**3. Flaxseed structure and chemical composition**

An embryo with two cotyledons that is enclosed by a thin endosperm and a smooth, frequently lustrous yellow to dark brown seed coat makes up mature flaxseed, which is oblong and flattened (hull) [15]. Flaxseed sprouts are edible, with a slightly spicy flavor. Whole flax seeds are chemically stable, but ground flaxseed can go rancid at room temperature in as little as 1 week, although there is contrary evidence. A longer amount of time will be prevented from ground flax going rancid by refrigeration and storing in airtight containers. When packed quickly without exposure to air or light, milled flax is impressively resilient to oxidation for 9 months at room temperature and for 20 months at ambient temperatures under warehouse conditions [16, 17].

The components of this herb are summarized in **Table 1**. Also, the content of vitamins, minerals, and active compounds in flaxseed is summarized in **Table 2**. The protein content in flaxseed has been reported as 10–30% in some studies [18–20]. In terms of protein distribution, the highest amount of protein is in the cotyledons of the flaxseed (between 50 and 70%), and about 30% is in the coat and endosperm [21]. The amino acid profile of flaxseed protein is comparable with that of soy. As shown in **Table 1**, higher amounts of the some amino specially arginine, valine, glycine, leucine, valine, and serine have been found in flaxseed [22]. In terms of essential amino acid index of flaxseed, it has been reported that the score for this index for flaxseed is 69, and this score is near the score for canola and soy, that is, 75 and 79, respectively [23]. Also, according to the Food and Agriculture Organization (FAO) reports, flaxseed score based on the limiting amino acid is 82; however, this score for soybean is 67 [24]. It has also been shown in some studies that 34.3% of the amino acids in flaxseed are essential amino acids, and the ratio of lysine to arginine, which is an indicator of cholesterolemic and atherogenic effects of a protein, is about 0.22–0.37, which reveals that flaxseed is less atherogenic than canola and soybean [25].

The Canadian Grain Commission examined brown Canadian flaxseed, finding that it typically included 41% fat, 20% protein, 28% total dietary fiber, 7.7% moisture, and 3.4% ash [3]. As reported, flaxseeds are rich in oil (40–46% by seed weight) with a healthy fatty acid profile. In terms of type of fatty acids, flaxseed contains about 73% polyunsaturated fatty acids (PUFAs) and 10%


#### **Table 1.**

*The most important flaxseed components.*


#### **Table 2.**

*The content of vitamins, minerals, and active compounds in flaxseed.*

saturated fatty acids. Also, in terms of the type of PUFAs, the evaluation results on different samples of flaxseed show the presence of about 52–57% α-linolenic acid (ALA) and an ω-3/ω-6 ratio of 1:0.3 [26]. Considering that higher amounts of ALA make the oil prone to oxidation and Linola seeds have about 5% less ALA compared with the brown flaxseed, therefore, this type of flaxseed is more resistant to oxidation than brown flaxseed and for applications such as frying and shortenings are more suitable [11].

As shown in **Table 2**, in terms of minerals and vitamins content, flaxseed is one of the appropriate sources of calcium, phosphorus, potassium, magnesium, and γ-tocopherol [3]. In addition, flaxseed is a good source of some active compounds such as phenolic compounds. It has been reported that some of the bioactive phenolic compounds are ferulic acid, chlorogenic acid, and gallic acid [27]. Cyanogenic glycosides (CG), phytates, lignans, and antipyridoxin factors are some of flaxseed's minor constituents that have been identified. Flaxseed has amounts of the lignan secoisolariciresinoldiglucoside (SDG) that are 75–800 times higher than those of any other documented plants or vegetables [26]. Flaxseed is one of the best dietary functional foods in terms of supply lignan, and the amount of lignan in it varies between 0.9 and 3% depending on various factors [11]. Based on the evidence in some experimental studies, the lignans in flaxseed have anti-estrogenic effects and may have a preventive role in the pathological process of some hormone-dependent cancers [28]. The most predominant lignan in flaxseed is SDG. Enterolactone and enterodiol, which have antiestrogenic properties and structural similarities to estrogen and can bind to cell receptors to inhibit cell proliferation, are formed from SDG [28]. Numerous items made from flaxseed are being sold in the market and promoted for their benefits as functional foods and nutraceuticals. Whole seeds, ground or milled flaxseed (flour), oil extracted from flaxseed (by pressing; cold-pressed or not), flax meal, the coat of the seed, a portion of the seed that has been removed, flaxseed hull, cyclic peptides

*Perspective Chapter: Flaxseed (*Linum usitatissimum *L) – Chemical Structure... DOI: http://dx.doi.org/10.5772/intechopen.108259*

(orbitides) from flaxseed oil, and lignan extracts from the flaxseed hull are among them [29]. Some of the experimental studies evaluated the digestibility of proteins in the flaxseed, and in some studies, the digestibility percent of this plant has been reported as 81.4–85.8% [30, 31], and this is near the soybean digestibility percent (digestibility percent = 84–85%) [32]. Also, the flaxseed biological value (BV) ranges from 66.4% to 77.4% [33]. The matrix elements, particularly mucilage and oil, and the earlier preparation of the seeds have an impact on the flaxseed protein digestibility [34]. It has been reported in some experimental studies that elimination of flaxseed mucilage improved protein digestibility to 50% compared with the 12.6% for full-fat and mucilage containing ground flaxseed [35]. Also, it has been reported that some of the other processing methods such as boiling, heat treatment can improve flaxseed protein digestibility [36]. One of the concerns related to flaxseed is allergy to the its proteins. However, to date, the results of several studies have reported rare cases of allergy to the proteins contained in flaxseed. Some researchers believe that this allergy observed in some cases is not due to flaxseed proteins, but the reason is the contamination in some seeds [37].
