**5.1 Blanching and kernel peeling**

Tree nut kernels with bright and white color are appealing to the consumers and are typically required for producing meals or milk, or consuming raw in salads [103]. For this purpose, blanching and peeling of kernels are usually needed. The peel on the kernel surface, also known as 'pellicle', has dark color and usually has high contents of natural antioxidants, such as tannins, phenolics and flavonoids [104]. They protect the kernel from natural oxidations and show good antibacterial activities [105]. However, due to the abundant antioxidants and fiber, pellicles usually have bitter taste, high chewiness, and low solubility in drinks, which lowers the sensory satisfaction and affect the palatability [106]. Kernel peeling is mainly accomplished by physical or chemical methods.

Physical peeling involves blanching and mechanical abrasive peeling [107] and is the recommended method in the almond industry [108]. However, abrasive peeling is not recommended for walnut kernels since walnut kernel has irregular shape and abrasion may also result in significant loss of nut flesh. During blanching, nut kernels are subjected to either water soaking or steaming at high temperatures, which cause the pellicle to swell and crack [109]. After blanched, the tree nuts go through soft rubber rollers to mechanically peel off the pellicle by abrasion [106]. Besides, blanching also showed the capability to protect the color of the tree nut kernels [110], and control the cross-contamination of aflatoxins in almonds [111]. The disadvantages of blanching include the increase of MCs, softening of the nut flesh and leakage of polyphenols into the water [112], which brings up the need to further dry the kernels after peeling and loss of antioxidant activities. The large water consumption is another concern for the sustainability, particularly in the drought regions.

Chemical peeling usually refers to the hot lye peeling by NaOH, Na2CO3, and Ca(OH)2, etc. [113, 114]. In a typical process, nut kernels are soaked in hot lye solutions for several minutes to corrode the pellicles, then rinsed with water [109]. Factors, such as alkali type, concentration, temperature, and soaking time, are important for the peeling performance [115]. Although lye peeling is efficient and effective, it may cause significant quality deteriorations, such as texture softening, surface browning, loss of crude protein and fat, decrease of antioxidant activities, and increased oxidation [113]. If the rinsing of peeled nuts was not performed adequately, the chemical residues on the nut surface cause additional food safety concerns. More importantly, disposal of the wastewater from lye peeling requires excessive use of chemicals and may cause severe environmental impacts.

For these reasons, novel and safe peeling methods have been studied and developed. IR radiation can penetrate the pellicle and heat up the kernel, which cause moisture evaporation and accumulation of vapor pressure under the pellicle. Meanwhile, IR heating may cause pyrolysis of pectin substances. When the vapor pressure reaches a critical level, the pellicles crack and can be peeled [109, 116]. Zhao et al. [117] developed a cryogenic peeling system for walnuts, in which the walnuts were held at −160°C by cold gas/liquid N2 and moved dynamically downwards, and the shrinking pellicles were removed by upflowed air. Studies have shown that walnuts and almonds with their pellicles peeled off had shorter shelf life compared to the unpeeled ones [106, 118]. Therefore, applications of edible coatings containing antioxidant substances, such as Mastic gum, chitosan incorporated with green tea extract, walnut phenolic extracts, and mango kernel starch, etc. are popular research topics in recent years [118–121]. Meanwhile, the sensory quality of the nuts with edible coatings should not be compromised.

#### **5.2 Roasting**

Roasting is a commonly used processing to improve the palatability of tree nuts, which is usually done at a temperature higher than 90°C [122]. Maillard reaction between the carbonyl group of reducing sugars and the amino groups of proteins in the nuts is responsible for the nonenzymatic browning and formation

#### *Processing of Tree Nuts DOI: http://dx.doi.org/10.5772/intechopen.102623*

of substances with desired 'roasted aroma and flavor' (e.g., pyrazines, furans, and pyrroles) [123–125]. During roasting, nuts are further dried and are subjected to some change in texture properties, which gives rise to the crunchy mouthfeel. The texture properties of tree nuts (hardness, fracture force, firmness, etc.) are significantly affected by the roasting temperatures [126, 127]. Meanwhile, roasting is also an effective measure to reduce the aflatoxin contents in tree nuts [128].

Conventionally, tree nuts are roasted by HA or oil [129]. HA roasting is usually performed at a temperature ranging from 100 to 180°C for up to 60 min [130]. The main problems with this method are the long processing time, high energy consumption and non-uniform roasting, which should be attributed to the slow convective and conductive heat transfer with HA heating. During oil roasting, tree nuts are immersed and fried in a vegetable oil at a temperature near 180°C for 7–8 min, followed by drying to remove the oil from surface [131]. Although the roasting process do not significantly reduce the contents of macronutrients, it also has some disadvantages. The major concern associated with roasting are the severe oxidation of polyunsaturated fatty acids that give rise to 'off-flavor' and the risk of toxin generation over the smoke point under the high temperature processing [132]. Some main chemical reactions related to the quality change during tree nut roasting are shown in **Figure 7**. In addition, oil roasting results in high oil intake in the products. Therefore, new roasting technologies need to be more efficient and cost effective, while not compromising the product quality and safety.

Formation of acrylamide, a group 2a carcinogen by WHO, from the free asparagine and reducing sugars in the nuts arises another food safety concern. Asadi et al. [133] found IR roasting caused the highest acrylamide content in pistachios, and MW roasting led to the lowest. Increasing the roasting temperature and time, and MW power facilitated acrylamide formation, since the formation rate of acrylamide increased with temperature [134]. Milczarek et al. [135] suggested that MW roasting was a promising method to replace the conventional HA roasting for almonds. Adding of salt during roasting can mitigate the acrylamide formation in tree nuts, which should be due to the prevention of intermediate (such as Schiff base) formation under the inhibition of cations [136]. In response, ABC [137] suggested that almonds should be roasted at the lowest possible temperature to minimize the acrylamide formation. Corradini and Peleg [138] found that using a stepdown temperature heating may reduce the acrylamide contents in baked goods. Bagheri [139] suggested that use of IR heating together with HA or MW heating reduced the roasting time while obtaining similar product quality. Future research


**Figure 7.**

*Main chemical reactions related to quality change of nuts during roasting.*

in tree nut roasting should focus on the development of novel roasting methods that combine the advantage of different heating methods, optimization of roasting parameters (using stepwise temperature roasting strategy) and addition of appropriate food-safe additives to preserve product quality, reduce acrylamide formation, and improve microbial safety and energy efficiency.
