Stability of Flax seed (Whole, ground, liquid)

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Good site for info about Flax Seed.

http://www.flaxcouncil.ca

 

STORAGE AND BAKING STABILITY Flaxseed

http://www.flaxcouncil.ca/pdf/stor.pdf

 

Flaxseed is recognized as a " functional food " because of its generous content of

the essential fatty acid,

alpha-linolenic acid (ALA) and the precursor of mammalian lignans,

secoisolariciresinol diglycoside (SDG). Each promises

to play a positive role in reducing the risk of a major health threat: ALA in

coronary heart disease1, and lignans with

respect to

cancer2. Health professionals have raised the question of how well these

components survive the hazards of oxidation and

heat under common conditions of flax storage and food preparation. The answer

is, surprisingly well.

 

Alpha-linolenic acid ALA, in isolated form or as a component of an extracted

purified oil, is generally considered

susceptible to oxidation3 because it is highly unsaturated; that is, it has

three double bonds in its 18-carbon chain.

Oxidation is encouraged by both warmth (autoxidation) and light

(photo-oxidation)4. However, ALA in the intact seed of

flax has proven remarkably resistant to oxidation despite the fact that it makes

up about 50-59% of the oil in flaxseed

which in turn is normally 35-45% of the seed weight3. Why this is so remains a

question but the facts are clear, as

represented by current research.

 

Storage effects:

 

Flaxseed, either whole or coarsely ground, appears stable to long-term storage

at room temperature. Even after 308 days

at 22°C (72°F) there was essentially no change in peroxide value as a measure of

oxidation by-products or in the

percentage of ALA in fat extracted from the stored flaxseed samples5. This

demonstration of oxidative stability in

common storage was later confirmed by direct measurement of oxygen consumption.

One gram samples of whole flaxseed,

milled flaxseed and extracted flax oil were held in individual sealed glass

tubes for 280 days at room temperature with

12h alternating dark/light cycles. All three preparations showed little change

in headspace oxygen during this time

although the flax oil sample

was more variable. The fatty acid composition of all three samples remained

unchanged, suggesting that flaxseed ALA was

stable to both heat and light6. These stability results with small samples have

been corroborated by studies on l kg

lots of milled flaxseed which were stored in closed packages at 23°C for 128

days. The samples were examined initially

and at

approximately thirty-day intervals. The packages were triple-layer paper bags

with plastic liners, much like those used

in the 60 lb. bags normally supplied to commercial bakers. Sensory tests by a

trained panel showed no difference in the

aroma intensity of water slurries of fresh and stored samples at any of the four

storage intervals. Changes in chemical

indices of

oxidation (peroxide values, free fatty acids and volatile compounds) were

negligible7. As further evidence of flaxseed's

storage stability, 36 consumers could not tell the difference between the taste

of yeast breads baked with the either

fresh or stored milled flaxseed included as 11% of flour weight in the recipe8.

 

Baking effects:

 

ALA in whole and milled flaxseed also appears to be stable to heat equal or

greater than the temperatures involved in

baking batters and doughs such as muffins and yeast bread. Thermal stability was

shown in 1992 by the absence of

significant changes in peroxide values and fatty acid composition when both

forms of flaxseed were heated for 60 minutes

at either 100°C (212°F) or 350°C (662°F). Furthermore, gas liquid chromatography

showed no signs of new trans isomers of

ALA or of cyclic fatty acid formation in samples subjected to these degrees of

heat5. In a follow-up study the

proportion of ALA in the fat of a muffin mix, where 28.5% of the formula was

milled flaxseed, was virtually unchanged

after baking at 178°C (350°F) for 2h (45.1% ALA before:45.0% after). This

stability was observed even though oxygen

consumption of the flaxseed muffin mix was considerably greater than that of the

control muffin mix6. A subsequent study

confirmed the stability of ALA in baked muffins containing the same amount of

milled flaxseed and noted that

thiobarbituric acid

values, as estimates of ALA oxidation were also unaffected by baking9. On

reflection, the baking stability of ALA should

not be surprising considering that the internal temperature of a muffin

approaching doneness would not be expected to

exceed the gelatinization temperature of starch. Wheat flour in the presence of

sugar, or honey in this instance, would

gelatinize around 95°C (203°F), much lower than the temperature of hot air in

the oven from which heat is transferred to

the baking product. A further margin of safety for the ALA-conscious consumer is

the fact that muffins are usually baked

for only 20-25 min. at 204-208°C (400-425°F) in contrast to some experimental

conditions4.

 

Biological evidence also supports the stability of ALA to baking temperatures:

 

Nine college women included 50g flaxseed in their daily diet for four weeks in

one of two ways. Five of them added

milled flaxseed, uncooked, to the food of their choice such as breakfast cereal,

soup, juice or yogurt. The other four

consumed bread baked with milled flaxseed (250g/kg) rather than their usual

bread. Plasma fatty acid profiles during the

four-week study were not significantly different between the women eating raw

milled flaxseed and those eating the same

amount of flaxseed baked in bread. Both subject groups exhibited a lowering of

serum total cholesterol and

low-density-lipoprotein cholesterol10. The implication is that baking had no

effect on the bioavailability of flaxseed

fatty acids.

 

Lignans:

 

SDG, the precursor of mammalian lignans in flaxseed, is a phenolic substance

associated with the plant fibre11. Because

phenolic structures are common to a number of commercial antioxidants4 and

because purified lignans as well as flaxseed

extracts have shown antioxidant effects in vitro and/or in vivo 12, it is

tempting to speculate that SDG may play a role

in

the storage and/or baking stability of ALA. The amounts of SDG in flax appear to

vary with the variety of flax, its

growing location and harvest year13. Each of these variables has been found to

affect the amount of mammalian lignans

produced from flax ingestion14.

 

Storage and baking effects:

 

While there is currently little information on SDG stability to common storage,

several studies suggest that there is no

significant loss of SDG from flaxseed during the baking process. Muir and

Westcott15 reported in 1996 that the amounts

of SDG measured chemically in both the crust and centre of a loaf of bread

agreed well with measurements of SDG in

flaxseed added to the dough. As well, the levels of SDG that they detected in

four loaves of flaxseed bread purchased at

local bakeries were within the range of values likely from the original addition

of 7% flaxseed. In 1998, Rickard and

colleagues16, using in vitro fermentation17, assessed total lignan production

from a number of commercial breads and

homemade products. Bread, muffins and pizza dough containing 6.9, 8.0 and 13.2%

flaxseed respectively, were baked at

190°C (375°F). In both commercial and homemade categories, lignan production

reflected the amount of flaxseed added.

This was also the case in pancakes enriched with 6.2% flaxseed which were

griddle-baked at 205°C (400°F). Furthermore,

urinary lignan levels from nine women who ate 25g flaxseed daily for eight days

were similar whether the flaxseed was

eaten raw in applesauce or baked in muffins or bread16. It appears that the

lignan availability from the SDG in flaxseed

is stable to customary baking temperatures.

 

References:

 

1. Schmidt EB, et al. In: Proceedings from the Scientific Conference on Omega-3

Fatty Acids in Nutrition, Vascular

Biology, and Medicine. Dallas, TX: American Heart Association, 1994, pp.

208-213.

2. Thompson LU. In: Flaxseed in Human Nutrition. Cunnane SC and Thompson LU,

eds. Champaign, IL: AOCS Press, 1995, pp.

219-236.

3. Kolodziejczyk PP and Fedec P. In Flaxseed in Human Nutrition. Cunnane SC and

Thompson LU, eds. Champaign, IL: AOCS

press, 1995, pp. 261-280.

4. Charley H. Food Science, 2nd ed. John Wiley & Sons, New York, 1982, pp.

33-45; 124-130; 232-245.

5. Ratnayake WMN, et al. J Nutr Biochem. 1992; 3:232-240.

6. Chen Z-Y, et al. J Am Oil Chem Soc. 1992; 71:629-632.

7. Malcolmson LJ, et al. Proc Flax Inst. 1998; 57:75-80.

8. Malcolmson LJ, et al. Flax Council of Canada, Internal Report, 1997, 15pp.

9. Cunnane SC, et al. Am J Clin Nutr. 1995; 61:62-68.

10. Cunnane SC, et al. British J Nutr. 1993; 69:443-453.

11. Harris RK and Haggerty WJ. Cereal Foods World 1993; 38:147-151.

12. Thompson LU. In: Flaxseed in Human Nutrition. Cunnane SC and Thompson LU,

eds. Champaign, IL: AOCS Press, 1995; pp.

219-236.

13. Westcott ND and Muir AD. Proc Flax Inst. 1996; 56:77-80.

14. Thompson LU, et al. Nutrition and Cancer. 1997; 27:26-30.

15. Muir AD and Westcott ND. Proc Flax Inst. 1996; 56:81-85.

16. Rickard. SE, et al. Proc Flax Inst. 1998; 57:8-14.

17. Nesbitt PD and Thompson LU. Nutrition and Cancer. 1997; 29:222-227.

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Good Health & Long Life,

Greg Watson,

http://www.ozemail.com.au/~gowatson

gowatson