## 1 Introduction  
Active packaging is a well-known approach for extending the shelf life of foodstuffs. This modern methodology can fulfil consumers' demand for freshness and flavour of food, without compromising its quality [1]. Latest research trends are focused on the activity and influence of new antioxidants in stored food by preventing uncontrolled oxidation reactions [2]. These reactions are one of the main causes of adverse changes in mushrooms during their manufacture and storage, significantly deteriorating their quality and safety [3]. The potential applicability of various antioxidants incorporated in packaging material has been widely studied [4; 5; 6; 7]. In a previous study [8], green tea combined with \(\alpha\)-tocopherol, blueberry extract, and sodium metabisulphite combined with citric acid were evaluated as antioxidant agents for extending the shelf life of mushrooms. However, the biggest challenge during our last study was the determination of colour changes of the mushrooms analysed. Certainly, surface colour (e.g., oxidative browning) is a significant attribute that influences the consumer acceptance and purchase decisions. Therefore, the impact of antioxidants on the shelf life of food products could be evaluated by determination of food colour [9; 10]. Over the last decades, the Commission Internationale de l'Eclairage (CIE) L*a*b* [11; 12; 13] and sensory analysis regulated by International Organization for Standardization (ISO) norms [14; 15] have been the most popular approaches for the determination of colour changes in food. The more sophisticated methods described in the literature, such as imaging spectrographs, acousto-optical tunable filters (AOTF), 3 CCD cameras or liquid crystal tunable filters (LCTF), can provide excellent results, but they require expensive and complex apparatus [16; 17; 18; 19]. On the other hand, cheaper and simpler approaches, such as the use of digital cameras, have been considered [20], but in the case of mushrooms, limited capabilities for determining subtle changes of colour have been reported. Concerning CIE L*a*b*, another disadvantage can be emphasized: a large number of measurements must be carried out to obtain representative results because of the non-uniform nature of mushroom samples, as well as the heterogeneous distribution of colours due to the oxidation process [21]. In the case of visual professional panels and trained testers, the evaluation is based on the subjective assessment of panellists, which may be unable to distinguish slight differences. The main objective of this work was to develop a digital software-based procedure for the determination of the surface colour of mushrooms in order to evaluate the effectiveness of the active packaging in which the mushrooms were stored. The procedure can capture and distinguish slight differences in the real colour of several samples, allowing for unambiguous discrimination of diverse treatments of mushrooms. As a result, a quantitative evaluation of the colour surface of fresh mushrooms packaged with an antioxidant material was proposed. The colour surface of blank samples, i.e., those without any antioxidant, was evaluated at different storage times using a MATLAB code developed specifically for this project. MATLAB was selected because it is a powerful tool that is easier to operate than other software packages. Histograms were processed by means of principal component analysis (PCA), and an oxidation pattern was created. The same procedure was applied to the mushrooms stored in the antioxidant packaging. The correlation between the colours of the samples was obtained, and the influence of antioxidant packaging on extending the shelf life of food was evaluated. The proposed method was compared with CIE L*a*b* and visual panel to emphasize the problems encountered during the colour analysis of food. This paper deals with the problem of colour evaluation of perishable food and the influence of active packaging on its shelf life.