**4. Food safety processing: disinfection and disinfestation**

Tree nuts are vulnerable to contamination by pathogenic microbes, such as *Salmonella* species, *Escherichia coli* strains and *Aspergillus flavus*, as well as molds and mildew [70, 71]. Two severe worldwide outbreaks of *Salmonella* occurred in 2001 and 2004 that were traced back to California almonds, which caused more than 200 illness cases in more than 15 US states or countries [72]. In response, ABC and United States Department of Agriculture (USDA) required all raw almonds to be pasteurized with at least 4-log reduction in the *Salmonella* population [73]. Additionally, aflatoxins are usually generated associated with the mold growth, which cause even more severe food safety risks. Insect damages (webbing, cast skins, frass, etc.) by field pests, such as Codling moth, (*Cydia pomonella* [*L*.]), navel orange worm (*Amyelois transitella [Walker]*), and storage pests, such as Red flour beetle and Indianmeal moth (*Plodia interpunctella [Hübner]*) have raised additional food safety concerns [74]. For example, navel orange worms lay eggs while the almonds are on tree, and hatch while they are dried on-ground. The insects could hide in dried nuts for a long time. In recent years, damaged nuts and live insects have been found in dried nuts or nut-containing products [35]. Insect infestation is also favorable for mold growth [75]. Although drying reduces water activity of nuts and can inhibit the microbial growth, drying alone is not enough to sufficiently

disinfect and disinfest the tree nuts [13]. Thus, additional disinfection processing is needed to ensure the microbial safety of the products.

## **4.1 Conventional disinfection and disinfestation technologies**

Conventionally disinfection and disinfestation technologies can be classified into chemical and thermal treatments, which are similar for different tree nuts. Chemical treatments refer to fumigation. The commonly used disinfectants and pesticides for fumigation of tree nuts include sulfuryl fluoride, aluminum phosphide and magnesium phosphide [74, 76]. Nowadays, there are increasing concerns of the chemical residues on tree nuts after fumigation as they are classified as 'probably carcinogenic to human', and excessive chemical use is not desired for the clean labeling of the products.

Thermal treatment is another type of disinfection and disinfestation method for tree nuts. High temperature heating kills the pathogenic microorganisms by denaturing their proteins and nucleic acids. For example, hot water blanching at 88°C for 1.6 min reduced 4-log *Salmonella* levels on the surface of almond kernels [73]. Oil roasting at 127°C for 53 s reduced 4-log *Salmonella* level on almonds and achieved effective disinfection of pistachios and walnuts [77].

#### **4.2 Emerging disinfection and disinfestation technologies**

In recent years, many emerging technologies have been studied and developed to improve the safety of tree nuts. It is generally agreed that 'moist heating' at high temperature and humidity conditions are effective for pasteurizing foods due to the high heat capacity and penetration depth. In addition, since 'moist heating' does not use harmful chemicals, it is more environmentally friendly and considered safer [78, 79]. Chen et al. [13, 37] applied step-down temperature HA heating with tempering for the simultaneous drying and disinfection of in-hull almonds that were harvested off-ground. When the average processing temperature was higher than 50°C, the insect eggs in the almonds were completely killed as no new larva was observed during the incubation and storage of dried nuts.

Radiative heating technologies have been proven to effectively pasteurize and disinfest the nuts. For example, IR preheating followed by temperature holding achieved 7.5-log reduction of *Salmonella enterica* on almonds owing to the intensive thermal effects [80]. SIRHA heating pasteurized pistachios and almonds with up to 6.1 and 4.7 log CFU/nut reductions in *Salmonella* level, respectively [22, 23]. MW and RF heating can penetrate the nuts and achieve rapid volumetric heating. As the results, 2–4 min of RF treatment was able to reduce 5-log *Salmonella* in the almonds [81]. RF heating could also kill the larvae of *Ceraphron cephalonica* and rice moth in walnuts [82], and *Indianmeal* moth in pistachios [83]. Nonetheless, due to the high temperature, thermal processing may damage the quality and reduce the shelf life of tree nuts, particularly causing lipid oxidation. Therefore, non-thermal disinfection technologies are gaining increasing interests.

Irradiation technologies have been used for disinfection for tree nuts. Ultraviolet (UV) illumination destructs the DNA structure of microorganisms and degrades the toxins. Particularly, Aflatoxin B1 (AFB1) absorbs UV radiation strongly at wavelengths of 362 nm and degrades rapidly under acidic (pH < 3) or alkaline (pH > 10) conditions [84]. Pulsed light treatment illuminates high intensity UV and/or visible light to target foods, which can also destruct the DNA structure in microbials and leads to effective disinfection [85]. Electron beam irradiation and Gamma irradiations have also been applied to disinfect tree nuts effectively [73, 86]. Low pressure

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

cold plasma can generate UV radiation and reactive chemical species that destroy the protein and DNA structure within bacteria and fungi that infect the nuts [87]. However, it has also been reported that increased irradiation dose caused decrease in the nutrient contents in tree nuts, such as α-tocopherol [88, 89]. Therefore, these irradiation disinfection technologies need further investigation to maintain both the safety and quality of tree nuts.

Some other emerging technologies, such as nanoparticles (NPs), electrolyzed oxidized water (EOW) and ozone treatments have also been researched. Photocatalytic NPs can be used as a disinfectant alone or combined with other materials due to the oxidative stress arising from reactive oxygen species (ROS) that are generated under visible or UV lights [90]. Among which, silver NPs show antimicrobial properties against several bacteria including *Escherichia*, *Pseudomonas*, *Salmonella*, *Bacillus*, *Clostridium*, *Enterococcus*, *Listeria*, and *Streptococcus* [91], and different fungus, including *Aspergillus niger*, *Candida albicans*, and *Saccharomyces cerevisiae* [92, 93]. Some metal oxide photocatalytic NPs, such as titanium dioxide (TiO2), also show potential disinfection functions [94, 95]. However, excessive use of NPs has raised concerns for their accumulation in foods that may cause toxicity.

EOW is normally obtained by passing the saltwater solution through an electrolysis system containing a cathode, an anode and a selective-permeable membrane [96]. The electrolysis of saltwater generates oxidizing species, such as O2 and Cl2 gases, and HOCl, at the anode. The redox potential of the EOW solution ranged from +700 to +800 mV with a pH of 4, which indicates high oxidizing ability [97]. These oxidizing species damage the cell wall and the metabolic process of a variety of pathogenic bacteria, such as *E. coli O157:H7*, *Listeria monocytogenes, Bacillus cereus*, and *Salmonella typhimurium* [98, 99]. Although EOWs are accepted as an antiseptic agent in food production, their toxicity needs further investigation.

Ozone is another ROS that has been used for food safety improvement. As a strong oxidant, ozone destructs cell wall, cell membrane and other cell constitutions in microorganisms [100, 101]. Ozone could also effectively degrade mycotoxins and aflatoxins in foods by reacting with the alkene double bonds [102]. Ozone is relatively unstable, and can spontaneously decompose into oxygen, thus do not generate hazardous residues in foods [101]. However, ozone is a greenhouse gas with strong global warming potential, and thus its application and emission may need to be regulated. Although not yet reported, these emerging technologies also show potentials to be used for the disinfection of tree nuts, and thus more research works are needed.
