- Storage and Baking Stability
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.
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.
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.
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
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.
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.
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.
- Thompson LU. In: Flaxseed in Human Nutrition. Cunnane SC
and Thompson LU, eds. Champaign, IL: AOCS Press, 1995, pp. 219-236.
- Kolodziejczyk PP and Fedec P. In Flaxseed in
HumanNutrition. Cunnane SC and Thompson LU, eds. Champaign,
IL: AOCS press, 1995, pp. 261-280.
- Charley H. Food Science, 2nd ed. John Wiley & Sons,
New York, 1982, pp. 33-45; 124-130; 232-245.
- Ratnayake WMN, et
al. J Nutr Biochem. 1992; 3:232-240.
- Chen Z-Y, et al. J Am
Oil Chem Soc. 1992; 71:629-632.
- Malcolmson LJ, et al. Proc
Flax Inst. 1998; 57:75-80.
- Malcolmson LJ, et al. Flax Council
of Canada, Internal
Report, 1997, 15pp.
- Cunnane SC, et al. Am J Clin Nutr. 1995;
- Cunnane SC, et al. British J Nutr. 1993; 69:443-453.
RK and Haggerty WJ. Cereal Foods World 1993; 38:147-151.
LU. In: Flaxseed in Human Nutrition. Cunnane SC and Thompson
LU, eds. Champaign, IL: AOCS Press, 1995; pp. 219-236.
- Westcott ND and Muir AD. Proc Flax Inst.
- Thompson LU, et al. Nutrition and Cancer. 1997;
- Muir AD and Westcott ND. Proc Flax Inst. 1996; 56:81-85.
SE, et al. Proc Flax Inst. 1998; 57:8-14.
- Nesbitt PD and Thompson
LU. Nutrition and Cancer. 1997; 29:222-227.
Flax Council of
Canada, 4654-167 Lombard Ave., Winnipeg, MB, Canada R3B
0T6, email: email@example.com
Web site: http://www.flaxcouncil.ca/