Investıgatıon Of Formatıon Of Α-Dıcarbonyl Compounds In Fruıt Based Products

Abstract

α-Dicarbonyl compounds and 5-hydroxymethyl-2-furfural (HMF) can be formed from sugars during the processing and storage of fruit products, due to their sugar-rich and acidic nature. Maillard reaction and caramelization are the reactions mainly responsible for the formation of these reactive intermediates in fruit products. α-Dicarbonyl compounds are the important intermediates for the flavor and browning development. On the other side, α-dicarbonyl compounds are significant precursors of toxic compounds such as advanced glycation end-products (AGE), furan, and acrylamide which are related to various degenerative and chronic diseases. During the formation of α-dicarbonyl compounds, the amino acid loss and vitamin degradation can also cause loss in the nutritional value of fruit products. Therefore, controlling these key intermediates during fruit processing and storage is crucial to maintain the quality and safety of fruit products. In this framework, this disertation aims to investigate the chemistry of α-dicarbonyl compounds in fruit products in depth during their storage and processing. To achieve this aim, firstly, occurence of the α-dicarbonyl compounds in fruit based products was measured. Secondly, efforts were put in understanding their formation mechanism, and lastly, the factors affecting their formation in fruit products were investigated. In addition, HMF has been considered as a quality marker in processed foods to date, however, in order to understand the importance of α-dicarbonyl compounds as quality and safety markers, HMF as well as α- dicarbonyl compounds was evaluated together. In the beginning, the content of α-dicarbonyl compounds and HMF in a large number of different fruit products (n=184) such as dried fruits, fruit juices, fruit juice concentrates, fruit puree concentrates, and fruit purees was determined. Among the α-dicarbonyl compounds, 3-deoxyglucosone (3-DG), glucosone, 1-deoxyglucosone, 3-deoxypentosone, threosone, diacetyl, methylglyoxal, and glyoxal were monitored. This study reported for the first time that the main α-dicarbonyl compound was glucosone (ranging between not detectable – 25.7 mg/L) in fruit juices. The other fruit products with mid- and low-moisture conditions contained 3-DG as the dominant one. The highest concentrations of α-dicarbonyl compounds and HMF were mainly found in dried fruits at concerning levels. Thus, the concentration of 3-DG in dried fruits varied between 21.9 – 4117.0 mg/kg, while HMF was ranging from not detectable to 2400.9 mg/kg. In general, the concentrations of α-dicarbonyl compounds were significantly (p < 0.05) higher than the levels of HMF. The daily intake level of α-dicarbonyl compounds and HMF from fruit products was also calculated in order to make a risk assessment. The dietary intake calculations showed that fruit juice products also pose a risk with high exposure, despite fruit juices contained low concentrations of α-dicarbonyl compounds and HMF compare to dried fruits. This study revealed that it was essential to investigate α-dicarbonyl compounds together with HMF in detail during the storage and processing of fruit products. During the storage of fruit products, the formation mechanism and the factors affecting the formation of α-dicarbonyl compounds was investigated in the following parts. In this regard, the changes in the concentrations of the reactants (sugars, amino acids) and the products (α-dicarbonyl compounds and HMF) were evaluated together with their formation mechanism by using multi-response kinetic modelling approach. This approach was applied to apple juices, orange juices, and peach nectars during the storage of 24 weeks at different temperatures. From the α-dicarbonyl compounds, glucosone, 3-DG, threosone, methylglyoxal and glyoxal were monitored during the storage. The main α-dicarbonyl compound was found as glucosone (ranging between 0.2 – 683.5 mg/L) in apple and orange juices during the storage, that was in accordance with the previous findings. In addition, HMF levels were found to be lower than the α-dicarbonyl compounds in stored fruit juice samples. A striking result to emerge from the data was that free amino acids showed no significant (p > 0.05) changes during the storage. Thus, it was first hypothesized that the sugar decomposition pathway rather than the Maillard reaction route was responsible for the formation of α-dicarbonyl compounds and HMF in fruit juices during storage. The use of multi-response kinetic modelling approach provided a better understanding of the most possible pathway of sugar degradation reactions in fruit juices by performing model discrimination and estimating the reaction rate constants. Accordingly, the formation rates of α-dicarbonyl compounds in peach nectar (sucrose-added beverage) were lower than that in apple and orange juices (no added-sugar juices). Isomerization of glucose and fructose via 1,2-enolization was found as a kinetically important reaction step in stored juice samples. HMF was mainly formed from the dehydration through fructofuranosyl cation rather than the 3-DG dehydration. One kinetic model for three different fruit juices was established that makes it easier to understand the formation mechanism of α-dicarbonyl compounds and HMF in acidic, sugar-rich, aqueous food systems in general. In the third part, how the factors (initial reactant concentration and pH) affect the formation of α-dicarbonyl compounds and HMF was investigated in fruit products with mid- and low-moisture content during storage. For this purpose, changes in the concentrations of reactants (sugar, amino acid) and products (α-dicarbonyl compounds and HMF) were monitored in fruit (apple, pomegranate) juice concentrates with different initial reactant concentration levels and in dried fruits (date, raisin, blueberry) with different pH levels during the storage of 20 weeks at 37 °C. Among the α-dicarbonyl compounds, glucosone, 3-DG, threosone, 3-deoxythreosone, 3,4-dideoxyglucosone-3-ene, diacetyl, methylglyoxal, and glyoxal were monitored in stored samples. Glucosone was the dominant one in 30 °Bx of fruit juice concentrates, similar to the previous findings. On the other hand, 3-deoxyglucosone was the major α-dicarbonyl compound in 50 °Bx and 70 °Bx of concentrates and in all dried fruits. HMF levels were also significantly lower than the concentrations of dominant α-dicarbonyl compounds during the storage, in support with the previous findings. The results also revealed that the decrease ratio of free amino acid concentration was increased from 34% to 77% when the initial reactant concentration increased from 30 °Bx to 70 °Bx in the fruit juice concentrates. Similarly, free amino acid loss was accelerated when the pH level changed from high-acidic (2.6) to neutral (6.6) in dried fruits, during the storage. With the increase in the loss of free amino acids, the concentrations of α-dicarbonyl compounds and HMF were increased in all fruit products. At the end of the storage, the level of 3-DG in dried date with pH 6.6 was found as 7251 ± 896 mg/kg which has been the highest level of α-dicarbonyl compounds reported in the literature until now. To understand the role of Maillard reaction in fruit products during the storage, the amino acid adducts of α-dicarbonyl compounds and HMF were confirmed by using high-resolution mass spectrometry. To the results, generally high mass accuracy ( Δ < 2 ppm) of the confirmation of amino acid adducts of α-dicarbonyl compounds and HMF proved the contribution of Maillard reaction to non-enzymatic reactions in the fruit products. In the end, it was revealed that during storage of fruit products, sugar degradation reactions mainly contributed to the formation of α-dicarbonyl compounds and HMF in aqueous fruit products, whereas Maillard reaction play important role in non-enzymatic reactions in mid-/low-moisture fruit products. In the last part, the effect of different processing stages of fruit juices on the formation of α-dicarbonyl compounds and HMF was investigated. For this purpose, changes in the concentrations of reactants (sugar, amino acid) and products (α-dicarbonyl compounds and HMF) were monitored in the samples of apple juice concentrate, orange juice, and peach puree concentrate collected from the critical process stages such as enzyme treatment, pasteurization, concentration during industrial processing. Among the α-dicarbonyl compounds, glucosone, 3-DG, 3-deoxypentosone, threosone, diacetyl, methylglyoxal, and glyoxal were monitored. The concentrations of sugars and free amino acids showed no significant (p > 0.05) changes during processing. The main α-dicarbonyl compound formed at each step of apple juice production was glucosone having a maximum concentration of 17.47 ± 0.16 mg/L at the end of the process. On the contrary, 3-deoxyglucosone was the dominant one present in orange juice and peach puree samples with a maximum concentration of 18.24 ± 0.86 mg/L and 29.71 ± 1.56 mg/kg, respectively. It was revealed that different production steps such as deaeration led to change in the formation of the main type of α-dicarbonyl compound in fruit products. This finding was the first reported in the literature. In addition, it was observed that continuous mild temperature conditions even below 100 °C can cause the accumulation of α-dicarbonyl compounds in aqueous fruit products. The presence of molecular oxygen, temperature, and the duration of the process were determined as the significant processing parameters affecting the formation of α-dicarbonyl compounds. Last but not least, the concentration of HMF was found to be quite lower than the level of α-dicarbonyl compounds in all samples during processing. This finding was in support with the previous findings obtained in this thesis study which showed the quite low or not detectable levels of HMF despite the high level of α-dicarbonyl compounds in aqueous acidic fruits. Therefore, it is suggested that not only HMF but also α-dicarbonyl compounds should be considered in order to make a reliable evaluation of the quality and safety of processed fruit products.