Chemical Changes Occurring in Nuts and Oilseed Varieties During Roasting

Abstract

The consumption of nuts and seeds is considered a part of a healthy diet. They can be consumed raw or as snacks after being roasted generally at 130-200 °C. Roasting provides desirable aroma, color, and also texture, thereby improving overall acceptability. Apart from the physical changes, chemical reactions such as Maillard reaction, sugar degradation, lipid oxidation, protein denaturation, and vitamin degradation also take place during roasting and are affected by the temperature and time of the roasting process. The thermal process can also cause potentially adverse health effects by promoting the formation of potentially toxic or carcinogenic components and causing a loss of nutritional value. The route of these chemical reactions is extremely dependent on the reactants as much as reaction conditions such as pH, temperature, and moisture content. The restricted amount of reducing sugar, low water activity, and neutral pH value of nuts and seeds lead Maillard reaction to follow a different pathway. Therefore the Maillard reaction-related studies carried out in aqueous food matrices and model studies are incoherent to describe the reaction in foods that have limited water content. Since the mobility of the reactants is necessary for these reactions to occur, the reactants gain reactivity with melting in the processes where water activity is limited, such as baking or roasting. In order to better understand these complex reaction systems that take place during roasting, it is necessary to monitor the change in the concentration of the reactants and formed products kinetically. The aim of this thesis is to reveal a kinetic model for Maillard reaction-caramelization and also for acrylamide formation in nuts and seeds as low moisture food systems during roasting by using a multiresponse kinetic modeling approach. To achieve this aim, sunflower seed (Helianthus annuus L.), pumpkin seed (Cucurbita moschata L.), flaxseed (Linum usitatissimum L.), peanut (Arachis hypogaea L.), and almond (Prunus dulcis) were studied due to their similar compositions to represent of low-moisture foods by considering these kinds of foods as sugar-limited lipid-rich reaction pools and with the available amount of amino acids and sucrose. In the first part, the proximate composition of nuts and seeds which is the reactants of the Maillard reaction were evaluated. Sucrose was predominant in selected nuts and seeds, while glucose and fructose were rapidly degraded upon roasting. While the highest sucrose concentration was found in peanuts, the least sucrose was found in pumpkin seeds. The highest amount of protein-bound lysine was found in peanut and pumpkin seeds whereas it was the lowest in almonds. Arginine, aspartic acid, and glutamic acid were found to be the most predominant amino acids in all samples. In the second part of this thesis, changes in the concentration of reactants and products of Maillard reaction and caramelization were evaluated during the roasting of samples at different temperatures for different times. Sucrose decreased gradually during roasting depending on the roasting temperature and time. Almost all free amino acids participated in the reactions and, the highest decrease in the total amino acid content (sum of free amino acids and protein-bound lysine) was found in flaxseeds (72%) and sunflower seeds (71%) followed by peanut (44%), almond (35%) and pumpkin seed (28%) with roasting, respectively. Changes in 5-hydroxymethylfurfural concentration in seeds and nuts showed an overall increasing trend in all samples in relation to the decrease in sugar content. The highest 5-hydroxymethylfurfural formation was observed in sunflower seed after 30 minutes of roasting at 180 °C whereas the least 5-hydroxymethylfurfural content was found in pumpkin seed due to the least amount of sucrose among other samples. The highest furosine content was found in pumpkin seed whereas no detectable amount of furosine was found in raw almond samples. The N-ε-carboxymethyl lysine concentration reached its highest value generally at 180 °C within 30 or 40 min in all samples with the highest concentration in sunflower seeds. The maximum N-ε-carboxyethyl lysine content was found in pumpkin seed roasted at 180 °C for 40 min. Among the raw samples, the highest total α-dicarbonyl compounds concentration was detected in sunflower and pumpkin seed probably due to formation prior to transportation or during sun-drying. α-Dicarbonyl compounds in pumpkin seed, peanut, and almond showed a similar increasing trend during roasting while in flaxseed and sunflower seed they showed a decreasing trend depending on the roasting conditions. 3-deoxyglucosone and methylglyoxal were the most predominant α-dicarbonyl compounds. The levels of acrylamide rapidly increased to a certain extent and started to decrease afterward due to the prolonged roasting times. The highest acrylamide content among all the samples was found in almonds after roasting at 200 °C for 15 min due to the higher amount of free asparagine compared to the other samples. In the last part, a kinetic model was proposed by using a multiresponse kinetic modeling approach for Maillard reaction and caramelization during the roasting of samples at 160 and 180 °C. Firstly, the Principal Component Analysis (PCA) was run to determine the differences between raw food matrices and the distribution of reactants among the samples. The pumpkin seed obviously separated from the others due to its high-water activity and also lower amounts of sucrose and free amino acids which are the two of the main reactants of the Maillard reaction. Almond, flaxseed, and sunflower seed were clustered together in the same region of the plot indicating similarities in their chemical composition. Peanut was located at the bottom part of the diagram due to its slightly high levels of sucrose and low levels of ash and protein whereas pumpkin seed appeared in a different part of the PCA plot. These results were the basis for the multiresponse modeling of Maillard reaction in low moisture, low reducing sugar-lipid rich systems. Accordingly, it is envisaged that all samples can be explained with a single model, whereas probably different reaction pathways will be dominant in pumpkin seed samples. Accordingly, 3-deoxyglucosone formation via sugar degradation; 5-hydroxymethylfurfural formation from 3-deoxyglucosone, and only in pumpkin seeds the conversion of N-ε-fructoselysine to glyoxal and Heyns product to 1-deoxyglucosone were found to be quantitatively important. N-ε-carboxymethyl lysine and N-ε-carboxyethyl lysine mainly originated through oxidation of N-ε-fructoselysine and the reaction of methylglyoxal with lysine residue, respectively. Additionally, another kinetic model was proposed for the acrylamide formation during the roasting of these kinds of low-moisture foods at 160, 180, and 200 °C for 5 to 60 minutes. According to the proposed model, sucrose degraded to glucose and fructofuranosyl cation; 5-hydroxymethylfurfural was mainly formed through the 3-deoxyglucosone pathway in all samples at 160 and 180 °C and the reaction of asparagine with 5-hydroxymethylfurfural was found as the predominant pathway for acrylamide formation. These models might enable us to understand the whole reaction mechanism and develop mitigation strategies for controlling the formation of thermal contaminants such as acrylamide during thermal processing of roasted nuts and seeds as sucrose-rich and low moisture foods by developing optimal thermal processing conditions. This approach also allows us to classify nuts and seeds according to their compositional characteristics, which is more critical for chemical reactions, instead of their botanical nomenclature.