamin A, 55 percent of thiamin, 10 percent of niacin and 56 percent of vitamin C. (Ref 9). The amount of loss is dependent on many factors, including storage time, drying temperature, and drying time. Surprisingly, sun drying causes the most nutrient loss. Indoor dehydration (using low heat and air in an electric dehydrator) causes moderate loss, and freeze-drying results in the least amount of degradation. Keep in mind that further use of heat in cooking to rehydrate the food will result in an additional nutrient loss.
Preparing peppers for dehydrating in an electric dehydrator
Nutrient Loss in Freeze Drying
The main difference between freeze-dried foods and fresh foods is water. Research has shown that while freeze-dried fruits and vegetables contain slightly lower amounts of certain vitamins, they are rich in antioxidants and fiber. Most researchers agree that the degree of nutrient loss from freeze-drying is minuscule. (Ref 10). Though it is impractical for most homeowners to freeze dry foods, purchasing food preserved by this method is an option for ensuring nutrient retention.
Cooking Methods and Nutrient Retention
A study published in the Journal of Food Quality identified the retention of vitamin C, iron, and β‐carotene in five vegetables cooked by household microwave‐steaming, stir‐frying with oil, stir‐frying with water, and boiling. Overall, the results showed higher nutrient retention values in vegetables cooked by microwave‐steaming and stir‐frying with oil. (Ref 11).
In another study, the influence of cooking on antioxidant activity evaluated in 20 vegetables has shown that pressure-cooking and boiling lead to the greatest losses. Griddling, microwaving and baking retained the most nutrients. (Ref 12)
Cooking does not always result in only nutrient loss, however. Research shows that cooked tomato products have higher available levels of cancer-fighting lycopene and antioxidants than tomato in its raw form. Cornell University researcher Rui Hai Liu found that lycopene levels rose 35% after cooking tomatoes for 30 minutes at 190 degrees F. Cooking vegetables also seems to have a positive effect on some antioxidants by increasing their bioavailability, particularly carotenoids found in carrots, cabbage, bell peppers, spinach, kale and asparagus. (Ref 13)
Heat changes the physical structure of food and therefore its digestibility and physiologic effect. For example, cooking vegetables causes an increase in the soluble dietary fiber content of vegetables and tubers and a decrease in insoluble fiber. Soluble fiber helps to decrease insulin levels. (Ref 14)
Conclusion and Key Takeaways
Depending on the fruit or vegetable of interest, the cooking methods, temperatures used, preservation conditions and different cooking technologies may have a positive, neutral or negative effect on nutrient retention. No single study or group of studies that I have found has done a comprehensive review of the nutrient changes for a wide variety of vegetables using all popular cooking and preservation methods. Interestingly, studies have shown both positive and negative effect of cooking on vegetables and fruits, depending on the nutrient being studied. Therefore, a mix of both raw and cooked vegetables appears to be the best option for a variety of nutrients.
The one clear takeaway shown by the research is that we obtain the most nutrient from vegetables and fruits harvested at the peak of ripeness and consumed as soon as possible thereafter.
Key Takeaways from the Research
The fresher and riper the food, the higher the nutritional content. If you grow your own or shop locally-grown and farmers markets for the ripest produce, your food will have a nutritional advantage over commercially grown food shipped long distances.
Steaming, stir-frying with oil, griddling, microwaving, and baking cooking methods appear to retain the highest level of nutrients when cooking. Boiling and pressure cooking degrades more nutrients than other cooking methods.
Canning, freezing, and drying are longer-term preservation options that retain some nutrients and result in the loss of others.
For the most nutrition content, obtain your fruits and vegetables as ripe as possible, and consume them quickly. A variety of both cooked and raw produce will provide balanced nutrient availability.
Dehydrating pepper photo by Bill Robinson, all other photos by Laura Ward
Laura Ward is a Tenth Generation Farmer, an Arizona Master Gardener, and an Arizona licensed Home Inspector and Termite Inspector (at least one job has to pay the bills). She has too many animals and poultry to mention them all, but is especially proud of her two Clydesdales, for the good work they do on the farm and their help teaching adults and children about heritage draft horse breeds and how gentle they are.
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