**1. Introduction**

Lipid oxidation is an important factor that limits the shelf-life of food. It reduces the nutritional value of lipids, some fat-soluble vitamins are lost because of its reaction with free radicals, some pigments disappear, and rancidity starts. This is one of the main causes of food product rejection by consumers, since it is associated with characteristic off-flavor due to the generation of volatile short-chain aldehydes and ketones, which are responsible for off-flavor, such as malonaldehydes (MDA) [1].

The development of active packaging provides an opportunity to extend the freshness of food products. Antioxidant packaging may act by absorbing the compounds that deteriorate the food, such as oxygen or free radicals. Oxidation process is a radical reaction initiated by the free radicals derived from oxygen, which are the primary free radicals. These free radicals are transferred to the lipid chains and their oxidation takes place. Previous works demonstrates that scavenging these primary free radicals is the best and the most efficient way to protect food against oxidation [2–8]. The antioxidant properties of selenium nanoparticles (SeNPs) are well known [9–12], and based on these concepts a new multilayer material containing nanoSe was built and optimized [13]. This new antioxidant polymer was optimized and studied at lab scale using oxidizable model compounds. However, tests with real food at both lab scale and industrial scale were not carried out. The scientific literature shows many approaches of antioxidant packaging materials, but most of them fail when trying to apply them at industrial level in a packaging line. Machinability of the new active materials, as well as stability, off-odor, color, and other physical characteristics, besides the antioxidant performance, are the main reasons why these new approaches are not common in the food market. Industrial scale up is always a big challenge and often the positive results from laboratory tests turn into a different panorama, where the gain of the new material is not acceptable. In this work, an antioxidant packaging based on selenium nanoparticles incorporated to a flexible multilayer was tested with food susceptible to rancidity. Then, in addition to the mentioned difficulties, Se nanoparticles had to be produced and handled at industrial level and incorporated in a homogeneous and reliable manner into the packaging material. The shape, size, and distribution in the packaging, as well as stability, had to be under control and the resulting material should be characterized and tested in real situations. The antioxidant capacity of the laminates with SeNPs incorporated have been previously demonstrated by Vera et al. at laboratory scale. It was confirmed that SeNPs are able to trap the primary free radicals derived from oxygen [13]. With this information, the new multilayer material containing nanoSe has recently been evaluated and it received a positive opinion from EFSA (European Food Safety Authority). The absence of migration of nanoparticles, together with the efficiency as an antioxidant at lab scale were the guarantee of food safety. Thus, it can be scaled up and launched into the market, as one of the few active materials in the EU market.

The first goal is to monitor the oxidation reaction of packaged food during its shelf life. One of the strategies to evaluate this is through a sensory panel. This methodology is very useful when the antioxidant packaging is designed to increase the time the consumer considers the food acceptable. Nevertheless, this methodology depends on the people that form the panel, and it is less reproducible than an instrumental method. Thus, it is important to combine this panel with analytical techniques that provide accurate measurements, such as the determination of the fatty acid composition of the food [14–16]. The method consists of derivatizing the fatty acids to their glyceryl esters, which will be analyzed either by GC-FID or GC-MS. This way, a quantitative value of each fatty acid before and after the oxidation will be obtained, and consequently the amount of fatty acids that disappear because of the oxidation will be available. Other techniques are based on the determination of the volatile compounds responsible for the unpleasant odors and in some cases propanal, pentanal, hexanal, 2-hexenal, 3-hexenal, or 2,4-heptadienal [17,18] release were also monitored to confirm rancidity. A common methodology in food is the thiobarbituric acid reactive substances (TBARS) assay. This method is based on the determination of malondialdehyde (MDA), which is a low-molecular-weight end product usually formed via the decomposition of certain primary and secondary lipid peroxidation products [19,20]. TBARS technique has been used in many oxidation studies of food [21]. To have an objective evaluation of the industrial production of this industrial nanoSe antioxidant packaging, all these methodologies were tested to demonstrate the efficiency of the new nanoSe active packaging on packaged food.

The main targets for oxidation in the lipids are the polyunsaturated fatty acids, which are vulnerable to the action of free radicals. This is due to the presence of double bonds that weaken the C-H bonds [22,23]. The hydrolysis and/or autoxidation of fats provide short-chain aldehydes and ketones responsible for rancidity, and rancidity is associated with the characteristics off-flavor and odor [1]. Nuts have a high content of unsaturated fatty acids [24]. Owing to that, walnuts and hazelnuts were the first foods selected for this study. According to the literature, walnuts have a content of monounsaturated fatty acids (MUFA) of around 9 g/100 g and a content of polyunsaturated fatty acids (PUFA) of around 48 g/100 g. Hazelnuts have a MUFA value of approximately 46 g/100 g and PUFA of approximately 8 g/100 g [25]. Therefore, they are good candidates to study their lipid oxidation and rancidity [26], and consequently nuts and walnuts were the first target of this study. Then, several foods susceptible to rancidity, packaged by different companies and packaging lines, have been analyzed and the results are shown and discussed.

#### **2. Materials and Methods**
