Effects of different extraction methods and process condition on fatty acid profile and stability of the avocado oil

Abstract

The avocado fruit is mostly consumed as a fruit rather than being processed to extract its oil. In addition to other constituents it has a content of oil which is the main component of its dry weight; which means that the fruit is found to be important for both food and oil. Avocado oil is important oil in the culinary, cosmetic and pharmaceutical industries. It finds its prominence due to its physicochemical properties in all these three industries. It is important to the food industry because of its high content of unsaturated fatty acids and high levels of antioxidants. In the cosmetic industry it is important because of its fast absorbing properties in the skin and the antioxidants properties, and in the pharmaceutical industry is found to be important as it’s a source of many bioactive compounds like sterols and vitamins. All these uses and benefits of the oil can only be employed once the oil is extracted from the fruit. However, the extraction method employed plays a major role on the properties of the oil which influences its application. Therefore, it is crucial to use an extraction method that can offer high yields and good quality parameters of the oil. In this study different solid-liquid extractions were done to recover the avocado oil from its flesh. The first research chapter explored two pre-treatment method of the avocado flesh, namely the oven drying and the microwave heating. The pretreated avovado probes were subjected to solid-liquid extraction using organic solvents. Both pretreatments were found to be effective and necessary for the process; but microwave heating was found to be the modern practice and requires much less time for pretreatment. Different process conditions were explored to evaluate how each parameter affects the oil yield from the avocado flesh. The selected parameters were the time, the solid to liquid ratio, as well as the modes of pretreatment and the modes of extraction, namely Soxhlet extraction, maceration and microwave aided extraction. The results found that oil yield increases with increase in extraction time, however, the process reaches equilibrium until there was no increase observed. Then maceration washes were evaluated as a mode of extraction by using fresh solvent with a number of different washes of the avocado flesh, this prevented the extraction reaching equilibrium but used a high quantity of solvent increase a recycling energy need. The optimum solid to liquid ratio found was the ratio 1:2 (w/w) to be best for the extraction, anything more was a waste of solvent. The pretreatment between microwave and oven drying found that the oil yield was almost similar between the two; however, the microwave heating was done only for 2 hours whilst the oven drying was done over 24 hours. Therefore, it was found that the microwave pretreatment would be best to save overall operational time. The second research chapter focused on the kinetics and thermodynamics of the extraction of avocado oil. The chapter evaluated how the extraction takes place when heat at different temperatures. It also explained the energy required for the process to take place. The extraction experiments were done at four different temperatures, 40 °C V 70 °C in increment of 10 °C, the results found that as the temperature increased the rate of process increased which resulted in the shorter extraction period. The addition of the heat to the extraction of avocado oil resulted in a positive enthalpy which meant that the process were endothermic. The entropy was also found to be positive which meant that the extraction was irreversible. As result, the Gibbs free energy was found to be negative. However, this meant that the process under those conditions took place spontaneously. The final research chapter evaluated both the yield of oil with different process conditions; it also further investigated the quality of the oil with different extraction conditions and methods. For the examination of the parameter effects on the extraction process, experimental design method was used. The experimental results were evaluated using response surface methodology (RSM) in order to optimize the extraction process, which aided identifying the optimum extraction conditions with the help of RSM. From the results of the analysis of variance (ANOVA) it was established that the results of the experiments were significant with a p-value < 0.05. This explained that the interaction of process conditions had a significant effect in the yield of the oil produced. The quality of the oil was studied by evaluating the fatty acids and the stability of the oil. The fatty acids results found that the unsaturated fatty acids were a high content in the oil as expected from avocado oil; however, it was also found that the fatty acids of the oil are dependent on the avocado fruit which varies in the content of unsaturated and saturated fats according to its geographical area provenience. The stability of the oil was also found to vary with different extraction methods and process conditions. It was also found that the stability of the oil can be compromised by some elements and compounds that are sensitive and prone to causing lipid oxidation. Hence extra virgin oils are sensitive as they are prone to photo-oxidation due to the content of chlorophylls.