# High-Pressure Technologies for the Recovery of Bioactive Molecules from Agro-Industrial Waste
## 3 Supercritical Fluid Extraction
### SFE of Bioactive Compounds from Waste
#### 3.2.1 Lipids  
Lipids have typically been extracted from fruit, plant, coffee, or seafood processing residues using scCO\({}_{2}\) without the need for cosolvents; indeed, lipids are well soluble in scCO\({}_{2}\). Fatty acids (FA), common components of complex lipids, can be extracted from red fruits' seeds and peels or from citrus plants' seeds. In some papers, the results obtained through supercritical extraction and extraction using organic solvents, such as hexane, have been compared [152, 153]. It is well known that red fruits such as raspberry, blueberry, wild strawberry, pomeranate, blackberry, and blackcurrant are very rich in antioxidants, in particular, flavonoids, polyphenols, carotenoids, anthocyanins, and vitamins C and E, which help counteract damage from free radicals, oxidative stress, and cellular degeneration. Waste from the industries that process these fruits (seeds and peels, for example) are rich in FA. Although the total yields in the extract are in some cases higher using hexane as the extracting solvent, the affinity of supercritical CO\({}_{2}\) for lipophilic compounds leads to purer extracts, with a higher percentage of fatty acids compared to hexane. For example, [PERSON] et al. demonstrated that when processing wild strawberry waste, 26.0% of fatty acids were obtained with scCO\({}_{2}\) and 1.4% using hexane [152]. Citrus (e.g., mandarin, lemon, lime, orange, tangerine, bergamot, and grapefruit) seeds are commonly considered as agro-industrial waste, but they are a source of by-products given they contain phenolic compounds, limonoids, carotenoids, and tocopherols [154, 155]. [PERSON] et al. [153] compared the extraction of fatty acid and triacylglycerol content by processing mandarin, lemon, and grapefruit seeds with scCO\({}_{2}\) extraction and Soxhlet extraction using hexane. Compared to what was observed for the extraction from seeds and peels of red fruits, and also citrus seeds, the total yield was higher for Soxhlet than SFE; the fatty acid profiles obtained through the two extraction techniques were similar but, using SFE, no traces of unwanted solvent were present. SFE has also been used to extract fatty acids from less common fruit-processing waste worldwide, such as tamarillo (native fruit from Peru, Ecuador, and Colombia), date (cultivated and consumed in African countries), rosehip, and mango. The processing residue from the preparation of tamarillo juices consists of the pericarp and seeds, which represents 24% of the fruit; rosehip seeds constitute about 30% wt of the fruit and is a waste product in the industrial production of herbal tea, fruit juice, jelly, jam, and marmalade; in the case of mango, 35-40% of waste is generated in the form of peels, kernels, and culled fruits [156, 157, 158]. Depending on the structure of the fruit waste, different SFE optimal operating conditions were identified. For tamarillo, the best results for the extraction of linoleic, oleic, and palmitic acids were obtained at 380 bar and 64 \({}^{\circ}\)C [156]; for saturated, monounsaturated, and polyunsaturated fatty acids (with oleic acid as the major constituent) extracted from date stones oil, the optimal conditions were 250 bar and 60 \({}^{\circ}\)C [159]; fatty acids were successfully extracted from rosehip seeds at 300 bar and 40 \({}^{\circ}\)C [157]; for mango kernel, the highest yield was achieved at a higher pressure (500 bar) and a temperature of 40 \({}^{\circ}\)C [160]. In relation to the valorization of agro- and food industrial waste, some research groups are trying to valorize coffee by-products, since coffee is one of the world's most widely consumed beverages and its processing chain leaves behind large volumes of waste [161]. In this framework, [PERSON] and [PERSON] extracted fatty acids from spent coffee grounds through supercritical fluid, Soxhlet, microwave irradiation, and ultrasonic irradiation extraction [162].