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Food Prep and Nutrition Loss

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A guest blog by Andrea Lewis.

In a perfect world, no vitamin, mineral or enzyme would be left behind. Nutrient preservation is one of the top 5 reasons given for adopting a raw vegan diet. However, cooking is not the only preparation method that can cause nutrient loss. Most food prep methods cause some degree of nutrient loss.

Prep Methods and Nutrient Loss

The following list is based on retention factors calculated using a protocol developed by the Association of Official Analytical Chemists (AOAC) for the USDA. Obviously, the amount/percentage of nutrient loss will vary depending on the type of nutrient and prep method used. For the sake of brevity, I've listed the maximum calculated range of nutrient loss for each method of preparation, separating the vitamins from the minerals.

Cooking – Vitamins 25-70%, Minerals 20-40%

Cooking and draining – Vitamins 35-75%, Minerals 25-70%

Reheating – Vitamins 5-45%, Minerals 0%

Freezing – Vitamins 0-30%, Minerals 0-10%

Drying – Vitamins 0-80%, Minerals 0%

The nutrients that are most depleted during prep are Vitamin C, the B vitamins – Thiamin, Riboflavin, Niacin, Folate, B6 and B12, the minerals Copper, Potassium, Magnesium, Sodium and Iron. Vitamin A, alpha and beta Carotene, Lycopene and Lutein are also significantly affected, but not to the same degree as the other nutrients.

Enzymes and other lesser known nutrients are also loss during these forms of preparation, but those nutrients are not usually measured and, therefore, the protocol used to calculate the retention factors for vitamins and minerals will not work for those unmeasured nutrients.

The AOAC/USDA retention factors are calculated using the “True Retention Method (%TR)”. The true retention method requires data on the weights of food before and after cooking, as well as nutrient content of the foods while raw and after cooking, cooking and draining, freezing and drying. The equation used is as follows:

%TR = (Nc*Gc) / (Nr*Gr) * 100

If weights of food before and after cooking are unavailable, the retention factor can be calculated on a moisture-free basis, the Apparent Retention Method (%AR):

%AR = [Nc(dry wt basis)] / [Nr(dry wt basis)] * 100

where:

Nc = nutrient content per g of cooked food,

Gc = g of cooked food,

Nr = nutrient content per g of raw food, and

Gr = g of food before cooking.

Dr. Brian Clement, the director of the Hippocrates Health Institute, has claimed that food processors and blenders can destroy more than 80% of a food's nutrients when making smoothies. There's currently no evidence that this is true, but it is probable that there is some nutrient loss when a blender or food processor is used; after all, exposure to the air results in some nutrient loss. However, I very much doubt that such an extreme degree of loss could happen simply by blending foods together at high speed. As a user of such devices, I look forward to any definitive scientific evidence on this subject.

It's interesting to note that drying foods can cause greater nutrient loss than cooking and draining, depending on the nutrient. This is especially true for Vitamin C. 80% of Vitamin C is lost through drying, 75% is lost by cooking and draining, 50% is lost through cooking and reheating, and 30% through freezing. Keep in mind that these percentages are the maximums established for each method, and some nutrients, particularly minerals, are impervious to the drying process.

Raw vegans needn't worry about nutrient loss through cooking, draining and reheating, but drying and freezing are useful and sometimes necessary for creating certain raw dishes. Drying is especially popular and important for long-term storage of raw foods; but dehydrated foods are not just a matter of convenience and taste, they are also somewhat protected against bacteria, yeast and mold growth, which makes drying a health and safety issue as well.

It's not my intention to discourage the use of dehydrators, refrigerators or freezers. As I mentioned earlier, exposure to the air itself will cause some nutrient loss. The only way to lose none is to eat straight from the ground, bush or tree, which is simply not feasible for most of us. What we can do is take what we have learned and use it to our advantage. Knowledge is power. Now, knowing that Vitamin C, for example, is easily depleted by all prep methods, one can then choose to eat their Vitamin C-rich produce fresh and raw. 

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