Supercritical fluid extraction of the omega-3 rich oil contained in hake (Merluccius capensis–Merluccius paradoxus) by-products: Study of the influence of process parameters on the extraction yield and oil quality

Abstract

The supercritical fluid extraction (SFE) of the omega-3 rich oil contained in by-products of the fish industry has been studied in this work. Specifically, the offcuts obtained from peeling hake (Merluccius capensis–Merluccius paradoxus) to obtain hake fillets have been used as raw material. This raw material was grounded to different sizes and freeze-dried to different moisture contents. Extraction experiments were carried out at different pressure, flow rate and flow sense in order to study the influence of such process parameters on the extraction rate and oil quality. The influence of continuous or intermittent contact solvent–subtract was also evaluated. The extraction temperature was maintained at 40 °C in all cases in order to avoid thermal degradation of the polyunsaturated fatty acids contained in hake oil. The results obtained indicate that internal mass transfer controlled the rate of the process under the conditions studied in this work. The optimum extraction conditions were found to be 25 MPa, 10 kg CO₂/h and upflow sense over freeze-dried and grounded hake. Under those conditions, more than 96% of the total oil contained in the raw material was extracted after 3 h. The oil extracted presented a high omega-3/omega-6 ratio (around 7) and a high EPA and DHA content (about 6% and 14%, respectively, of the total fatty acids contained in the hake by-product used in this work). Quality and stability of the hake oil were evaluated along 14 days of storage under different conditions. The acidity of hake oil obtained by SFE was high, but it was similar or even lower than the acidity of the oil obtained by conventional solvent extraction. Acidity suffered little further increment along storage. Hake oil was very prone to oxidative modification, enhanced by increasing temperature and light. The oil oxidation process is complex affecting the concentration of parameters related both with primary and secondary steps of oxidation and to sensory properties as smell and colour.

Introduction

Polyunsaturated fatty acids (PUFA), especially omega-3 fatty acids, have been very often subject of scientific studies in the last years. There is voluminous new information available on PUFA and their presence in food and food products. There is also plenty of new knowledge on the biological effects and health implications of this type of fatty acids [1]. Nutrition experts have recently recommended to increase the PUFA consumption in order to maintain an omega-6/omega-3 ratio between 5:1 and 10:1 in the diet [2]. This ratio is actually maintained only in some diets as Japanese [3] or Mediterranean [4], both of them characterized by a high consumption of fish, whereas the majority of the diets extended in Western society are characterized by a great consumption of saturated fatty acids and a ratio omega-6/omega-3 much higher than recommended [5]. Since changing the nutritional habits of a whole society is a difficult task, many products enriched with omega-3, as nutritional supplements or functional foods (milk, juice, butter, cookies, etc.), have been developed in the last years. For that reason, looking for natural sources of omega-3 fatty acids, designing new methods for obtaining omega-3 fatty acids at production scale and trying to include omega-3 fatty acids in food products of habitual consumption are nowadays important tasks in which many scientists are involved all over the world.

Fish is the most important natural source of omega-3 fatty acids in the diet, specifically fish oil, which has been extracted at production scale from the 19th century by means of traditional methods based mainly in four steps: fish cooking, pressing, decanting and centrifugation [6]. Nowadays, some alternative methods are being developed, among them, methods based on supercritical fluid extraction (SFE) and methods based on enzymatic processes with proteases [7].

Supercritical fluid extraction is characterized by the use of supercritical solvents. The most used SC solvent is carbon dioxide. CO₂ has low toxicity and economical cost, is not flammable and is environmentally friendly. In addition, CO₂ has mild critical conditions (T_c = 304.15 K, P_c = 7.38 MPa), which make it suitable for processing thermo-degradable compounds as polyunsaturated fatty acids.

In the last decade, several studies have been published about SFE of oil from different types of fish or fish by-products: Esquível et al. [8] studied the extraction of sardine oil with SC-CO₂. Dunford et al. [9] studied the effect of moisture on the extraction of mackerel oil with SC-CO₂ and proposed a mathematical model to explain such effect on the extraction yield. Recently, Letisse et al. [10] studied the SFE of omega-3 rich oil from by-products of the fish industry, specifically from sardine heads.

The aim of this work was to study the supercritical CO₂ extraction of oil from hake by-products. A semi-pilot plant was used to study the influence of the process parameters on the extraction yield and on the quality of the oil extracted.