Science-based precision processing for future healthy, structured and tasty fruit and vegetable-based foods

Aims of HST FoodTrain

HST FoodTrain (project EU-FP7-ITN-264470) is a multi-site Marie Curie Initial Training Network (ITN) funded by the European Commission under the FP7-People Programme which ran from January 1, 2011 until December 31, 2014. The objective was to train multi-disciplinary young scientists in the field of fruit and vegetable processing and at the same time provide them with multi-sectorial experience both in academia and in the private sector. The consortium of HST FoodTrain consisted of two universities (Katholieke Universiteit Leuven, Belgium and the University of Reading, UK), a private research institute (DIL, Germany), two large companies (Coca-Cola Services, Belgium and Unilever R&D Vlaardingen, The Netherlands) and an SME (Hiperbaric, Spain).

The project offered research and training opportunities for early-stage and experienced researchers. Besides research-based training, recruited researchers also received local and network-wide training in complementary skills via workshops, winter schools and conferences, and they have been exposed to both the academic and the private sector via secondments.

Due to their health-related benefits, 600g daily intake of fruit and vegetables is recommended by the WHO and the European Commission¹. However, this criterion is met only by a limited number of consumers. This issue can be addressed by creating a large diversity of new and improved fruit- and vegetable-based products that concomitantly preserve health-promoting components, as well as structural and sensory quality attributes, which fulfil consumer preferences and needs. To develop this new generation of fruit- and vegetable-based products, mechanistic knowledge on the impact of food processing on product quality attributes and functionalities was needed.

In this context, the HST FoodTrain project aimed to gain insight into the impact of novel and existing structure-enabling and preservation processes, including high pressure homogenisation, pulsed electric fields, thermal processing and high pressure (thermal) processing, on the composition and (micro)structure of fruit and vegetable-based food products. The relationship between those properties and product quality parameters, such as texture/rheology, flavour, colour and nutrient bio-accessibility was investigated. Among others, citrus fruits (Citrus spp.), mango (Mangifera indica), watermelon (Citrullus lanatus), tomato (Lycopersicum esculentum), carrots (Daucus carota), and broccoli (Brassica oleracea) were considered. Additionally, a new method for process impact comparison among different process technologies was developed. Some of the main outcomes of the research activities performed within the project are presented here.

Processing, food structure and physical functionalities

Food products often contain structuring additives (such as thickening, emulsifying, and/or stabilising agents) that are used to engineer their physical properties and appearance. Within the HST FoodTrain project, new processing methods were investigated to exploit the endogenous structuring potential of plant-based systems, thereby avoiding the use of additives. Traditional processing methods for vegetable-based food products include thermal processing and mechanical processing (such as cutting, blending). In addition to these currently applied processes, within HST FoodTrain new methods have been explored such as the use of high pressure homogenisation, the use of enzyme technology and the use of ion exchange resins and sequences thereof to specifically engineer targeted rheological properties of the vegetable-based end product². In this way, it has been demonstrated that certain vegetable-based food sauces can be made more consistent, thicker and creamier, which makes the addition of structuring additives unnecessary.

Besides the research line on structure engineering of vegetable-based food systems, researchers within HST FoodTrain have specifically investigated the texture evolution of mango fruit (including different varieties and different ripening stages) upon processing (including freezing and thermal processing) and its relation with changes in cell wall biopolymers (such as pectin) and starch. Texture of mango pieces seemed to largely decrease at increasing stages of ripening and after a freezing process.

Results also indicated that the texture of unripe mango pieces in mango juice was significantly improved by thermal pasteurisation, probably due to starch gelatinisation, and unripe mango pieces were able to retain sufficient textural properties during storage in the juice. On the other hand, despite a similar starch gelatinisation, no improvement in texture was observed after thermal treatment of semi-ripe mango pieces in juice most likely due to a more degraded cell wall structure and a lower starch concentration at the later ripening stage.

Processing, food structure, composition and bioaccessibility

Fruit- and vegetable-based products are a major source of carotenoids, which are lipid soluble pigments that account for their characteristic colours. In recent years, carotenoids have received great attention due to their health-related benefits, including the prevention of cardiovascular diseases, certain types of cancer, as well as eye diseases^3,4.

In order for carotenoids to exert their health-related functions, they need to be absorbed by the human body and thus they need in the first place to be bioaccessible. Bioaccessibility is defined as the fraction of a nutrient that is released from the food matrix during digestion and made available for intestinal absorption. Several factors can affect carotenoid bioaccessibility in fruit and vegetable-based products. Carotenoids in the plant tissue are stored within subcellular substructures, namely the chromoplasts, which are located inside the plant cells, surrounded by the cell wall.

Results obtained within the HST FoodTrain project confirmed that the organisation and localisation within the chromoplast substructure, as well as the cell wall, constitute the main natural structural barriers governing the release and thus the bioaccessibility of carotenoids in tomato and carrot-based products respectively⁵. Different processing strategies, i.e. thermal treatments and high pressure homogenisation, were exploited to disintegrate these barriers and thus enhance carotenoid bioaccessibility. It is noteworthy that while disrupting the structural barriers naturally present in the plant matrix, processing can create novel process-induced barriers entrapping carotenoids. To this regard, thermal processing and high pressure homogenisation could either enhance or impair carotenoid bioaccessibility in carrot and tomato-based products^6,7.

The impact of processing on carotenoid bioaccessibility turned out to depend mainly on the chemical structure and localisation of carotenoids within the matrix. In addition, contrarily to what was expected, no unequivocal relationship between structure and carotenoid bioaccessibility could be established. Finally, after carotenoid release from the food matrix, its solubilisation in the oil phase and transfer into mixed micelles during digestion are crucial steps determining the overall carotenoid bioaccessibility⁸.

Processing, food structure, composition and sensory properties

Sensory properties such as flavour and colour of food products are crucial parameters for consumer acceptance, preference and repeated purchase. Processing can directly or indirectly affect the flavour of fruit and vegetable-based systems by inducing conversion reactions of volatile components and structural changes respectively. Therefore within HST FoodTrain, the impact of processing on the flavour of fruit- and vegetable-based products, including the understanding of the relations between flavour perception and food (micro)structure, was investigated. For instance, the impact of thermal (HT) and high pressure high temperature processing (HPHT) on the volatile profile and structural quality parameters of carrot purees was compared⁹, including the effect of oil addition.

The results demonstrated that HPHT processing exerts a distinct effect on the volatile fractions compared to thermal processing. From a structural point of view, limited or no difference could be observed between carrot purées (without and with oil) treated with HPHT or HT processes⁹. In addition, the potential of novel processing techniques (i.e. sequences of cold-break or hot-break processing and high pressure homogenisation) in modulating the flavour profile of vegetable systems containing tomato, carrot and broccoli was investigated. Results obtained include the identification of different process types and/or sequences that can be used to generate distinct headspace volatile profiles in a mixed vegetable system.

These results show some promising indication of how changes in process conditions can impact the volatile profile of a mixed vegetable system. This can be a starting point for more targeted tailoring of the flavour profile of mixed vegetable products. Moreover, optimised process conditions for reducing off-flavour in broccoli purée and the correlation of aroma profile with the microstructure formed due to the process were identified. Understanding the change in flavour profile as a function of different process conditions will help to further improve the quality of food products.

Besides flavour, colour is a very important quality attribute of food products. During processing and storage, colour of food systems can change depending on the conditions that favour certain degradation reactions. Within the HST FoodTrain project, the colour degradation during processing and shelf-life at ambient and elevated temperature (for accelerated shelf-life testing) of pasteurised orange juice has been investigated in detail. In view of understanding colour instability of pasteurised orange juice during storage, changes in several quality parameters were kinetically investigated as a function of time and temperature in the range of 20°C to 42°C. Visual colour degradation is rather complex, and this study clearly demonstrated that there is more than one pathway in the development of browning in stored orange juice.

There was only a limited contribution of carotenoid degradation to colour change, and other mechanisms such as acid-catalysed degradation of sugars and ascorbic acid degradation reactions appeared to be important for colour degradation in pasteurised orange juice during storage¹⁰. Additionally, another study was carried out exploring the role of sucrose content on flavour perception in reduced sucrose orange juice-based drinks and whether a salting-out effect could be observed in the sugar levels normally used in commercial beverages (10% w/w). Quantitative Descriptive Analysis (QDA) profiles were conducted in two different systems consisting of sugar with or without sweeteners. Overall the findings suggest that the perceived sensory profile of orange flavour-model varies with sugar content, and it is hypothesised that this may be related to the ‘salting-out’ effect of flavour volatiles into the sample and not due to sweetness enhancement.

Process impact comparison of novel processing techniques

To compare traditional (thermal treatment) and novel preservation technologies (pulsed electric fields (PEF) and high pressure processing (HPP)) for pasteurisation of juices, equivalent microbial inactivation has been selected as a starting point for the identification of process conditions for these three process technologies. Two food matrices have been investigated: tomato and watermelon juice. In order to compare the impact of the selected traditional and novel pasteurisation processes of the two plant-based juices, HS-GC-MS chemical fingerprinting was used as an untargeted approach and combined with appropriate multivariate data analysis.

Generally, some different fingerprint markers have been identified for the different technologies. Also, all three pasteurisation technologies caused loss of several newly generated volatiles compared to the control sample. Beside observed differences, some similarities between PEF and thermal processing have been observed as well. The main quality-related chemical reactions that occurred after processing and during shelf-life can be classified as: (1) oxidation of fatty acids, (2) carotenoid degradation, (3) amino acid degradation. Most of the detected and selected compounds have been already reported in tomato products and no undesirable substances have been detected. The untargeted approach used in this study gives a broader insight into chemical reaction pathways occurring during processing and shelf-life and can also identify several targeted components for further investigation¹¹.

Conclusions

Overall, the research activities performed within the HST FoodTrain project, showed that novel processing techniques can be exploited to obtain safe fruit and vegetable-based products with improved quality characteristics. The results obtained within the project constitute an important set of scientific information that can be used to steer the structural, nutritional and sensory properties of fruit- and vegetable-based products. An update on publications and other dissemination activities resulting from HST FoodTrain is available on the project website: http://hstfoodtrain-itn.eu/dissemination/publications.

References

1. Food-based dietary guidelines in the WHO European Region. WHO/Europe, 2003. http://www.euro.who.int/
2. Sankaran, AK et al. Food Hydrocoll. 2015, 43, 442–450.
3. Rao, AV; Rao, LG. Res. 2007, 55, 207–216.
4. Perera, CO; Yen, GM. J. Food Prop. 2007, 10, 201–230.
5. Palmero, P et al. Food Chem. 2013, 141, 2036–2043.
6. Ribas-Agustí, A; Van Buggenhout, S; Palmero, P; Hendrickx, M; Van Loey, A. Food Sci. Emerg. Technol. 2014, 24, 113–120.
7. Palmero, P; Lemmens, L; Hendrickx, M; Van Loey, A. Food Chem. 2014, 157, 275–282.
8. Palmero, P; Panozzo, A; Simatupang, D; Hendrickx, M; Van Loey, A. Food Res. Int. 2014, 64, 831–838.
9. Grauwet, T et al. Food Res. Technol. 2014.
10. Wibowo, S et al. Food Chem. 2015, 171, 330–340.
11. Aganovic, K et al. Food Sci. Emerg. Technol. 2014, in press

Acknowledgements

The authors gratefully acknowledge financial support from the European Commission’s 7th Framework Programme (FP7).

About the author

Ann Van Loey obtained a PhD in Applied Biological Sciences at KU Leuven, Belgium in 1996, and is currently Professor at the Laboratory of Food Technology, KU Leuven, Belgium. Her main research topic is understanding and quantifying process-structure-function relationships in food systems (particularly fruit- and vegetable-based products) during processing. She is co-author of 275 international peer review papers and 22 book chapters. In 2003 she received the Young Scientist Award from the European Federation of Food Science and Technology. She is also involved in several national and international research projects and is currently coordinating the FP7-Marie-Curie Initial Training Network HST FoodTrain.