## 1 Introduction  
Over the past few decades, consumer concern about the freshness and quality of meat increased significantly [1]. Meat and meat products are an example of food products that are susceptible to deterioration due to their composition [2]. In addition to the bacterial spoilage, the main degradation process of ground pork is caused by protein oxidation, which is characterized by loss of red color and lipid oxidation, associated with off-flavors and off-odors [3, 4]. Lipid peroxidation occurs via free radical chain reactions, with reactive oxygen species (ROS) such as hydroxyl (OH-) and hydroperoxyl (OOH-) radicals being the major initiators [5]. Several factors such as fat content, storage temperature, kind of meat, oxygen availability, and type of packaging can greatly influence the extent of lipid oxidation [6]. Since the half-life of ROS is extremely short, their direct measurement is not an easy task. Several methods based on the detection of different fat oxidation products are used. One of the most common is the thiobarbituric acid reactive substances (TBARS) assay. In this case, malondialdehydedehyde (MDA), considered to be one of the main end products of lipid peroxidation, already present in the meat is measured, as well as that generated by the hydrolysis reaction. However, several considerations must be taken into account. First, not all fats produce MDA. Second, other substances may be generated by fat peroxidation. In addition, some food additives may react with MDA, thus underestimating the final result. Finally, thiobarbituric acid can react with other food components [7, 8]. For these reasons, and despite its popularity, TBARS results must be considered and interpreted with caution [9]. Recent advances in active food packaging allow for the extension of the shelf life of fresh foods [10; 11; 12; 13; 14]. Most researchers focused their work on delaying the lipid oxidation of fatty foods and meats by adding active agents such as antioxidants to the packaging materials [15; 16; 17; 18]. Therefore, a new simple method for the determination of lipid oxidation would be highly appreciated, not only because fatty acids are important for the nutritional value of meat (especially the polyunsaturated ones, such as linoleic acid, around 11.8% and linolenic acid, 1.0% in pork), but also to evaluate the efficiency of the new active packaging in the preserved food. Raman spectroscopy can be considered as a useful tool for such purposes. Its main advantages are: little or no sample preparation, very weak scattering of water and carbon dioxide, rapid analysis, selectivity, versatility and the possibility of qualitative and quantitative analysis of both organic and inorganic compounds. On the other hand, limited sensitivity and high detection limits are relatively common. To overcome these disadvantages, surface-enhanced Raman spectroscopy (SERS) is a technique in which a special substrate, such as surfaces covered with roughened metals, or colloidal particles greatly enhances the Raman signal. The most common SERS substrates are made of silver, gold, or copper [19] and allow enhancement factors (EF) up to \(10^{11}\)[20; 21]. SERS allows both quantitative and qualitative analysis of chemical species. Today, there are two reliable theories that explain the SERS phenomena, by considering either electromagnetic or chemical mechanisms [21; 22]. Conventional Raman spectroscopy was already applied to evaluate lipid oxidation [23], to discriminate and characterize oils [24; 25; 26; 27] and to control fat content or changes in meat [28; 29], with mixed results. Although some SERS studies focused on lipids [30; 31; 32], to the best of our knowledge, SERS was never used as a method for the determination of lipid oxidation in meat or meat products. In this work, a different approach to evaluate fat degradation is presented. For the first time, a home-made SERS substrate, much cheaper than commercial ones, was developed and used for the analysis of lipids in ground meat.