By: Marta Barral-Martinez1, Maria Fraga-Corral1,2, Pascual Garcia-Perez1, Jesus Simal-Gandara1,* and Miguel A. Prieto1,2,*
1 Nutrition and Bromatology Group, Analytical and Food Chemistry Department. Faculty of Food Science and Technology, University of Vigo, Ourense Campus, E-32004 Ourense, Spain.
2 Centro de Investigação de Montanha (CIMO), Instituto Politécnico de Bragança, Campus de Santa Apolonia, 5300-253 Bragança, Portugal
The Nutrition and Bromatology Group at the University of Vigo was formed by researchers who develop their activity in the different links of the food chain and share the goal of improving the production, quality and safety of raw materials and food monitoring, as other leading centers at the international level. It was born with a focus on the efficient valorization of byproducts, both non-food waste (crop residues) and low-quality food products, whether vegetable or animal by-products.
Almond, Prunus dulcis (Mill.) D.A.Webb, is the most produced tree nut worldwide. In fact, according to the Food and Agriculture Organization of the United Nations (FAO), more than 3 million tons of almonds are produced annually in an area of more than 2 million hectares. These relevant production rates are also accompanied by an increase in waste production, such as hulls, shells, skins, and blanch water, which are hardly exploited and merit greater attention leading to their industrial valorization [1]. Concerning the importance of the main wastes generated in the almond industry, almond hulls represent the major almond by product covering about 52% of the total fresh weight, followed by almond shells that represent the 33%, the blanch water (11%) and almond skins (4%). With respect to their applications, hulls and skins have been widely reported for their content in bioactive compounds, being mostly applied to biomass production and livestock feed, whereas shells are often incinerated or applied for fuel generation, and blanch water constitutes a by-product that has gained more attention recently and is still considered as an underexploited resource [1,2].
Almond consumption has recently been associated with the development of real health benefits, demonstrated by multiple scientific studies in which almonds have been used traditionally to treat several diseases, including neurological disorders or even respiratory and urinary disorders [2]. Such health-promoting properties are a consequence of the presence of bioactive compounds, as revealed by numerous studies where different properties have been associated with almonds and their by-products, acting as antioxidant, antitumor, antimicrobial, anti-inflammatory, prebiotic, antidiabetic, and anti-obesity agents (Table 1). Therefore, the valorization of almond by-products can allow the recovery of natural ingredients to promote new products in different sectors of the food, pharmaceutical and cosmetic industries.
Among the bioactive compounds present in almond by-products (Figure 1), phenolic compounds constitute the most studied family, especially phenolic acids, flavonoids, and proanthocyanidins, together with other families, including polysaccharides, terpenoids like ursolic, betulinic and oleanoic acids, sterols, and fatty acids, either saturated or unsaturated [1,2]. Concerning the source of these compounds, polyphenols are mainly found in the hulls and skins, whereas the shells are mainly rich in polysaccharides, essentially represented by xylooligosaccharides and hemicelluloses, together with a high proportion of lignin. It is also important to note that the concentration of bioactive compounds is highly dependent on different factors, including the crop, ripening degree, and seasonal and geographical variations, and such influence has been also assessed in the case of almonds and their derived by-products [2]. For instance, the polyphenolic content of almond hulls was reported to depend on the irrigation regime, presenting a higher concentration of total phenols, ortho-diphenols and flavonoids when trees were subjected to continuous irrigation [3]. In parallel, the phenolic content of skins is significantly affected by different processing stages during almond production, such as blanching and roasting, both having a negative effect on the accumulation of polyphenols; in fact, the blanch water obtained as a result of this process has been shown to retain such compounds, exhibiting a very similar phytochemical profile than that of the kernels, containing naringenin 7-O-glucoside and catechins as the most remarkable compounds [1].
Lipids are another relevant family of compounds found in almonds and their by-products. According to the European Food Safety Authority (EFSA), the fat content of almonds is between 40 and 50%, being mostly represented by monounsaturated fatty acids (70%), especially oleic acid, followed by polyunsaturated fatty acids (22%) whose major constituent is linoleic acid, and a little proportion of saturated fatty acids (8%) [4]. As a matter of fact, several authors agree that the sum of oleic, linoleic, palmitic, and stearic acids contributes to 99% of the total fatty acid content of almonds. Due to this composition, the relative higher amount of unsaturated fatty acids with respect to the low concentration of saturated fatty acids makes almond consumption an exceptional diet regime to prevent the onset of cardiovascular diseases [2].
Besides the potential application of almond by-products by the food industry, there is an urgent need to establish novel strategies facing the industrial exploitation of these resources in other sectors. Due to the economical relevance of this nut worldwide, sustainable approaches are required to implement a circular economy model around this industry, with the aim of improving its economical projection and meeting ecological principles in terms of recycling, waste management, and greenhouse gas emission. Consequently, almond shells have been lately characterized as efficient adsorbents for heavy metals during wastewater treatment and for synthetic toxic dyes in the textile industry. Moreover, due to the lignocellulosic nature of this by product, they have been used in the production of wood and bioplastic materials, as well as natural substrate in agriculture [2].
Concerning the production of bioactive compounds, greener strategies are also expected to meet the requirements of such a sustainable circular economy system, by conferring novel methodologies committed to the environmental-friendly extraction and purification of these phytochemicals, as it is the use of next-generation techniques, including ultrasound-assisted and microwave-assisted extractions, or the application of pressurized liquids or supercritical fluids.
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