Phytosterols or plant sterols are natural constituents of plants. They resemble mammalian cholesterol, both in their chemical structure and their biological function (Piironen et al., 2000). Phytosterols regulate the fluidity and permeability of membranes and also play many other functions in plants, for example as precursors of brassinosteroids, an important group of plant hormones involved in many aspects of plant growth (Hartmann, 1998).
Phytosterols are also important dietary components. Vegetable oils and oil-based products are the richest dietary sources of phytosterols followed by cereal grains, cereal-based products, and nuts (Piironen et al., 2000). Sterols in foods exist as free sterols, fatty acid esters, steryl glycosides, and acylated steryl glycosides (Phillips et al., 2005a). Because of the similarity in the chemical structures of phytosterols and cholesterol, dietary phytosterols reduce intestinal absorption of both dietary and endogenously produced cholesterol, thus reducing serum cholesterol levels (Plat and Mensink, 2005). Because of their cholesterol-lowering properties, phytosterol-derived products such as phytostanol esters have become important ingredients in a wide range of functional foods (Bacchetti et al., 2011).
Almond is the most important tree nut crop in terms of commercial production, which is limited to areas characterized by a Mediterranean climate (Kester and Asay, 1975). Kernel quality has become an important criterion for modern almond cultivars (Socias i Company et al., 2008). Almond germplasm collections have been evaluated for variation in kernel quality traits such as oil content, fatty acid composition (Kodad et al., 2011), and tocopherol content (Kodad et al., 2006). However, there is little information on the variability in phytosterol content in almond germplasm, because the literature only provides results from analyses of commercial almond samples. For example, the analysis of a single sample of almond kernels from a local market (Normén et al., 2007) revealed a phytosterol content of 2080 mg·kg−1. Another study on four different samples of almond kernels from the U.S. market (Phillips et al., 2005b) reported an average phytosterol content of 1990 mg·kg−1 (from 1930 to 2080 mg·kg−1). Similarly, the analysis of three almond samples from the U.S. market (Robbins et al., 2011) showed an average kernel phytosterol content of 2107 mg·kg−1. The U.S. Department of Agriculture (USDA, 2011) National Nutrient Database reports a kernel phytosterol content of 1720 mg·kg−1. Oil phytosterol levels between 2178 and 2750 mg·kg−1 have been reported for raw almond oils (Miraliakbari and Shahidi, 2008), whereas a phytosterol content of 1999 mg·kg−1 has been reported for commercial almond oil (Dulf et al., 2010). The USDA (2011) National Nutrient Database reports an almond oil phytosterol content of 2660 mg·kg−1. Phillips et al. (2005a) found that 78% of the phytosterols in almonds are in the form of free and esterified sterols, whereas the remaining 22% are in the form of steryl glycosides and acylated steryl glycosides. The latter compounds have similar cholesterol-lowering properties to free and esterified phytosterols (Lin et al., 2009). However, their relevance for oil quality is limited, because very low levels of steryl glycosides and acylated steryl glycosides are extracted by conventional hexane extraction (Moreau et al., 2003).
There are some discrepancies in the literature about the composition of almond phytosterols. Although all the studies have identified β-sitosterol as the predominant phytosterol in almond, they differ in the proportions of the other phytosterols. Several studies (Dulf et al., 2010; Normén et al., 2007; Phillips et al., 2005a, 2005b; Robbins et al., 2011) have reported a phytosterol fraction mainly made up of β-sitosterol (from 72.1% to 83.9%) and Δ5-avenasterol (from 9.9% to 12.9%) and to a lesser extent campesterol (2.5% to 3.4%) and stigmasterol (1.6% to 3.4%). However, other studies have reported different phytosterol profiles characterized by absence or lower levels of Δ5-avenasterol (Cherif et al., 2009; Maguire et al., 2004; Miraliakbari and Shahidi, 2008).
One of the most important world collections of almond cultivars is located at Centro de Investigación y Tecnología Agroalimentaria (CITA) of Aragón, Spain, with ≈250 accessions introduced from all over the world (Espiau et al., 2002). This collection shows a very large variability reflecting the wide genetic diversity of almond (Socias i Company and Felipe, 1992). The objective of this research was to assess genetic diversity for total content and profile of free and esterified phytosterols in the CITA world germplasm collection.
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