Tuesday, February 3, 2015

Frozen Fruit & Vegetables: Excellent Sources of Dietary Vitamins C, E & B2, Even After 90 Days of Storage - At Least If the Cold Chain Remains Intact | Only β-Carotene is Lost

Don't fall for the "trick! In fruit A loss of water weight can cause "increases" in relative vitamin content.
You will probably be aware that freshly frozen vegetables and fruit retain large amounts of its natural vitamin content. "Large amounts", however, is not an exactly accurate figure. The results of a very recent study by Ali Bouzari, Dirk M. Holstege, and Diane Marie Barrett may yield some quantitative insights into the exact nutrient loss due to freezing and subsequent storage in several fruit and vegetable commodities (Bouzari. 2014).

To be more specific, the scientists evaluated the ascorbic acid, riboflavin, α-tocopherol, and β- carotene in corn, carrots, broccoli, spinach, peas, green beans, strawberries, and blueberries.
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As you can see in Figure 1, the the loss of vitamin C is minimal compared to non-frozen foods, where much of the vitamin C content is lost after a few days of storage. commodities.
Figure 1: Changes in vitamin content of selected vegetables after freezing and storage for 90 days (Bouzari. 2014)
For peas, the ascorbic acid (vitamin C) and alpha tocopherol (vitamin E) level did even increase during storage.
The distribution cold chain has to remain intact! Theoretically frozen vegetables are an excellent source of vitamins. Unfortunately this is true only if the foods are handled properly. When the cold chain is broken, the advantage of freezing fruit and veggies is lost and the vitamins are lost, because the drip loss increases by ~760% (Gonçalves. 2011)!
That sounds great, but just like similar changes in strawberries, these effects are brought about by a loss of water weight during freezing and storage and are thus not actually good news. Furthermore, the scientists were able to show that...
  • none of the commodities showed significant differences with respect to riboflavin content
  • three commodities had higher levels of α -tocopherol in the frozen samples, 
  • β carotene was not found in significant amounts in blueberries, strawberries, and corn,
  • peas, carrots, and spinach were lower in β -carotene in the frozen samples
Overall, the vitamin content of the frozen commodities was comparable and occasionally higher than their fresh counterparts. Only for beta carotene there were significant decreases in some commodities, you should keep in mind, when you go grocery shopping.
Changes (%) in vitamin C content of fresh and frozen spinach during storage (Gil. 1999; Bouzari. 2014).
Bottom line: With the exception of beta carotene, frozen commodity foods are excellent sources of vitamins and can - in case of vitamins like vitamin C - deliver even more of the important nutrients than their "fresh" counterparts that lose large amounts of their precious vitamin content during storage - the same goes for phytochemicals, where this was studied (Mullen. 2002).

One thing we should keep in mind though, is that the exact values are thwarted by the dehydrating effect of freezing which is also at the heart of the sudden "increase" in vitamin C in spinach leaves upon freezing measured in the study at hand | Comment on Facebook!
  • Bouzari, Ali, Dirk M. Holstege, and Diane Marie Barrett. "Vitamin Retention in Eight Fruits and Vegetables: A Comparison of Refrigerated and Frozen Storage." Journal of Agricultural and Food Chemistry (2014). 
  • Gonçalves, Elsa M., et al. "Degradation kinetics of colour, vitamin C and drip loss in frozen broccoli (Brassica oleracea L. ssp. Italica) during storage at isothermal and non-isothermal conditions." International Journal of Refrigeration 34.8 (2011): 2136-2144.
  • Gil, María I., Federico Ferreres, and Francisco A. Tomas-Barberan. "Effect of postharvest storage and processing on the antioxidant constituents (flavonoids and vitamin C) of fresh-cut spinach." Journal of agricultural and food chemistry 47.6 (1999): 2213-2217.
  • Mullen, William, et al. "Effect of freezing and storage on the phenolics, ellagitannins, flavonoids, and antioxidant capacity of red raspberries." Journal of Agricultural and Food Chemistry 50.18 (2002): 5197-5201.