Aroma characterisation and retention after heat treatment and drying of fruits using extraction and GC-MS analysis

Abstract: This study concerns the identification and characterisation of volatile components of fruits, and evaluation of the effect of heat treatment and drying on retention or loss of volatiles of fruits. The investigation included developing a procedure to extract volatile components from the fruit matrix, a purification step, separation, identification and quantification. Initial experiments with Vangueria infausta L. showed that some components, especially sugars, degrade during heating in the GC analysis, producing furfural, hydroxyl methyl furfural (HMF) and other volatiles. These compounds are obtained together with the native aroma components of the fruit, making analysis difficult. We developed a procedure using a hydrophobic column that could retain the hydrophobic aroma components and eliminate sugars that could disrupt the analyses. The extract was analysed by GC. The volatile components found in pulp of Vangueria infausta were primarily hexanoic acid, octanoic acid, ethyl hexanoate, ethyl octanoate, methyl hexanoate and methyl octanoate. Based on the odour activity values, it could be concluded that the odour of the fruit is mainly attributed to ethyl hexanoate and ethyl octanoate (paper I). Drying is often used to process and preserve food stuff but many food attributes including aromas which are important for palatability and consumer interest are affected by the process. Our research showed that the principal aroma components are well preserved during the initial phase of drying (down to about a relative water activity of 0.65). However the aromas are lost after more extensive drying. A possible explanation for volatilisation is the sugar crystallisation that occurs below a RH of around 0.70. (paper II). Also we evaluated the effects of drying with or without blanching on volatiles of mango (Mangifera indica L.). Fresh, blanched and dried mango samples were analysed (paper III). The fresh sample presented a very large number of peaks. The blanching was carried out in water and microwave at 70°C during 10 minutes and at 90°C during 2 minutes. Blanching increased the levels of aroma components. Both blanching procedures had no dramatic effect on the impact of the blanching. Prolonged hot air drying (aw=0.65) reduced most of the aroma. α-pinene vi and 1-butanol were strongly affected due their volatility. Drying had no great effect on components with high boiling points, which displayed significant retention even after extensive dehydration. Ethyl butanoate was high retained despite its high volatility. To evaluate a possible influence of sugar crystallisation on aroma retention a further study involved three model matrixes based on carbohydrates aqueous solution plus oils: I) pectin-sucrose-oils, II) pectinmicrocrystalline cellulose-sucrose-oils and III) microcrystalline cellulosesucrose- oils. The oil phase comprised the reference materials of the most powerful aroma components found in Vangueria infausta (hexanoic acid, ethyl hexanoate and ethyl octanoate). The model mixture was dried at 80°C, 3 m/s for 60-420 min prior to GC analysis (Paper IV). The aroma components were preserved in all models throughout the drying process (until aw ≈0.8). So sugar crystallisation did not induce the loss of volatiles. However noticeable sugar crystallisation was observed in model II. We assume that the presence of pectin and microcrystalline cellulose increased the ability of the matrix to compact, as water activity decreased during drying. So sugar crystallisation is probably the reason for aroma retention within the matrix. The results of the studies in this thesis illustrate what happens to volatiles during thermal processing of fruits. These results could help design a better strategy for aroma isolation and characterisation, and explain the aroma entrapment due to sugar crystallisation during drying of fruits. The results can also be used to design a strategy for sustainable utilisation of volatile components of fruits like Vangueria infausta, one of the wild fruits to be included in a formulation of new industrial food products. However, more studies are needed in order to learn more about sustainable utilisation of various wild fruits growing in Mozambique and southern Africa.