Colour and appearance analysis of fruit and vegetable soup using a digital colour imaging system

Within Food R&D scientists, chefs and engineers aim to develop natural fruit and vegetable based soups with optimal flavour, texture, appearance and health benefits whilst maintaining product safety. The colour and appearance has a major influence on the perceived quality (e.g. nutrition and freshness). This paper describes a method for the determination of the colour of soups containing vegetable particles. Digital images were made under controlled lighting using a DigiEye imaging system. It allows documenting the appearance by making colorimetrically accurate images which are suitable for the measurement of colour uniformity, size and shape. The uniformity of the light under diffuse and directional illumination was investigated using different targets and soup samples. For accurate colour analysis the images have to be corrected for non-uniform illumination and the colour has to be measured at a fixed location outside the centre of the lighting cabinet. Directional illumination (angled light with additional mirrors) was used to introduce gloss resulting in images matching the actual appearance of soups and vegetable particles very closely. Imaging glossy soups under diffuse illumination resulted in dull images with dark spots. No significant difference was found between the colours of soups analysed under diffuse or directional illumination. Under directional illumination a better repeatability was observed. Additionally, for 36 different soups, the measured results from the DigiEye system were compared to two different colorimeters (0/45 and diffuse/0 geometry). Linear relations were found between the CIE Lab values measured by the DigiEye system and those measured by two different colorimeters. Best correlations were obtained between DigiEye and 0/45 colorimeter (r=0.980-0.996). The short term precision of the DigiEye system is somewhat better than those of the colorimeter. Introduction Foods have to be visually appealing, tasteful and healthy. Colour is regarded as a key attribute of foods. The overall multisensorial appearance strongly affects the consumers’ perception of food products and finally the consumption, enjoyment and purchasing behaviour. Consumers prefer bright coloured fruits and vegetables. The colour and appearance has a major influence on the perceived quality (e.g. freshness). Browning or yellowing of raw green vegetables such as broccoli is considered undesirable. Processing of green vegetables may turn them into a non attractive dark olive green colour (degradation of chlorophyll). The colour may be a measure for over processing (heat treatment) and the accompanying decomposition of the nutrients and the change in bioavailability. The consumer’s perception of colour does not only effect the interpretation of product appearance, but also extends to how product flavour and overall quality is perceived. The colour of tomato soup may be associated to creaminess or to the strength of the tomato flavour [1]. The perception of colour-texture is finally linked to vitality. Fruit and vegetable based soups are an important source of nutrition [2]. The colour can be linked to the presence of specific antioxidants and other phytochemicals that neutralise free radicals in the body (e.g. lycopene in tomatoes and beta carotene in carrots and pumpkin). There is a general agreement among the scientific and nutritional communities that to gain the maximum benefit and protection from antioxidants, our diets should be as colourful as possible. Food research aims to develop fruit and vegetable soups with optimal flavour, texture, appearance and health benefits whilst maintaining product safety. Visual colour assessment is subjective and difficult to control (e.g. depends on observer and viewing conditions). Colour can easily be measured by instrumental methods such as colorimeters or spectrophotometers [1,3]. For homogeneous opaque materials with a smooth surface the measured colour will be close to the consumer perception of colour. However, only very few food products are really homogeneous. The surface texture, gloss, shape and form of food products has a dramatic influence on the human perception of colour. Figure 1. Influence of lighting conditions on the images of tomato / bell pepper soup [diffuse (left) and directional (right) illumination]. Image size = 10cm * 10cm. This paper describes the development of a method for capturing and analysing of colour images of soup products containing fruit and vegetable particles. These images have to be colorimetrically accurate and in agreement with the visual appearance of the product allowing direct and immediate comparison of products produced at different global locations to investigate the influence of processing parameters and colour stability during shelf life. They have to be used for colour (uniformity), size, shape and structure analysis, communication and archiving, on-screen panelling and artificial colour mapping. Accurate colour images can be made using flatbed scanners (FBS) or camera based systems. FBS is limited to relative thin samples like rice [4] or textiles [5]. For soups and vegetable particles the DigiEye colour imaging system from Verivide was used. This system has been developed for the textile industry [6, CGIV 2010 Final Program and Proceedings 399 7, 8] and is recently also used in the food industry. Normally for colour analysis an even diffuse illumination is needed avoiding bright specular reflections in the image. However under these conditions very dull images without surface texture were obtained by which these images are not suitable for appearance analysis (Figure 1). By using directional illumination (angled light and mirrors) gloss was introduced resulting in an image matching the actual appearance of the soup and vegetable particles very closely. In this study it was investigated if these images can be used for colour analysis.