Loss of minerals and vitamins during processing of whole foods is well reported and is likely one of the reasons that research shows that consumption of whole grain food provides health benefits when compared to refined cereals. For example, researchers1 have investigated the losses of certain nutrients during the processing of whole grain wheat to white flour and found that losses of seven vitamins amounted to 50 to 86.3%. This included loss of 72% of the vitamin B6, 86.3% of the α-tocopherol and 66.6% of the folate. The total mineral bulk content of wheat flour was reduced from between 60.0 to 84.7% during the refining process, while the trace mineral content was reduced by between 40.0 to 88.5%. White bread compared to whole grain bread had 40% less magnesium, 71.4% less chromium, 69.4% less cobalt, 69.8% less copper and 77.4% less zinc.
The same researcher also investigated the losses of various types of processing on the losses of vitamin and minerals. Because minerals are inorganic, it was not expected that canning or freezing would result in losses. However, the study authors reported that the canning of spinach resulted in a manganese loss of 81.7%, a cobalt loss of 70.6% and a zinc loss of 40.1%. High zinc losses were also reported in canned beans (60%) and canned tomatoes (83.3%). The study analysed the content of 552 foods and edible products for levels of vitamin B6 and 507 foods for the content of pantothenic acid. The foods with the highest concentrations of the B vitamins were generally nuts, condiments, seeds and yeasts as well as dried meats and fish. When nuts, legumes and seeds were excluded only 13.9% of foods had levels of B6 over 2µg/kg.
Canned seafood and canned meats had vitamin B6 contents that were 48.9% and 42.6% less than the raw equivalents. However, losses of vitamin B6 in dry meats and dairy products were small. The canning of vegetables results in large losses which ranged from 57 to 77% of the vitamin B6, and the of nine vegetables with more than 2 µg/kg of vitamin B6, none maintained this level after canning or freezing. Losses of pantothenic acid during the canning of animal food ranged from 20 to 35%, whereas losses during the canning of vegetables ranged from 46 to 78%. The mean loss of vitamin B6 during the canning of fruit was 37.6% and during freezing was 15.4%, whereas mean losses of pantothenic acid were 7.2% and 50.5% during freezing and canning, respectively. Generally, enriched or raw foods contained the highest levels of B vitamins.
The polishing of rice resulted in the loss of all trace minerals except copper. This is interesting because it was the feeding of polished rice that originally lead to the discovery of the discovery of thiamine. Refined white sugar had negligible levels of chromium, cobalt, molybdenum and zinc compared to raw or cane sugar. The authors concluded that planning a diet would need to be difficult if adequate amounts of vitamin B6, pantothenic acid, chromium, manganese and zinc were required. They suggested that any processed food ingested would require compensation with highly nutrient dense foods in order to avoid deficiencies. The small quantities of vitamin B6 and pantothenic acid found in most processes, canned or frozen foods cast doubt on the average Western diet providing adequate intake of these important B vitamins. Large intakes of frozen, canned, or processed food would be unlikely to meet nutritional requirements.
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