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Abstract:

For shelf-stable pasteurised fruit juices, colour degradation (browning) is a known problem, which is, however, scientifically still not completely understood. Besides colour, also other chemical changes can occur in fruit juices during shelf life, thereby affecting consumer acceptance. The objective of the present PhD work is to obtain insight into quality changes of high-acid pasteurised fruit juices during shelf life. The following strategic choices have been made: (i) starting raw materials; (ii) processing conditions; (iii) storage conditions; and (iv) analytical approaches. Orange and mango juice were chosen as representative high-acid, pasteurised, shelf-stable products. As it was known from the start of the PhD that the colour stability of pasteurised mango juice during storage is much better compared to orange juice, the effect of product composition on colour stability during storage has been investigated. For that purpose, mango juice was studied as plain (i.e. without added precursors) and as reformulated juice (i.e. with the addition of different reaction precursors such as sugar, ascorbic acid and citric acid, making the formulation more similar to the composition of orange juice). For all fruit juices, the same intense pasteurisation process (92 °C for 30 s) was selected to destroy microorganisms and to inactivate thermally resistant endogenous enzymes. After pasteurisation, all juices were stored at ambient and/or accelerated storage conditions. The quality evolution of orange and mango juice during storage was studied by a priori selected targeted attributes (i.e. targeted approach) and by an untargeted multivariate ‘fingerprinting’ approach. Both approaches were applied individually or were combined. Changes in quality characteristics were investigated ranging from single response (univariate) to multiresponse (multivariate), and attributes or compounds clearly affected by shelf life were kinetically modelled. The experimental work of the PhD is presented in two parts, (i) investigating shelf life quality changes in pasteurised orange juice as affected by extrinsic factors (storage time and temperature) (Part I) and (ii) investigating shelf life quality changes in pasteurised mango juice as affected by intrinsic factors (different juice compositions) (Part II). In Part I, pasteurised orange juice samples were stored at four different temperatures, 20 °C and 28 °C for a total of 32 weeks, 35 °C for 12 weeks and 42 °C for 8 weeks. A targeted approach was used to gain insight in the chemical reactions responsible for quality changes, particularly, colour changes. The effect of storage time and temperature on a wide range of quality attributes (colour, carotenoids, acidity, sugars, oxygen, vitamin C, furfural and 5-hydroxymethylfurfural (HMF)) was investigated using a kinetic approach. In addition, multivariate data analysis (MVDA) was applied to describe the correlation between the multiple targeted responses and colour. Browning was clearly observed during storage, particularly at elevated storage temperatures, and could be associated to non-enzymatic browning reactions, such as the oxidative ascorbic acid degradation and acid catalysed degradation of sugars. It was observed that the total carotenoid content was relatively stable during storage which indicates that the contribution of carotenoid degradation to colour changes was limited. No changes in the acidity of orange juice were observed at all storage temperatures. Significant decreases were observed for ascorbic acid, oxygen and sucrose, while furfural, HMF, glucose and fructose significantly increased. Next, insight into other possible reaction pathways responsible for overall quality changes in stored orange juice was obtained through an untargeted fingerprinting-kinetics approach. For this purpose, the headspace fraction of the matrix was analysed. Volatile compounds classified as sulphur compounds, terpenes, esters, and aldehydes were clearly changing during storage and/or were highly correlated to storage time (i.e. shelf life markers). Changes in the selected volatile compounds can be linked to various chemical reactions, such as acid-catalysed degradation, Strecker degradation, oxidative reactions, and hydrolysis of esters. Also, flavour scalping could have played a role in changes in the volatile compounds during storage. To quantify the changes of the selected volatile markers as a function of time and temperature, kinetic modelling was applied. Results showed that terpenes (α-pinene, α-terpineol, and linalool) and aldehydes (octanal) had clearly temperature and time dependent kinetics within the investigated temperature range. In Part II, mango juices were prepared by adding different potential precursors for non-enzymatic browning (ascorbic acid, citric acid and sugars) to the control juice (mango puree and water). Control, ascorbic acid-enriched (AA250 and AA500), citric acid-enriched (CA, CA+AA250 and CA+AA500) and sugar-enriched (S) samples were stored at an accelerated storage condition (i.e. 42 °C for up to 8 weeks). The effect of different mango juice compositions on quality changes was investigated using a combined targeted and untargeted multivariate approach followed by kinetic modelling. With this advanced analytical approach, shelf life markers consisting of quality attributes (colour, °Brix, ascorbic acid, sugars, furfural, HMF) and volatile compounds (terpenes, sulphur compounds, acids, ketones, and esters) were selected. The markers could be linked to acid-catalysed reactions (e.g. terpene rearrangement, hydrolysis of esters and hydrolysis of sugars), ascorbic acid degradation and oxidation reactions. By modelling the kinetics of the selected shelf life markers, an increase in acidity of mango juice seems to enhance several reactions (e.g. sugar degradation, HMF formation, hydrolysis of esters and terpenes rearrangement). Also, faster ascorbic acid degradation and furfural formation in AA-enriched samples was observed. To determine which quality degradation reactions play important roles in predicting shelf life, the kinetic parameters of the selected markers of orange and mango juice were compared. Under the investigated storage conditions, colour degradation could be the main reaction determining shelf life of pasteurised orange juice, while in mango juice, changes in the volatile compounds could limit its shelf life. This hypothesis should be confirmed with a sensory evaluation which determines the level of acceptability of the juice (i.e. end of shelf life). In conclusion, insight into the chemical changes of pasteurised juices during storage was obtained through (combination) of targeted and untargeted multivariate approaches followed by kinetics. Multiple degradation reactions are responsible for quality changes in orange and mango juice, of which the reactions are influenced by storage temperature, oxygen and acidity. Future research can be carried out in different areas, for example, conducting a sensory evaluation in relation to the juice acceptability, developing analytical methods to characterise the brown components, improving the identification of the volatile compounds, and using an advanced multiway data analysis technique.