Nuts are a rich source of proteins and essential amino acids as indicated by the USDA National Nutrient Database for Standard Reference [10]. The major sources of proteins are peanuts, almonds, and pistachios, while chestnuts are the poorest in proteins. Chung et al. [11] reported higher protein content for some of these nuts, which can be ascribed to different geographic regions. The protein content also varied within the same nut species, denoting a significant effect of cultivar [12,13,14,15]. Other factors, such as the harvest year, post-harvest storage, and even the processing method [16] can affect the content of proteins in nuts. For example, Dodevbka et al. [17] reported differences between raw, boiled, and roasted nut samples from Serbia. The seed storage proteins are the main type of proteins present in nuts and are responsible for nut allergies [18].

Regarding the amino acid profile of each nut, there is a considerable variation in the content of essential and non-essential amino acids. The nut protein composition is dominated by hydrophobic amino acids, followed by acidic, basic, and hydrophilic amino acids [19]. Among the non-essential amino acids, glutamic acid is the most important, ranging from 0.02 g/100 g in chestnuts to 6.21 g/100 g in almonds. The second major non-essential amino acid is arginine ranging from 0.12 g/100 g in chestnuts to 3.08 g/100 g in peanuts, followed by aspartic acid that ranges between 0.03 g/100 g in chestnuts and 3.15 g/100 g in peanuts. Leucine is the most essential amino acid, followed by phenylalanine and valine. Chestnuts present the lowest values of these essential amino acids (0.10, 0.07, and 0.09 g/100 g for leucine, phenylalanine and valine, respectively), while peanuts are the richest source of leucine and phenylalanine, and pistachios are the richest source of valine.

The essential amino acid contents and their digestibility determine the nutritional value of a food protein. Although nut proteins are often recognized as incomplete proteins (i.e., do not contain all essential amino acids) when compared to animal proteins, their consumption is strongly associated with cardiovascular health [22]. Moreover, the presence of large quantities of arginine in all tree nuts has positive effects on immune response and inflammation, and cardiovascular function, including its key role in reducing the risk of cardiovascular disease and reproductive performance [23]. The health benefits of nut consumption can be enhanced by combining different protein sources to provide adequate levels of all essential amino acids.

Vitamins are essential for a balanced and healthy diet. Nuts contain fat-soluble vitamins (ascorbic acid, B1, B2, B3, B6) and antioxidants such as α-tocopherol (vitamin E), promoting better health, playing an important role against the aging process, improving brain function, and helping consumers to have healthy skin [24,25]. According to studies carried out by several researchers, the existence of vitamin C (ascorbic acid) is an important antioxidant for human colon cells [26,27]. The nut’s nutritional value depends on its chemical composition, and this is the result of the interaction of the cultivar (genotype), meteorological factors such as temperature and radiation, and production practices [28,29,30].

Nuts are also rich sources of minerals such as magnesium and potassium. In recent years, increased consumption of nuts has been considered good for human health to increase the intake of certain minerals, and they are considered a heart-healthy snacks when eaten in moderation [34]. Nuts are an important food source of minerals such as copper and magnesium. These two minerals may be protective against coronary heart disease. Nuts are also fairly high in potassium, particularly pistachio and cashew nuts. Most nuts have a decent amount of zinc and iron, but pine nuts, cashews, and almonds stand above the rest. In contrast, nuts do not have a high content of calcium, still, some nuts such as almonds are better in terms of calcium content.

Fiber is a health-promoting nut ingredient. The intake of dietary fiber is inversely related to obesity, type two diabetes, cancer, and cardiovascular disease according to epidemiological and clinical studies [38]. Among nuts, almonds present the highest content of fiber, with a clear effect of genotype influencing its amount recorded [39]. Some works have highlighted the influence of genotype on the fiber content found in almonds [40], ranging from 6.88% to 9.74% in blanched almonds [41], showing that almond skin is also responsible for the fiber content of this nut, as it is composed of around 60% of fiber [42]. However, not all available data follow the same trend, with similar values of fiber recorded for different cultivars [43,44,45]. Cashews have the lowest fiber content among the referred nuts, with recent works pointing at values always around 3% to 4% [46], with no apparent significant effect of the cultivar on its content, although comprehensive studies are lacking for this specific nut. Chestnuts are considered to be a good source of dietary fiber [47], with similar values to those of cashews. The content of fiber in chestnuts has been the subject of studies that cannot find a trend on the factors behind their variation. Some authors point out the clear effect of cultivar on fiber content [43,44,48,49] or area of production [50] or year [43,44]. However, other works clearly state the similar content of fiber, regardless of cultivar [51,52,53]. The hazelnut fiber content is usually referred to as ranging from 6.5 g/100 g to 9.7 g/100 g [54]. Researchers have found higher amounts of fiber in some cultivars, such as the Turkish tombul hazelnut (12.9 g/100 g) [54], or other cultivars, with fiber values ranging from 9.8 g/100 g to 13.2 g/100 g (dry weight basis), being lowest in Tonda di Giffoni and highest in Campanica [55]. This also shows the variation of fiber content among cultivars, also recorded in the comparison of sixteen hazelnut cultivars [56]. For pistachio, the available works dealing with fiber content are few. However, early data indicate a content of 1.1–2.0% [57,58] although more recent works show considerably higher values. Dreher [59], Bulló et al. [60], and Terzo et al. [61] refer to values of fiber as around 10%, with Rabadán et al. [39] suggesting that the major factor between variations is the crop year and related weather conditions. Finally, walnut presents an intermediate amount of fiber when compared to other nuts. Although the majority of available works indicate values ranging from 4% to 6% [59,62,63,64,65], some authors have found considerably different amounts of fiber, namely Özcan [66], which indicates 1.8%, and Özcan et al. [67] that reports values between 3.8% and 3.9%. Again, the major factor affecting the fiber content of walnut is the genotype, with a slight effect also found to be caused by the crop year and related weather conditions [39].

Nuts are rich in several nutrients, although with great differences between them and minor but sometimes still significant variations within cultivars. Lipid content and fatty acid profile are two of the parameters that can change considerably when discussing nut composition. Besides these great variations between nut species, changes in lipid content and profile can also occur due to several other factors, with genotype as one of the most important that influences nut composition. Recent works show that genotype and the environment are key factors behind changes in several compositional parameters of some nuts, namely fat content [39].

For almonds, Summo et al. [69], working with samples from a germplasm collection under the same growing condition, recorded variations of lipid content, depending on the cultivar, from 42.4% to 56.2% (fresh weight). Barreca et al. [70] also reported a significant cultivar effect on the content of lipids in almonds. Almonds are also known for their interesting fatty acid profile, which is mainly composed of monounsaturated (MUFA) (60%) and polyunsaturated (PUFA) (30%) fatty acids, with a predominance of oleic, linoleic, palmitic, or stearic acids [43,44,71,72]. The work of Summo et al. [69] also shows the effect of the genotype on the fatty acid profile. Although major fatty acids are the same across the studied cultivars, changes can be observed in the individual amount of each fatty acid, as well as for the sum of unsaturated (mono- or polyunsaturated) and saturated (SFA) fractions. For cashew nuts, recent studies show great variability in fat content and associated fatty acid profiles when comparing different production regions. The work of Rico et al. [46], analyzing 11 cashew origins, shows that fat content can vary from 45.05 g/100 g in Vietnamese samples to 50.40 g/100 g in samples from Kenya. In the fatty acid profiles, oleic, linoleic, and palmitic acids are the three major ones. Although monounsaturated fatty acids represent the major fraction in all samples, followed by saturated fatty acids, at least in one sample, the second most important fraction is polyunsaturated fatty acids.

Chestnuts are featured with low-fat content and compared to other nuts, such as hazelnut, macadamia, pecan, or almond, chestnuts, exhibit the lowest fat content [73]. However, in this minor chestnut fraction, fat-soluble bioactive compounds, such as tocols and phytosterols, are present in higher quantities when compared to fat-rich nuts. They contain a high quantity of essential fatty acids (those that must be provided by food intake, as they are not synthesized in the body but are necessary for health) [74,75,76], either saturated or unsaturated, linked to several processes involved in health and chronic diseases [77]. Among them, the most important unsaturated fatty acids are linoleic and linolenic acids [75,78]. Fat content and fatty acid profiles can, as for other nuts, change significantly among cultivars. A thorough study of 17 chestnut cultivars produced in Portugal shows significant variations ranging in fat content from 1.67% to 3.50% [76]. Chestnut fat is primarily composed of three fatty acids, namely linoleic, oleic, and palmitic acids, with a predominance of polyunsaturated fatty acids. However, when comparing samples, significant variations of these fractions can be seen, with some presenting almost the same amount of mono- and polyunsaturated fatty acids. Similarly, the amount of saturated fatty acid also recorded significant variations across cultivars.

Among nuts, hazelnut presents one of the highest contents of fat, above 60%, with some authors indicating the amount of fat above 70%, depending on the cultivar [79] or even on the canopy position of the fruits [80]. The fat present in hazelnuts is mainly composed of MUFA, representing around 80% of the total fatty acid content, and oleic acid is the major individual monounsaturated fatty acid [81,82,83]. Polyunsaturated fatty acids represent the second major fraction in hazelnut fat, almost exclusively due to the content of linoleic acid [84,85]. However, some works have found that SFA can represent the second major group of fatty acids [66,69], influenced by the higher content of palmitic acid.

Like most other nuts, pistachio is rich in fat, the available works indicating values around 50% [86,87,88,89], although some cultivars can have increased fat content, reaching values as high as 74.15% [90]. Following the trend of other nuts, pistachio fat is rich in unsaturated fatty acids, namely MUFA. This fraction is mainly composed of oleic acid, with a contribution from palmitoleic acid, while the second most important fraction, PUFA, is mainly composed of linoleic acid [83,89,91]. Regarding SFA, the minor fatty acid fraction is made almost entirely of palmitic acid [82,86].

The fat content of walnut is very high, with average values that can be surpassed only by hazelnuts [82]. Although the fat content is in the 60% range, considerable variations have been observed when comparing cultivars. Values varied between 49% [92] and 82% [93]. However, as referred before, most of the works show values of fat around 60%, with some variations associated with the studied cultivar [94,95]. Walnut fat is mostly composed of unsaturated fatty acid, namely PUFA, while MUFA is the second most important type of fatty acid [96,97,98]. Linoleic and linolenic acids are the ones responsible for the high amount of PUFA, with oleic as the major MUFA. Regarding SFA content, palmitic and stearic are the ones present in higher amounts [92,94,95].

Like in numerous other crops, phenolics are present in nuts. Many studies are reporting the beneficial effects of nut consumption on human health, including cardioprotective, neuroprotective, antidiabetic, anti-inflammatory, and antioxidant properties [99,100,101,102,103]. Studies have shown that the consumption of nuts improves flood lipoprotein profile [104,105] and gut microbiota [106]. These health effects are mainly due to the presence of several type of compounds, including phenolics, as reported by Lamuel-Raventos and Onge [102]. Each nut species presents its typical phenolic profile and content. For example, Liu et al. [107] found a high content of phenolics, such as vanillic acid, catechin, naringin, quercetin, and ellagic acid, in chestnuts, while Smeriglio et al. [108], in almonds, found a high content of phenolics, such as quercetin, kaempferol, and isorhamnetin. Instead, Taş and Gökmen [109] reported high levels of procyanidins A and B, trimers and tetramers, and prodelphinidin in peanuts.

The most abundant phenolics in almonds are catechin, epicatechin, protocatechuic acid, ferulic acid, kaempferol, and isorhamnetin [108,117]; in chestnuts are gallic acid, vanillic acid, syringic acid, catechin, and ellagic acid [119]; while in hazelnuts, the preponderance is for the catechin, epicatechin gallate, and gallic acid [121,122]; in peanuts, p-hydroxybenzoic acid, p-coumaric acid, ferulic acid, and epicatechin dominate [124]; in pistachios, gallic acid, syringic acid, catechin, and epicatechin [126]; while pecans and walnuts have in common high contents of chlorogenic, caffeic, p-coumaric, ferulic, ellagic and syringic acids [118]. In general, all nuts have in common the presence of high amounts of phenolic acids and flavonoids. The anthocyanins are present in vestigial amounts and are therefore not considered.

All these compounds are highly important because they have been associated with important beneficial effects on human health, as reported in the review of Lamuel-Raventos and Onge [102] and De Souza et al. [103]. Consumer perception of their beneficial effects has increased the intake of nuts. Different important findings from researchers have also contributed to the increment of such products in the human diet. For example, Brown et al. [130] found that higher nut consumption was associated with a reduced prevalence of high cholesterol and blood pressure, diabetes, and gallstones, due to the richness of phenolic compounds. In addition, Musarra-Pizzo et al. [131] tested a mix of phenolics present in natural almond skin and found that epicatechin and catechin were able to stop the growth of Staphylococcus aureus, suggesting that extracts from almond skins can be used to develop novel products for topical use. Neuroprotective effects against Alzheimer’s disease were found in almonds, hazelnuts, and walnuts due to their richness in tocopherols and phenolics [132].

The aroma compound profile of nuts is dependent on geographical origin and thermal processing and the presence of microorganisms. In almonds, several studies indicate aldehydes as the major volatiles, namely benzaldehyde [133,134,135] with a characteristic bitter-almond flavor, although this compound might not be found in several cultivars [136,137]. Besides terpenoids and substances derived from amino acids, volatiles are usually present as a result of the oxidation of fatty acids [138].

Processing causes several modifications, either in the number of compounds, but also in the chemical classes present [136,137]. In the work of Elmore et al. [139], they verified that walnuts from China and Ukraine contained high levels of lipid-derived volatiles from the linoleic acid breakdown (hexanal, pentanal, 1-hexanol, and 1-pentanol) and α-linolenic acid breakdown (1-penten-3-ol), whereas Chilean walnuts contained high levels of alkylbenzenes. Pyrazines are the major group of aromatic compounds in peanuts. They are formed by the thermally induced the Maillard reaction. The same applies to other nuts, such as pistachio and hazelnut. It is the roasting process that makes the fruit commercially viable and valuable, improving the nut’s sales and sensory characteristics [86]. Two pyrazines represent peanut flavor: 2,5-dimethyl pyrazine (with a characteristic nutty aroma) and 2-methoxy-5-methyl pyrazine (roasted nutty aroma).

In hazelnuts, the results from Kiefl and Schieberle [140] showed that the aroma-active compounds 2-acetyl-1-pyrroline, 2-propionyl-1-pyrroline, 5-methyl-(E)-2-hepten-4-one (fibertone), 2,3-diethyl-5-methyl pyrazine, 3,5-dimethyl-2-ethyl pyrazine, and 2-furfurylthiol are appropriate odorant indicators to distinguish the several nut aromas. Specifically, the roasted or nutty aroma of roasted hazelnuts was developed if both 5-methyl-(E)-2-hepten-4-one and 3-methyl-4-heptanone were higher than 450 μg/kg, whereas the sum of the two 2-acyl-1-pyrrolines and two pyrazines should not exceed 400 μg/kg to avoid an over-roasted odor. A favored aroma can be obtained for each cultivar if specific temperatures, roasting techniques, and roasting times can be applied.

One major quality concern related to nuts is the development of off-flavors due to the formation of oxidative degradation products [141,142]. Various volatiles are involved in off-flavor; 1-Pentanol, 1-hexanol, and hexanal are the most important volatiles involved in off-flavor, and their presence at the highest levels is a synonym of nut degradation.