Aroma characterisation and retention after heat treatment and drying of fruits using extraction and GC-MS analysis
Abstract: Popular Abstract in English Fruits are a key complement to the diet of many people in the southern region of Africa, especially in rural areas. maphilwa (Vangueria infausta L.), maçanica (Ziziphus mauritiana), maçala (Strychnos spinosa), mapsincha (Salacia kraussi), cajú (Anacardium accidentale L.), mavungwa (Landolpia kirki) are some of the most commonly found fruits in sub-Saharan Africa, particularly in Mozambique, Botswana, Madagascar, South Africa, Zambia and Zimbabwe. They play an important role in the diet and gastronomy of the rural communities. Several investigations of nutritional aspects have been carried out, and showed that the fruit is rich in dietary fibre and sugars and have a high micronutrient content in the form of minerals and vitamins. Vengueria infausta L. belongs to the family Rubiaceae. The common names are African medlar in english or maphilwa in ronga (one of local language in southern Mozambique). The fruit is usually harvested between February and April. It is brownish orange when ripe and is spherical in shape. The fruit is about 2-5 cm in diameter and contains 3-5 seeds. The fresh fruit is sweet and tastes like medlar (Mespilus sp.), although with some similarities to green apple and pineapple. The fruit can be eaten fresh, cooked or dried. It’s used also to prepare juice, jam puddings and marmalade. We assume that the fruit may be useful and the taste and aroma profile attractive and appreciated. Information about identification of aroma on African medlar is limited. The investigation included developing a procedure to extract volatile components from the fruit matrix, a purification step, separation, identification and quantification. The extraction procedure used solvents (ethanol, diethyl ether and pentane). Initial experiments showed that some components, especially sugars, are degraded during the heating in the Gas Chromatography (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 with a capability to retain the hydrophobic aroma components and wash out the hydrophilic components (sugars) using water. The aromas were released using a mixture of pentane and diethyl ether prior injection viii into the GC. The aroma components were separated on the basis of their retention times, followed by identification through MS. The identification was verified using standards. Each peak was quantified, taking into account the peak areas of components relating to the internal standard. The main aroma components identified in Vangueria infausta were hexanoic acid, octanoic acid, ethyl hexanoate, ethyl octanoate, methyl hexanoate a n d methyl octanoate. The esters (methyl hexanoate and ethyl octanoate) are the main contributors to the aroma of the fruit (Paper I). The second aspect of this work was to evaluate the effect of drying upon aroma components of the fruits. Samples of Vanueria infausta pulp were convectively dried at 80°C, 3 m/s for up to 480 min. The results showed that the principal aroma components of pulp are well preserved during the initial phase of drying down to a relative water activity of about 0.65, but are lost after more extensive drying. This is due to the volatilisation induced by sugar crystallisation that is likely to occur below a relative humidity of around 0.70 during the drying process (Paper II). The third study of this thesis aimed to evaluate the effect of blanching and drying on the aroma of mango (Mangifera indica L.). Three samples of mango (fresh, blanched and dried) were analysed (Paper III). The blanching was carried out in water at 70°C during 10 min and in a microwave at 90°C during 2 min. The most relevant aromas Mangifera indica are 1-butanol, α-pinene, 3-carene, myrcene, limonene, terpinolene, and ethyl butanoate. The experiments show that the levels of aroma components are increased when the material is blanched while hot air drying reduced most of the aroma when the drying is prolonged below 0.65 in aw. Also the study shows that water blanching, microwave blanching, long period/low temperature or short period/high temperature had no marked effect on the impact of the blanching. Another goal of this thesis was to investigate the influence crystallisation of carbohydrates on retention or loss of aroma. (Paper IV). Three models were evaluated: I -pectin-sucrose-aromas; II -pectin-microcrystalline celulose-sucrose-aromas and III -microcrystalline cellulose-sucrosearomas. The aroma fraction was composed of the main aroma components identified in Vangueia infausta: hexanoic acid, ethyl hexanoate and ethyl octanoate. Each model was dried on over at 80°C, 3 m/s during 60-420 min. GC results showed considerable aroma retention in all models at least ix starting when the aw value reached 0.8. Model with pectin and microcrystalline cellulose rapidly exhibited low aw values and more consistency. Our assumption is that the crystallisation of sugars could play a role on aroma retention during the drying of fruits. The results from these studies show what happens to aroma during heat treatment of fruits. It is useful to understand the encapsulation of aroma due to sugar crystallisation during drying. The results can help design a better strategy for sustainable utilisation of aroma components of fruits, like the African medlar, one of the wild fruits now included in local industrial processing of new products. We believe that greater knowledge on volatiles can be useful in sustainable utilisation of wild fruits grown in Mozambique and southern Africa.
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