Stability of strawberry and apple phenolics and changes in other metabolites after different processing techniques and storage conditions in industrial set-up

Abstract

(Poly)phenols are important bioactive compounds with potential beneficial effects on human health, these are commonly found in fruits and vegetables. This research was focused on the (poly)phenols of apple (Malus domestica) and strawberry (Fragaria x ananassa) as these are two of the fruits more consumed worldwide, both fresh and processed (juices, jams, purees, smoothies, dried). Although the consumers increasingly look for less processed products, fruit products need to be offered and available in the market all year long. Therefore, fruit processing is essential to extend their shelf life, together with guaranteeing their nutritional and bioactive content while maintaining their sensory characteristics. Overall, there is a considerable variability among the (poly)phenols, which depends firstly on the matrix, the part of the fruit (e.g., flesh, peel, achenes) and the inherent fruit characteristics (e.g., origin, variety, ripeness degree). However, the different technologies for fruit processing and the storage conditions also play a role in the changes of the (poly)phenols concentration and sensory characteristics of the final product. Simultaneously, the processing technologies and conditions also influence the metabolic profile of both fruits. The objectives of the present Thesis were to determine to what extent different processing technologies, freezing, thermal treatments, and high-pressure processing, tested at an industrial scale and storage conditions, -20, 4 and 24 ºC for twelve months, affect the (poly)phenols, color and sensory attributes in the selected strawberry and apple products. As well as, to determine how the fruit processing influenced the metabolic profile and identify possible markers that help assess the degree of processing in these products. All this in order to potentially facilitate the use of customized processing and storage conditions to limit the degradation of (poly)phenols while maintaining the best quality and organoleptic characteristics. For achieving the separation, identification and quantification of the different (poly)phenols compounds characteristics to each fruit, the samples were extracted and analyzed by specific chromatographic methods and conditions. In addition, a non-targeted metabolomics approach based on UPLC-ESI-QTOF-MS was performed for the identification of processing markers. Methods and specific conditions are detailed in CHAPTER III. The results reached from this Thesis and included in the CHAPTERS V, VI and VII concluded that : 1. In strawberries, mild and standard thermal treatments showed similar patterns for most phenolic groups; an increase in proanthocyanidins, no change in ellagic acid conjugates, and a major decrease in flavonols and anthocyanins. Nevertheless, the process to obtain strawberry concentrate, vacuum concentration, had a detrimental effect on all the compounds. On the other hand, the storage conditions had a stronger detrimental impact on the phenolic compound levels, as well as on color and sensory attributes than the processing techniques. 2. In apples, mild treatments and high-pressure processing had similar effects in all phenolic groups, with increases in dihydrochalcones, hydroxycinnamics, and proanthocyanidins and decreases in flavonols. However, the standard thermal treatment increased dihydrochalcones and even flavonols concentrations. Thus it can be said that the initial processing techniques exerted a greater influence than storage conditions. 3. In general, proanthocyanidins were the major phenolic group and the most stable during storage, while anthocyanins were the group most affected by both processing and storage. 4. Overall, the storage at -20 and 4 ºC barely affected the phenolic level and the quality characteristics during storage. Nevertheless, storage at 24 ºC highly degraded the polyphenolic content and the quality and sensory attributes in strawberry, although not that drastic in apple products. 5. Through a non-targeted metabolomics approach, seven metabolites were identified and proposed as potentially powerful markers to evaluate the processing degree of strawberry and apple puree products. 6. Pyroglutamic acid, and 2-hydroxy- 5-methoxy benzoic acid were identified as upregulated markers, whereas pteroyl- D-glutamic acid, and 2-hydroxybenzoic acid β-D-glucoside were identified as down regulated markers of thermal processing in strawberry products. 7. Dihydroxycinnamic acid glucuronide, caffeic acid, and lysoPE(18:3(9Z,12Z,15Z)/0:0) were identified as upregulated markers, showing an increasing trend correlated with thermal processing in apple products. 8. The selection of the right variety together with both processing technology and conditions should be customized accordingly by industries, to preserve or even increase the (poly)phenol content in the final product as close as possible to that of fresh fruit. 9. The use of these markers may potentially help to objectively measure the degree of food processing and help to clarify the controversial narrative on ultra-processed foods.