In Part 1 of this blog, I provided an overview of the tremendous antioxidant capacity of flaxseed for human health. In this article, the role of these antioxidants in providing stability to flax, and the significance of this for food applications will be described.
While it is well accepted that foods and dietary supplements prepared with polyunsaturated fatty acids (PUFAs) such as plant based alpha-linolenic acid (ALA) are healthier, they also have an increased vulnerability to rancidity which is caused by oxidation, the chemical reaction of lipids with oxygen (1).
Despite the high level of ALA in flaxseed, research has shown that ground (milled) flaxseed can be stored at room temperature for at least 4 months (2). In this study, two samples of flaxseed, were milled and stored at 23 ± 2°C for 128 days in paper bags with plastic liners. Samples were evaluated at 0, 33, 66, 96, and 128 days for chemical, sensory, and volatile indicators of quality. None of the samples showed a significant increase in measures of oxidation – peroxide values or conjugated double bonds – throughout the 128 day storage period. Overall, the levels of total volatiles in the milled flaxseed samples were much lower than levels reported in stored vegetable oils containing significantly lower levels of ALA. Dienals, formed during PUFA oxidation, and hexanal, a compound used as an indicator of oxidative deterioration, were found at very low levels in both samples and did not reach high levels throughout the 128 day storage period. A trained sensory panel could not detect any differences in the odor properties of fresh or stored milled samples. No differences in flavor could be detected between bread made with 0 and 128 day milled flaxseed.
In a follow-up study, it was reported that ground flaxseed showed remarkable stability when stored at ambient temperatures for 20 months, indicating the presence of a strong protective system preventing oxidation (3). The stability of ground flaxseed has been attributed in part to the lignans since both secoisolariciresinol and enterodiol, the lignans found in flaxseed, exhibit greater antioxidant activity than Vitamin E (4).
The Importance of Processing
Proper processing to protect the antioxidant cocktail in flaxseed is important. Preventing the oxidation of ALA is the key to using flax ingredients in food formulations. Milled flaxseed should be purchased from reputable companies that understand the role that seed selection and processing play in overall quality.
The color of flax varies from golden brown through to a dark red brown usually due to differences in growing conditions. Some of the seeds however, have a darker color which is visibly distinguishable. Immature flax seeds have a green color. A positive correlation between the percent of ‘dark’ colored seeds in a seed sample and the level of oxidation products has been reported (5). Analytical research has shown that dark and damaged seeds are more likely to oxidize, leading to rancidity and deterioration in health benefits (6, 7).
Companies that provide shelf life guarantees for milled flaxseed of upwards of two years can do so because they ensure the removal of ‘dark’ seeds and of immature seeds. Gentle processing also preserves the very active and stable antioxidant system which naturally occurs in flaxseed (7).
Mother Nature has provided flaxseed with a large number of diverse antioxidants to protect the fragile ALA for the developing seed. These antioxidants are not only extremely healthy, but when the seed is properly processed yield ingredients with stability characteristics of tremendous value to food processors. Thank you Mother Nature!
References:
- Fitzpatrick, K. and Eskin, M. 2005. Functional lipids within the Global Functional Food and Nutraceutical Sector. In: Handbook of Functional Lipids. ed. C.A. Akor. CRC Press. NY, New York, USA.
- Malcolmson LJ, et al. J. Am Oil Chem Soc. 2000;77:235-238.
- Przybylski R, et al. J. Am Oil Chem Soc. 2001;78:105-106.
- Hu, C, et al. Food Chem Toxic. 2007; 45(11): 2219–2227.
- G.R. Pizzey. 2006. U.S. Patent Application. Flax-Stabilized Fatty Acid Compositions and Methods.
- Daun JK, Barthet VJ, Chornick TL, Duguid S. 2003. Structure, composition, and variety development of flaxseed. In: Flaxseed in Human Nutrition, 2nd ed. Thompson LU, Cunnane SC, eds. Champaign: AOCS Press: 1-40.
- Westcott ND, Muir AD. 2003. Chemical studies on the constituents of Linum spp. In: Flax—The Genus Linum. Muir AD, Westcott ND, eds. New York: Routledge; 55-73.