In general, tree nuts are dry fruits with one seed in which the outer wall becomes hard upon maturing. The most popular tree nuts in the world are almond, Brazil nut, cashew, hazelnut, macadamia, pecan, pine nut, pistachio and walnut. Considering the production of world’s most popular tree nuts, cashew nut ranks first on a global basis with a production of 2,760,000 MT, followed by almond (2,560,000 MT), walnut (2,550,000 MT), Brazil nut (1,000,000 MT), pistachio (940,000 MT) and hazelnut (860,000 MT) in 2010. The production of remaining tree nuts (pecan, macadamia, and pine nut) was around 1.85 MT in the same year [1]. In addition to the main composition (Table 1), some phytochemicals such as tocopherols and phenolic compounds are present in tree nuts [2–6]. Although peanut is a legume, it shares a similar nutrient profile with tree nuts. Therefore, it is used as comparison with tree nuts for the purpose of this review. With today’s busy lifestyles, tree nuts are nutritious, tasty, convenient, and easy snack that contribute to a healthy lifestyle. They are typically consumed as whole nuts (either raw or roasted or salted) or used as ingredients in a variety of processed foods, especially in spreads, bakery, and confectionary products, among others. Tree nut oils are also used for several purposes such as cooking, salad dressings, and flavoring ingredients, among others. In addition, tree nut oils are also components of some skin moisturizers and cosmetic products [2]. According to FAO (2012), the average nut consumption in the world is 2.1 kg/person per year [1]. Tree nut consumption varies both among and within the regions where there is tree nut production. For instance, the European region is shown as having consumption of 2.8 kg/person per year. However, when looking more closely at individual country consumption within that region, Spain and Greece (tree nut producers) have consumption levels of 7.3 and 9.9 kg/person, respectively. The variations in consumption between North and South Europe are even more apparent in the food availability (as kg/cap/year) with EU (4.0), Spain (6.7) and Greece (10.0). Tree nuts are five times more available to Greeks than U.S. consumers. Consumption levels in Asia, with vast population levels, are even more skewed [7].

The consumption of nuts in roasted and salted form is limited. Children and elderly people cannot easily open the nuts (such as pistachio) and consume them. Another problem after processing and before consumption of nuts is their storage and handling which influence the quality of the product. There are many reports about presence of mycotoxins, especially aflatoxin in nuts [9–11]. Increases in the moisture content, air temperature, and air relative humidity are the main reasons for increasing the fungal growth and as a result of aflatoxin production [12]. There is some evidence that contamination occurs during the export process, the sea transport or storage at an imported countries [13]. If the total mycotoxins (especially aflatoxin B₁) level of nuts increased to more than the maximum allowance level, the nuts cannot used by the consumers. There are some reports of rejection of exported nuts due to aflatoxin levels [13,14]. Development of new products (such as nut spread) from nuts and using suitable packaging materials can reduce the risk of losses of product due to contamination to mycotoxins. Development of nut spread would potentially increase the food uses of nuts and introduce consumers to a healthier, non-animal breakfast snack food. In this review, the production of the stable nut spreads and factors affecting their quality will be explained. The effect of processing conditions (roasting and milling) on the rheological behavior, sensory acceptance and oxidative stability of nut spread were also presented in this review.

The term “nut butter” refers to a product that contains at least 90% nut ingredients whereas, the term “nut spread” refers to a spreadable product having at least 40% nut ingredients, which can be added in various forms, e.g., as nuts, a paste and/or a slurry [15,16]. Nut butters and nut spreads are spreadable products made from nuts that are ground into paste. Both nut products can be spread like commercially available butter. They can be produced from almond, cashew, hazelnut, macadamia nut, peanut, pecan, pistachio and walnut. Similar spreads can also be made from other seeds such as sesame seed, pumpkin seed, soybean and sunflower seeds, but they are not categorized as nut spread [17]. Nut spreads have a variety of uses, and the most common use of nut spreads is in sandwich preparation. Other uses include as toppings for edible crackers or as dips for vegetable pieces. Besides that, nut spread is also used in a variety of baking and cooking applications. Nut spread is popular and widely accepted by consumers due to its flavor, good nutritional values and suitability for consumption either alone or in combination with a variety of other foods. The nut-based spreads market in the US increased at a compound annual growth rate of 6.2% between 2004 and 2009 [18]. Since the most important characteristic of nut spread is spreadability, it is of utmost importance that the product should have a soft texture and be easily spreadable to avoid tearing the bread or crumbling the crackers. In addition, since children are the most popular user of nut spread, soft and spreadable product characteristics will help to facilitate the application of nut spread by this age group without assistance from their parents. For this reason, creamy and smooth nut spreads are preferred.

In general, the quality of nut spreads depends on the formulations used (Table 2).

There is a wide variation in the quality of nut spreads as many types of ingredients can be used for the production of spread. Proper combination of these ingredients during production is required to ensure a stable nut spread product. The basic formulation of nut spread usually contains the following ingredients: (a) selected, blanched, dry roasted nuts; (b) sweeteners; (c) vegetable oils; (d) emulsifiers; (e) protein sources; and (f) flavorings [23,25].

One of the famous nut spreads is prepared from peanuts. According to the USDA, peanut butter and peanut spread shall conform to the classification shown in Table 3.

Peanut spread is a paste made from ground roasted peanuts, with or without addition of oil. It is popular across the world and is produced in some emerging markets. It is used as a sandwich spread. Besides peanuts, other types of nuts can be used to make nut spreads [26–28]. Nut spreads are sometimes presented with layers of either jam or jelly in a bottle [29]. These types of layered spread are normally consumed with bread as sandwiches. Figure 1 shows a simple flowchart of nut spread production.

Table 4 summarizes the steps involved in nut spread production, their functions and characteristics of process. Roasting and milling (grinding) are two important stages in nut spread productions.

The overall quality of nut spread is related to the quality of nut paste which is used as the main ingredient. The quality of nut paste is influenced by raw kernel quality, processing conditions such as roasting temperature and time, and storage conditions [29]. Efforts have been made to improve the flavor and texture of nut spreads by addition of moist ingredients such as honey or several types of flavorings. In product development, roasted nut may be used in high moisture systems such as commercial nut spread and jelly. It had also been reported that the shelf life of nut spread depends on the type of ingredients added to the nut paste during production of the nut spread [53,54]. Table 5 lists the quality parameters or attributes of different types of spreads found in the literature.

Rheology is the science of flow and deformation of materials under stress and strain. In the food industry, rheological data are needed for studying the functionality of ingredients in product development, determination of food texture by correlation to sensory data, immediate or final product control and process engineering calculation for equipment such as pumps, heat exchangers, extruders and mixers. Many semi solid food products are made of dispersions of colloidal sized particles such as solids or immiscible liquids and their polymers. The presence of these suspended particles and polymers may affect the stability and the rheology of the suspensions as a result of interactions between them. Several works on the rheological properties of semi solid pastes such as peanut butter [45] and pistachio butter [68,69] had been reported. Citerne (2001) reported that peanut butter behaved like plastic material and apparent yield stress of 24 Pa and 370 Pa for the unstablized and the stabilized suspensions, respectively [45]. Taghizadeh and Razavi (2009), in their study on the time independent rheological properties, reported that pistachio butter behaves like a non-Newtonian pseudo-plastic fluid under yield stress [68]. The yield stress can be used to calculate whether a sample is likely to settle in situ, or whether it will be difficult to start pumping or stirring. Good rheological product design will enhance processing and end use. Although many researchers have investigated the time-dependent rheological behavior of materials, in general, the thixotropic characteristics of many other foodstuffs in food processing are not extensively studied. Viscoelasticity is also one of the rheological properties of materials. In viscoelastic tests, the elasticity part is defined as storage modulus (G’), the viscosity part as loss modulus (G”) and ratio of G”/G’ as damping factor. Shakerardekani (2012) reported that storage modulus, loss modulus and damping factor for pistachio paste were 42943 Pa, 6370 Pa and 0.15, respectively [70]. According to Rao (2007), in any flow process, whether during manufacturing and masticating of the food product, the flow stress affects the structure of the system, which, in turn, affects its rheological characteristics [71]. A rheological analysis of structured water-in-oil emulsions consist of a network of solid fat particles in a continuous oil phase with water droplets captured in it. Chocolate is an example of fat continuous food dispersion, where a continuous network is formed by solid fat, sugar, protein particles and ground cacao particles. All of these systems contain emulsifiers as stabilizers and rheology regulators or fat crystallization regulators. The emulsifiers are effective mainly because of their ability to absorb to different interfaces [72]. All emulsion exhibited a gel-like characteristic with storage modulus higher than loss modulus. Understanding the rheology of emulsions is crucial in predicting the long-term instability of the product particularly due to flocculation and coalescence. Rapid increase in the storage modulus upon ageing can be an indication of strong flocculation [73]. The flocculation of oil droplets usually resulted in liquid entrapment and increases the effective volume fraction of the emulsion. Hence, the increase in the net attraction among droplets resulted in the increase in storage modulus. In contrast, ageing of the emulsions decreases the storage modulus due to droplet rearrangements that led to a weaker structure [74,75].

A broad spectrum of sensory characteristics, including appearance, aroma, flavor, and texture are used by consumers to make purchasing and consumption decisions related to foods [76]. Trained panelists describe a product’s behavior in their mouth in terms of quality and quantity through its mechanical, geometrical, and fat and moisture characteristics from the first bite through complete mastication. Several studies had reported on the usage of Quantitative descriptive analysis (QDA) for characterization of the sensory properties of peanut butter, peanut spread and peanut soy spread [55,66,77,78]. According to McNeill et al. (2002) among important qualities of peanut butter include texture, color, flavor and nutritive value [66]. Texture is one of the sensory properties of foods that play a major role in consumer appeal, buying decisions and eventual consumption. It was found to be the single most dominant attribute of consumer preference of foods [79]. Spreadability is an extremely important attribute of semi-solid food texture. Spreadability is a subjective term related to how easy a sample is uniformly distributed over a surface. Gills & Resurreccion reported that descriptive attributes spreadability highly correlated with consumer attribute spreadability [77]. Beside textural attributes, overall liking of spreads such as hazelnut spread is related to the flavor of the product [27]. Flavor liking cannot be measured directly by instruments; it is an interaction of consumer and product [80]. With a wide range of materials and processing methods in peanut butter manufacturing, processors may not have a clear understanding of the most desirable roasting or storage conditions that contribute to the flavor of their product [81,82]. Pattee et al. (1991) [83] showed that the peanut flavor (nutty, sweet, salty, roasted and rancidity) from sensory analysis is related to the roasted peanut. In the case of roasted nuts, the volatile profiles are highly complex and are composed of compounds arising not only from lipid oxidation, but also from Maillard reaction, Strecker degradation, and caramelization of sugars [84]. Color is another important attributes used by consumers to judge the acceptability of food products [85]. It can be concluded that sensory evaluation can be used to provide further information on the texture, color, flavor and as a result acceptability of nut spread by consumers.

Lipid oxidation is initiated by compounds known as sensitizers which include heat, light and metal ions. Lipid oxidation produces undesirable flavors, aromas and compromises the nutritional quality of fats and oils leading to the production of toxic compounds [86]. The changes in fatty acid composition of lipids provide an indirect measure of susceptibility to lipid oxidation. Nuts may be held for up to 2.5 years under optimum conditions, but under unsuitable storage conditions they become inedible within a month either due to insects, mold, absorption of foreign flavors, discoloration, staleness or rancidity. Fat content alone is not a good indicator of storage stability [87] but the degree of unsaturation or polyunsaturation [88,89], tocopherols [87], chlorophyll and beta carotene, moisture content [90] and temperature [91,92] affect primary lipid oxidation and oxidative stability of intermediate moisture foods during storage. In the presence of oxygen, oxidative reactions are usually of the greatest importance and hence, the storage life is then limited by the development of oxidative rancidity of fat in the food product [93,94].

The protective effects of various antioxidants on the oxidative stability of plant-derived oils and fats have been the subject of intense research, showing great potential to protect oils and fats against oxidation. The use of natural antioxidants and natural products has been widely studied by several researchers [95–97]. Judde et al. (2003) found that the addition of 1% (w/w) soy lecithin was effective in delaying the oxidation in rapeseed, soy, walnut, and palm oils by increasing the induction time by 1.7–1.8 times when measured at 110 °C and decreasing the peroxide values by 2.2–4.6 folds when heated at 40 °C for 35 days [98]. According to Miraliakbari & Shahidi (2008), the oils of pecans and pistachios were the most stable, whereas oils of pine nuts and walnuts were the least stable due to unsaturated fatty acid level [99]. The oxidation of oils and fats can be controlled by application of antioxidants, using processing techniques that minimize tocopherol and other natural antioxidant losses [86,100,101].

Table 6 shows strengths, weakness, opportunities and threats of nut spread industries. Some Strategies for maximizing strengths and opportunities and mitigating weaknesses and threats of nut spread industry are listed in Table 7.

The main gaps in nut spread production are as follows:

Recently, much consumer attention has focused on those who follow reduced carbohydrates and fats or protein and vitamin fortified nut spreads. Low-calorie pistachio butter [117], milk-based fortified spreads [118], peanut spreads fortified with soy flour [119] and protein fortified peanut butter [120] are examples of these nut spreads. While nut spreads tend to be relatively low in carbohydrates, in view of the recent focus by many on minimization of carbohydrate intake, it would be desirable to reduce further the level of carbohydrates in such products. Unfortunately, consumers show little liking for losing organoleptic properties in their favorite foods. Therefore, low carbohydrate products should have good organoleptic properties similar to their full carbohydrate nut spreads. Much of the peanut butter literature focuses on achieving good spreadability and mouth feel, avoiding oil separation, and reducing fat or total calorie content. Particularly in view of the recent attention paid to carbohydrate and fat reduction, there is still a need for nut spreads which are neatly and easily applied and which have reduced levels of fats and carbohydrates.

Nut spreads are spreadable products made from roasted nuts. In addition to the ingredients (nuts, sugar, vegetable oil, and emulsifier), the roasting conditions of nuts, particle size distribution and type and amount of stabilizer affected consumers acceptability, rheological behavior and oxidative stability of nut spreads. The oxidation of oils and fats can be controlled by application of antioxidants, using processing techniques that minimize tocopherol and other natural antioxidant losses. Recently, much consumer attention has focused on reduced carbohydrate and fat nut spreads. Although much of the literature has reported on nut spread production, most studies were related to peanut butter and peanut spread. It is recommended that further research be undertaken to develop other types of nut spreads with prolonged shelf life.