Vitamin loss: canning, freezing & more

[Korn]

Hi,

In a book I just got about homocysteine (called 'The Heart Revolution', by Kilmer S. McCully, M.D., and Martha McCully on Harper Perennial), there are some tables showing the effect food processing has on the vitamin levels in various foods:


Some excerpts:

Losses of B6 through various processes:
Root vegetables / canning: -63%
Beans & peas / canning: -77%
Green vegetables / canning: -57%
Beans & peas / freezing: -56%
Green vegetables / freezing: -37%
Fruits & juices / freezing: -15%
Fruits & juices / canning: -38%

Loss of B6 from processing of grains:
Whole wheat / made it into white flour: -82%
Whole wheat / made into cake flour: -86%
Brown rice / made into white rice: -69%
Brown rice / made into precooked rice: -94%
Raw corn / made into white cornmeal: -87%
Raw corn / made into yellow cornmeal: -47%

Losses of Folic Acid through food processing:
Raw corn / made into yellow or white cornmeal: -62%
Whole wheat / made into white flour: -79%
Brown rice / polished into white rice: -20 %
Fresh asparagus / canning: -75%
Lima beans / canning: -62%
Green beans / canning: -57%
Beets / canning: -72%
Corn / canning: -73%
Mushrooms / canning: -84%
Chickpeas / canning: -37%
Green peas / canning: -55%
Tomatoes / canning: -54%
Spinach / canning: -35%
Fresh vegetables / freezing: -10-15%

[Korn]

The results above seem to be confirmed by this study, which found that water blanching will reduce the B6 levels by 10-15% even in canned garbanzo beans - beans that already are reduced in B6. (Steam blanching of the same, canned chick peas only reduced the vitamin B6 levels with 5-8%).

Some more links/info on the effect freezing has on nutrient levels in food:

http://www.salugenecists.com/genpage...rient&dbid=108

Effects of cooking, storage and processing on vitamin B6

While vitamin B6 has historically been described as one of the most stable of the B vitamins, cooking and processing actually greatly reduce the vitamin B6 content of foods. Canning vegetables results in approximately 60-80% nutrient loss; canning fruit, approximately 38% loss; freezing fruit, approximately 15% loss; converting grains to grain products, between 50-95% loss; and converting fresh meat to meat products, between 50-75% loss.

The amount of vitamin B6 lost during the heating that takes place in home cooking processes is dependent upon the acidity of the food with the more acidic foods, in general, being poorer at retaining their vitamin B6 content. Freezing vitamin B6-rich foods in a home freezer can result in reductions of approximately one-third to one-half of the total content.

B6 is an important, but water-soluble vitamin. It isn't stored in the body, so it's a nutrient you need to consume/absorb regularly. Water-soluble vitamins can be problematic when it comes to freezing food, because freezing normally means blanching - using boiling water for up to several minutes (see below).

Blanching will also reduce the enzymes that will cause that may cause fruits to look old (develop brown colors) and loose vitamin C. (Confusing, I know...).

According to this site, both freezing (for one week)and refrigerating human breast milk (for 24 hours) "caused a statistically significant decline in levels of vitamins C, A and E. Nevertheless, the values of all nutrients were still within the international reference ranges for mature breast milk."



On the other side; under the title "Fresher than fresh", National Frozen & Refrigerated Foods Association claims that "All available experimental data shows that frozen products often contain more nutrients than fresh foods." They also claim that "Freezing, per se, does not injure vitamins. Air exposure is much more destructive, particularly to volatile nutrients such as vitamin C and thiamine." Interesting statements... why do they use more destructive when comparing two ways of storing food if one of them isn't destructive at all? And - while lost of people would eat food that's been frozen for a month, would they eat a one month old plant that's just been lying on the kitchen table?

According to what http://www.vegetableexpert.co.uk writes, freezing retains all the vitamin C, but causes the loss of vitamins E and B6. The World Cancer Research Fund's info is more detailed:

Some vegetables deteriorate quickly after they have been picked, and freezing can halt this process, keeping levels of vitamin C higher. A survey by Health Which? magazine found that frozen green beans contained more than double the vitamin C content of fresh. Frozen peas were also as nutritious as fresh.
Not all frozen vegetables are higher in vitamin C though, which is why buying fresh is still a priority. Frozen broccoli, green peppers, cauliflower and spinach tend to be lower in vitamin C; however, if you aren’t able to buy fresh vegetables every couple of days, they are still an excellent choice.

Some plants will loose weight/volume after blanching/freezing and become more compact, which is the main reason that some plants have a higher level of vitamin C 'pr. unit' (weight or eg. a cup) than they had before the processing.

Here's info from a study (http://www.blackwell-synergy.com/doi...1980.tb07496.x) on 'thermal degradation and leaching of vitamin C from green peas during processing':

Pea samples were collected after various unit operations in a commercial cannery and the vitamin C losses at these points were investigated. Blanching and retorting accounted for the major losses. A substantial amount of vitamin C in peas leached into the brine. Approximately two-thirds of the original vitamin C in fresh peas was lost during processing.

A study on turnip green blanching showed that boiled water blanching caused the loss of 16% ascorbic acid, and 100% folic acid, thiamin and riboflavin while microwave blanching caused the loss of 28.8% ascorbic acid, 25.7% folic acid 16.9% thiamin and 7.2% riboflavin.

Another quote (http://www.onhealth.com/script/main/...iclekey=56510), based on info from Journal of the Science of Food and Agriculture, November 2003:

Beware of Blanching Before Freezing

In the second study, researchers looked at the effects of blanching 20 different types of vegetables before freezing and storing vegetables.

They found that blanching (briefly immersing them in rapidly boiling water) of vegetables prior to freezing caused a loss of up to one-third of their antioxidant content, including vitamin C. Slight additional losses were detected during freezer storage.

Folic acid was also very sensitive to the effects of blanching and more than half of this vitamin was lost during blanching.

More info here. Folic acid is an important, especially for people with low B12 levels.

About vitamin C and sweet peppers:

The vitamin C content for green mature and breaker peppers stored at room temperature (20°C) increased up to 10 days of storage, reaching similar values as those obtained for red peppers direct from the plant. However, stored red ripe peppers showed a significant loss in vitamin C content, around 25%. Refrigeration at 4°C for up to 20 days did not change the ascorbic acid content, except for red peppers, which showed losses around 15%. The ascorbic acid content was altered in response to the preservation procedures assayed. Reductions of 12% and 20–25% during the water blanching and canning process, respectively, were observed. Ascorbic acid retention during freezing was 60%, increasing when the product was previously blanched (87%). Dehydration of peppers resulted in an 88% decrease in ascorbic acid content, whereas freeze-drying did not cause significant losses.

Here's a quote from a study of stability of anthocyanins and ascorbic acid in raspberry and blackcurrant cultivars after freezing (from International Society for Horticultural Science):

Significant changes of ascorbic acid and anthocyanin content during treatment were found in both raspberries and blackcurrants. The losses of ascorbic acid after the treatment in blackcurrants were insignificant, whereas in almost all raspberry cultivars the losses were from 83 to 88%. The content of anthocyanins in raspberries decreased after frozen storage, however, it increased in most of the blackcurrant cultivars, which can be explained by easier extraction of anthocyanins from partly degraded berry skins. In conclusion, raspberries are very sensitive to temperature regimes during frozen storage, whereas blackcurrants are tolerant to frozen storage even in temperatures above optimum.

In other words, the truth about how freezing affects food varies from plant to plant. As almost always, a general statement about nutrient levels in frozen/blanced food would be wrong. The only general statement I've seen that's probably 100% correct is this one: "Contrary to myth, Freezing foods (fruits and vegetables in particular) does NOT deplete 100% of that food's vitamins and minerals. The process of freezing, if kept at a constant 0 degree temp does not contribute to a TOTAL loss of nutrients - if it did, then the majority of the population would be malnourished as we rely on and eat a large amount of frozen foods." The problem is only that 'not 100%' isn't good enough...

Here's another specific study, from the Electronic Journal of Polish Agricultural Universities:

Fresh dill leaves contained 116 mg vitamin C, 0.196 mg thiamine, and 0.638 mg riboflavin in 100 g fresh matter and whole plants 77 mg, 0.115 mg, and 0.433 mg, respectively. The treatment of blanching affected a decrease in the level of vitamin C by 35-48%, thiamine by 43-45%, and riboflavin by 27-33%. After blanching smaller losses were recorded in whole plants than in leaves. Freezing induced a decrease in the level of the investigated vitamins but only in non-blanched samples. During a 12-month refrigerated storage of the blanched material a decrease in the content of the analysed vitamins was smaller than in samples, which were non-blanched before freezing although in the case of thiamine, it was not small enough to equalize the losses affected by blanching. The lower storage temperature favourably affected only the preservation of vitamin C.

According to 'Staying Healthy with Nutrition: The Complete Guide to Diet and Nutritional Medicine', "rancid oils and fats, X-rays, radiation, aspirin, air pollution, and freezing of foods all destroy vitamin K, and mineral oil binds with K and rapidly eliminates it from the intestines." (Vitamin K is necessary for normal blood clotting). Drgourmet.com has a different opinion, and writes that "the reported amount of Vitamin K in 1 cup of frozen chopped broccoli is 143 mcg. This would be considered a very high amount of Vitamin K. A cup of raw chopped broccoli, by contrast, has only 93 mcg" - but also explains that this is because of the difference in volume that occurs during the freezing process.


Here's what http://www.annecollins.com/nutrition/vitamin-b5.htm writes about Vitamin B5 (Pantothenic Acid):

Enemies:
Food processing. Canning. Up to 40 per cent lost if boiled. Destroyed by freezing.

(No source listed).

Vitamin E is also known to be destroyed by freezing (and extreme heat). Here's one site discussing this:

Freezing food containing vitamin E can significantly reduce its levels once defrosted. Foods containing vitamin E exposed to heat and air can turn rancid. Many common sources of vitamin E, such as bread and oils are nowadays highly processed, so that the vitamin E content is significantly reduced or missing totally, which increases storage life but can lower nutrient levels. Vitamin E is an antioxidant which defensively inhibits oxidative damage to all tissues.

Some info about potatoes and cow peas from http://www3.interscience.wiley.com/c...TRY=1&SRETRY=0 :

The effects of the commercial thermal processing and storage on the retention of ascorbic acid, thiamine and riboflavin in potatoes and cow peas have been studied. Samples have been collected after washing, blanching and thermal processing operations. Washing had no significant effect on the levels of these vitamins in both potatoes and cow peas. Either blanching (at 100°C for 2 min) or thermal processing (at 121°C for 20 min) had a great effect on the retention of ascorbic acid, thiamine and riboflavin. While the retention of ascorbic acid and thiamine had been affected by the storage conditions (at 25°C for 6 month), there was no significant change of riboflavin content in potatoes and cow peas. Based on a laboratory study, leaching was largely responsible for losses of these vitamins during water blanching, thermal processing and storage.

http://www.nutritionaustralia.org/Fo...eshveg_faq.asp

Freezing is a very efficient method of preserving the nutritional value, texture and flavour of many vegetables. Most vitamins will keep well in frozen vegetables. Carotene (a compound that is converted to vitamin A in the body) may actually be better preserved in frozen produce because packaging keeps the vegetables away from light (which destroys carotene).

For example, frozen peas typically have about 60% more carotene than 'fresh' peas (that have been exposed to light during their trip to the market and while awaiting sale).

Some losses of vitamin C and folate (also known as folic acid) occur during commercial freezing. About 25% of the vitamin C, and perhaps a greater percentage of the folate, will be lost during the blanching process that precedes commercial freezing. A smaller quantity (perhaps 10%) of the thiamin (formerly called vitamin B1) will be lost during blanching. Little further loss occurs during the time the food is kept frozen, provided that it has been stored properly (-18 degrees C for no more than six months).

However, the vitamin losses associated with blanching and the thawing/cooking process are similar to those that occur during normal cooking of fresh vegetables.

http://www.annieappleseedproject.org/howfoodprepa.html:

Blanching and Storing

Many people, pressed for time, resort to frozen foods instead of fresh. But what are the effects of blanching foods, i.e., soaking them in hot water, which is commonly done before commercial freezing?

In a separate study, Finnish scientists found that blanching and long-term freezing of 20 commonly used vegetables also affected the level of various beneficial compounds in different ways.

Blanching, they discovered, destroyed up to one-third of the vitamin C content of vegetables, and this was followed by a further slight loss during storage. Folic acid turned out to be particularly sensitive to blanching, with more than half of this important B vitamin being lost, although levels remained stable during freezer storage.

Carotenoids and sterols (also common antioxidant compounds) were not affected by either blanching or freezer storage.

Dietary fiber was not adversely affected and minerals in general were stable. But phenolic antioxidants and vitamins were much more sensitive. There was a 20-30 percent loss of antioxidant activity detected in many vegetables.

Total Effect

From this pair of studies we can see that if you buy a package of frozen broccoli in the supermarket and then microwave it according to instructions you will be getting almost NONE of the antioxidants and vitamins you expected from this food.

And, just to add some confusion:

http://www.organicgardening.com/feat...3-1272,00.html
"Tests have shown that, after nine months, vegetables that were blanched before freezing retain up to 1,300 percent more vitamin C and other nutrients than vegetables frozen without blanching."



Here are a couple of sites that list recommended water blanching times for various plants:
http://hgic.clemson.edu/factsheets/hgic3063.htm

Water Blanching Times:

1½ minutes: Cabbage (shredded), green peas

2 minutes: Asparagus (small stalks), blackeye peas, carrots (diced or sliced), greens (except collards)

3 minutes: Beans (snap, green or wax), broccoli and cauliflower flowerets, celery, collards, sweet peppers (halves), rutabagas (cubed), summer squash (½-inch slices).

4 minutes: Whole kernel or cream corn (blanched on cob, cooled and cut off cob), eggplant (1/3-inch slices)

5 minutes: Carrots (whole, small)

The following times depend on size:

2-4 minutes: Asparagus, beans: lima, butter, pinto

3-4 minutes: Okra

3-5 minutes: Brussel sprouts, Irish potatoes (new)

3-7 minutes: Onions (until center is heated)

7-11 minutes: Corn-on-the-cob

Microwave Oven-Blanching: Directions for microwave-blanching of vegetables are not based on any published research. Recent research indicates some of these times are not sufficient to adequately blanch some vegetables. Inadequate blanching does not cause a food safety problem, but quality may suffer. Water is a more reliable method of blanching.

Here's another list, again confirming that frozen food often is blanched (kept in boiled water for several minutes) before the actual freezing takes place. (http://www.aces.edu/dept/extcomm/new...blanching.html)

Here are recommended blanching times for the following vegetables:
Beans (lima) 2 to 4 minutes
Beans (snap) 3 minutes
Corn on cob 7 to 11 minutes
Corn 4 minutes
Peas (garden or field) 2 minutes
Okra 3 to 4 minutes
Squash 3 minutes