Investigation Of Chemical Reactions In Hazelnut Induced By Roasting

Abstract

High lipid content, proteins and their reactive amino side chains, sugars including the reducing sugars as well as vitamins, minerals and the other constituents make hazelnut a complex reaction medium. The constituents of this reaction medium are precursors of many chemical reactions induced by roasting. Among the chemical reactions proceeding during roasting of hazelnuts; Maillard reaction, sugar degradation and lipid oxidation are the most prominent ones. As a consequence of these reactions, not only desirable properties like color, flavor and aroma are improved but also unavoidably undesirable compounds like 5-hydroxymethylfurfural and advanced glycation end products or toxigenic compounds like furan are formed. In order to understand the proceeding of these reactions during roasting of hazelnuts in depth, the formation and elimination of the main compounds of Maillard reaction, sugar degradation and lipid oxidation should be introduced, quantified and explained by a kinetic approach. The objective of this PhD thesis is the investigation of the chemical reactions occurring during roasting of hazelnuts. To achieve this aim, the proximate composition and profiles of constituents of hazelnuts belong to fourteen Turkish hazelnut varieties harvested in two consecutive years were evaluated at first. Then, the formation of Maillard reaction and sugar degradation products as well as their common products was determined at different roasting temperatures and times. Effect of oil on the formation of Maillard reaction products, early glycation and advanced glycation end products, was also investigated by heating hazelnut resembling model reaction mixtures under the conditions of hazelnut roasting. Finally, a multiresponse kinetic model was proposed, after testing many of them, by using the data of roasted hazelnuts and the important reaction steps of Maillard reaction and caramelization were revealed. At the beginning of this study, proximate composition of hazelnut varieties was determined as well as their profile analysis. By doing so, each component taking part in the chemical reactions during roasting was entirely identified. The lipid content of hazelnuts was found to be higher than the other constituents and rich in mostly oleic acid, followed by linoleic acid and saturated fatty acids namely palmitic and stearic acid. Hazelnut protein was found to contain all essential amino acids and 20% of total amino acids were arginine, glutamic and aspartic acid. The most predominant sugar in hazelnut was sucrose, constituted of 80% of total sugars. The others were glucose, fructose, raffinose, stachyose and myo-inositol. Phytic and malic acids were the organic acids found in hazelnut. Hazelnut was found to be rich in both water-soluble vitamins like panthotenic acid and nicotinic acid and lipid- soluble vitamins like alpha-tocopherol. Potassium, magnesium, calcium, sodium, manganese, zinc, iron and copper were the minerals found in hazelnut. Hazelnut skin, as a part of hazelnut, was found to be rich in phenolic compounds, 74% of which were in conjugated soluble form. Gallic and ferulic acids were the phenolic acids found in hazelnut skin. Common products of Maillard reaction and sugar degradation quantified in roasted hazelnuts were alpha-dicarbonyl compounds including, 3-deoxyglucosone, 1- deoxyglucosone, 3,4-dideoxyglucosone, glyoxal, methylglyoxal and dimethylglyoxal, as well as 5-hydroxymethylfurfural and furan. An early glycation product, fructosyllysine, and an advanced glycation product, carboxymethyllysine, as well as acrylamide were the Maillard reaction products analyzed in roasted hazelnuts. 3-deoxyglucosone, glyoxal and methylglyoxal reached to the highest concentrations when compared with the other dicarbonyl compounds during roasting of hazelnuts. The maximum furosine concentration was almost four times higher than carboxymethyllysine concentrations of hazelnuts. Acrylamide, furan and 5-hydroxymethylfurfural were found to be not a great concern as process contaminants in roasted hazelnuts. The effect of oil on the formation of early and advanced glycation products during roasting of hazelnuts were enlightened by heating hazelnut resembling mixtures. Fructosyllysine was found to degrade immediately after its formation within 15 min, indicating the predominance of advanced glycation products during prolonged roasting. The concentration of pyrraline, among the advanced glycation end products, was the highest indicating that the modification of lysine was mostly achieved by 3-deoxyglucosone. The concentration of other advanced glycation end product was carboxymethyllysine>formyline>MP-lysine>maltosine. MP-lysine, as an advanced lipation end product, was the only advanced stage product whose concentration increased depending on the amount of oil. A multiresponse kinetic model of Maillard reaction and caramelization occurring during roasting of hazelnuts was proposed. Isomerization of glucose and fructose, sucrose degradation, 5-hydroxymethylfurfural formation, reaction of sugars and amino acids, degradation of Amadori/Heyns product, formation and degradation of 3-deoxyglucosone and 1-deoxyglucosone, formation of glyoxal, methylglyoxal, dimethylglyoxal and their further reactions were included in the proposed model. Model discrimination was performed and explained until to find the best model fits to the experimental data. The important reaction steps in the proposed model were found to be as isomerization of glucose and fructose through 1,2-enolization, formation of 5-hydroxymethylfufural from the fructofuranosyl cation which was formed via sucrose degradation, Amadori product formation from reaction of glucose and amino acids, glyoxal formation through glucose oxidation as well as methylglyoxal and dimethylglyoxal formation from 1- deoxyglucosone. The reactions occurring during roasting of hazelnuts leaded to the proceeding of Maillard reaction to the advance and final stages with the contribution of caramelization reaction as revealed by both mass balance of the compounds and the color of the hazelnuts.