Walnuts, a precious food to be preserved

Rich in polyunsaturated fatty acids, walnuts are a consumer favourite, but industrial use is limited due to their high susceptibility to oxidation, which results in the loss of nutrients and taste. New experimental applications may counter this problem.

According to FAO (data 2014 – average 2008-12), the world produces about 14 million tons of nuts per year, of which 3 million are in-shell walnuts produced primarily in China, Iran and the US. Over the past several years, the sector has boomed thanks to growing consumer market demand. Yet despite the walnut’s favourable nutrient and sensory profile, industrial use is quite limited due to their high susceptibility to oxidation which causes the loss of nutrients and taste. While the high polyunsaturated fatty acids content (PUFA) on one hand benefits consumer health, on the other hand it promotes deterioration caused by oxidation (rancidity), leading to the development of undesirable volatile compounds and off-flavours. Walnuts are widely recognized as an important source of fatty acids. The main components of the oil content are monounsaturated fatty acids (oleic acid) and polyunsaturated omega-3 and omega-6 fatty acids (linoleic and alpha-linolenic acid). The latter make up 70 to 80% of the total fatty acid content, so that walnut oil is the vegetable oil with the highest PUFA content. An important source of fibre and minerals, walnuts are particularly rich in tocopherols (Vitamin E), fat-soluble antioxidants mainly present in form of γ-tocopherol which ranges from 194 to 297 mg/kg. Among the compounds with antioxidant properties there are also phenolic compounds, which are highly concentrated in the external integument of the kernel. Genetic and environmental factors, as well as aspects relating to harvesting and storage, particularly affect the content of non-flavonoid poliphenols (ellagic tannins). If walnuts are damaged or submitted to specific technological treatments, which may damage the particles that enclose fats, there is a higher risk of alteration. In these conditions the lipids which are no longer protected by the membranes, may come into contact with oxygen, light, (exogenous and endogenous) lipolytic enzymes, and with metals present in plant tissues, acting as catalysts, and potentially leading to an oxidation process. Thus, the oxidation process is favoured by the use or low-quality raw materials (damaged or contaminated with lipolytic micro-organisms), unsuitable storage conditions, processes (thermal treatments or grinding) that result in an increase in the surface area exposed to oxygen. The enzymes strongly involved in this phenomenon are lipases which, acting on the triglycerides, free fatty acids and increase acidity. Lipases’ activity is influenced by various factors, such as humidity, temperature and pH. Among these, humidity and free water (aw) play a key role. In fact, they trigger the reaction and influence the thermodynamic equilibrium: a 5% humidity and 0.25 aw are sufficient to start the deterioration process. Free fatty acids, which are released, are then exposed to the lipoxygenase, which mainly affects the linolenic, linoleic and arachidonic acids, as well as to an autoxidation process catalysed by iron and copper. Temperatures above 50°C, however, can induce the denaturation and, hence, inactivate this enzyme. Studies carried out on walnuts have shown that short thermal treatments determine substantial reductions of the lipoxygenase activity. Treatments at 55°C for 2 minutes reduce the enzymatic activity by 62%, whereas treatments at 60°C for 2 or 10 minutes result in a reduction of the lipoxygenase activity by 75 and 81% respectively. Hydroperoxydes formed due to the action of the lipoxygenase, are typically highly unstable, and generate a range of by-products (such as volatile carbonyl compounds or non-volatile compounds), which over time may experience modifications.

Application aspects
Due to their reduced oxidative stability, hence, walnuts are not widely used for industrial applications. The aim is to enhance the stability of nut-based semi-finished and finished products, in order to make their use simpler and safer. To encourage processing and increase the shelf-life of walnuts, roasting can be considered one of the most effective methods. Thermal treatment, in fact, has positive impacts like enzymatic inactivation, stronger aromatic characteristics, and improved yield of the oil extraction procedure. When looking at the chemical-physical properties of roasted and non-roasted walnuts, stored in the dark at 60°C for 12 days, it can be seen that the fatty acid content is not subject to substantial variations during the storage period. The effects of the roasting process are more clearly visible by analysing the number of peroxides, the dienes percentage, and the tocopherol content. A study conducted by Bipin Vaidya and his staff (2012), in fact, demonstrated that the content in peroxides and dienes in roasted and non roasted walnuts tends to grow over time when stored at 60°C. However the increase in roasted walnuts is lower compared to that of non-roasted ones, and due to this, at the end of the storage period, the values indicating the oxidation stability are worse in non-roasted walnuts. This confirms the fact that roasting improves stability by slowing the oxidation process. Instead, looking at the tocopherol level, it can be observed that their content decreases over time. Tocopherol’s bioavailability increases in thermal treated products since the exposure to high temperatures can causes breakdown of the cell walls and hence a higher release of these compounds. However, in this way they are more exposed to light, oxygen, and heat, and, hence to a higher risk of deterioration. Comparing the values of roasted and non-roasted walnuts, it can be seen that the decrease is greater in non-roasted walnuts. Several studies pointed out the antioxidant behaviour was also present in the products of the Maillard reaction, which may involve an increased retention by tocopherols. The interaction between the two antioxidants can increase oxidation stability in roasted walnuts. Walnuts shelf-life depends on a delicate balance between oxidizable phytochemical substances, pro-oxidizing compounds and antioxidants. Normally walnuts are eaten whole, fresh or roasted, and only a small share is used for industrial applications. Next to oil, on the market there are just a few walnut-based products, and in most cases they are flavour substitutes. The possibility of developing stable semi-finished products over time (in form of powders, pastes, etc.) would allow their use in a variety of preparations.

Testing
To overcome the problem of rancidity, micro-encapsulation methods of the fat phase have been developed (one example is nut oil) which involves the use of spray drying or freeze-drying technologies. The treatment of fats requires first the creation of an emulsion by adding specific compounds capable of creating a net that can incorporate and stabilise the tiny drops of fat. The effectiveness of this process depends mainly from the composition and structure of the encapsulating material, which must protect from oxygen, light, and water, and prevent the contact with the other ingredients of the fat matrix. The aim is to protect polyunsaturated fatty acids by stopping or retarding oxidation. The compounds used for the encapsulation must show high stability and low viscosity, emulsifying properties and high solubility in water; they should not allow phase separation of the emulsion during the dehydration, and should form a dense network during the drying process. Commonly used compounds include proteins (sodium caseinate, serum protein and soy proteins), hydrocolloids (modified starches, Arabic gum, tragacanth) and hydrolyzed starches (maltodextrins). There is, however, no encapsulating material featuring all these properties, and thus, for the purpose of obtaining an optimum encapsulation, it is essential to use combinations of different materials. Studies conducted on microencapsulated walnut oil indicate that the process does not affect the polyunsaturated fatty acid profile. Notably, the linolenic acid content is very similar to that of natural oil and the addition of encapsulating compounds does not affect the ɣ-tocopherol content. Hence, the use of these agents, combined with an adequate dehydration method, is an effective tool for extending the shelf-life of walnut oil. Lyophilization is then one of the most useful processes for removing water from emulsions. This process, where water vapor is removed by sublimation from the frozen product, provides the food product with a reasonable added value and maximises its quality parameters. Studies are currently in progress to obtain innovative walnut-based food products by applying a lyophilization process which involves the addition of different types of matrix polysaccharides in different formulations. The aim is to obtain a stable product, pleasing to the senses, with just a slight variation of the content of natural antioxidants (polyphenols and tocopherols). To enhance the specificity of the fruit (and not only of the oily phase) the products are developed starting from a paste obtained from roasted walnut kernels added with dietary fibres, with the double aim of replacing common encapsulating materials necessary for microencapsulation, while further improving the nutritional value of the product. It is therefore possible to widen the market and applications of walnuts by developing methods that preserve the precious qualities of the fruit, and to create innovative products suitable for use in the food industry – confectionery and non-confectionery, both as an ingredient in other food products, or as a product in its own right.

Bibliography
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• Buranasompob A., Tang J., Powers J.R., Reyes J., Clark S., Swanson B.G. (2007). Lipoxygenase activity in walnuts and almonds. LWT – Food Sci. Technol., 40: 893-899
• Calvo P., Castaño A.L., Hernàndez M.T., Gonzàlez-Gòmez D. (2011). Effects of microcapsule costitution on the quality of microencapsulated walnut oil. Eur. J. Lipid Sci. Technol., 113: 1273-1280
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By Milena Casali, Davide Barbariga, Roberta Dordoni, Istituto di Enologia e Ingegneria Agro-Alimentare Università Cattolica del Sacro Cuore – Piacenza