*3.1.3 Microwave assisted extraction*

*Innovation in the Food Sector Through the Valorization of Food and Agro-Food By-Products*

**3. Extraction and purification of bioactive compounds in strawberry** 

The extraction with solvents is a technique to isolate a substance from a solid or liquid mixture. This technique is currently used in combination with other techniques such as microwaves and ultrasound since the solvent only extracts soluble compound. Due to the strawberry extrudate nature, the solid–liquid extraction can be carried out with a Soxhlet extractor, which is one of the most commonly used conventional extraction techniques [38]. The extraction efficiency depends mainly on the choice of solvents [39]. The polar character of the bioactive compounds allows their solubility in various solvents, such as water,

Recently, numerous studies have explored the extraction of bioactive compounds using deep eutectic solvents from various groups of natural sources [41]. The formation of eutectic solvents is the result of the complexation of a halide salt, which acts as a hydrogen bond receptor, and a hydrogen bond donor [41]. Some eutectic solvents have been developed from the combination of primary metabolites and bio-renewable starting materials, e.g., sugars, alcohols, amino acids and organic acids [41]. Eutectic solvents produce less adverse effects on the environ-

There is a long variety of studies on solvent extraction focusing on the extraction of bioactive compounds. An evaluation of the effect of different solvents and acids in the extraction of anthocyanins from strawberry fruits concluded that acetone provided an efficient and reproducible extraction, avoiding problems with pectins and allowing the concentration of the sample at low temperature [42]. In another study, it was observed that the acetone/acetic acid mixture (99:1, v/v) reached good results for the qualitative and quantitative evaluation of polyphenols present in

Hydrothermal extraction is a process in which the matter is treated by adding hot water or water vapor [38]. Steam explosion is another kind of hydrothermal treatment where the matter is treated with saturated water vapor at high pressure followed by rapid depressurization [44]. The disadvantage of using hydrothermal treatments is that they affect thermosensitive compounds and might form undesir-

Hydrothermal extraction at low temperature, i.e. ranging between 50 and 90°C, mainly induces the de-flocculation of macromolecules [46]. Hydrothermal extraction at medium temperature, i.e. ranging between 150 and 180°C, solubilizes cellulosic and hemicellulose biomass [47]. The steam explosion treatment, with temperature ranging between 180 and 260°C and increase in pressure of

Several studies confirm the successful extraction of bioactive compounds by these hydrothermal extractions [48, 49]. Extraction of bioactive compounds in strawberry extrudate has been studied by applying hydrothermal treatments in the range of 90–200°C, [15]. Thermal treatment at 150°C for 60 minutes was the most efficient process based on the solubilization of sugars and phenols as well as the

0.69–4.83 MPa, it is able to solubilize lignocellulose biomass [44].

antioxidant capacity of the liquid phase produced [15].

ment, allowing to replace conventional chemical methods [41].

**extrudate**

**3.1 Extraction techniques**

alcohols, and acetone [40].

strawberries [43].

able compounds [45].

*3.1.2 Hydrothermal extraction*

*3.1.1 Solvents extraction*

**276**

Microwaves are electromagnetic fields in a frequency range of 300 MHz to 30 GHz, which are generally operated at a frequency of 2.45 GHz [10]. Microwaves can access biological matrices and interact with polar molecules, such as water, which vibrate or rotate by the effect of microwaves and generate heat and can enhance the processes of extraction of bioactive compounds [10, 50]. Microwave assisted extraction has been successfully applied in anthocyanin extraction processes in grape skins [51], the recovery of pectins from press residues of various berries, i.e. red and black currant, raspberry and elderberry [52] and to extract phenolic antioxidants from peanut skins [53].

#### *3.1.4 High hydrostatic pressure extraction*

High hydrostatic pressure extraction is a method that works at high pressures ranging from 100 to 1000 MPa. These high pressures cause cell deformation, cell membrane damage, protein denaturation, deprotonation of charged groups, and the breakdown of bonds, making bioactive compounds more accessible for extraction [54]. High hydrostatic pressure extraction is considered to be a faster and more efficient technique than other conventional extraction methods [55, 56]. In addition, high hydrostatic pressure extraction has the advantage of not increasing the temperature during the processing time, so it would be an ideal method to extract thermosensitive compounds.

Several high hydrostatic pressure extraction studies have been carried out with strawberries for the extraction of bioactive compounds. The impact of high hydrostatic pressure extraction on total strawberry puree phenols was observed by Patras et al., [56], which reported that the amount of total phenols increases as the pressure in high hydrostatic pressure extraction increases [56]. In another study, the change in kaempferol, and quercetin quantity in strawberries pulps were tested at different pressures and for different processing times [57]. According to this study, the change in the amount of kaempferol was not very significant and the amount of quercetin increased with increasing pressure [57]. Another study showed that the nutritional and sensory qualities of strawberry puree after high-pressure processing at 500 MPa and 50°C for 15 min were much better than after a heat treatment at 90°C for 15 min [58].

#### *3.1.5 Pulsed electric fields extraction*

Pulsed electric fields or high intensity pulsed electric fields consist of a short time electrical treatment, between nanoseconds to milliseconds, in which the material located between two electrodes is exposed to a strong electric pulse of intensity field of 100 to 300 V/cm Pulsed Electric Fields or 20 to 80 kV/cm high intensity pulsed electric field, the operation parameters being the duration and number of pulses [59, 60]. Pulsed electric fields can produce the electrical rupture of the cell membranes producing the formation of pores, what is known as electroporation [60, 61]. Pore formation improves cell permeability allowing the recovery of bioactive compounds [62]. Compared to other non-thermal treatments such as the high hydrostatic pressure extraction method, pulsed electric field extraction methods require a much shorter processing time, higher extraction efficiency and these techniques can be easily applied in continuous operation [59]. Therefore, pulsed electric fields is a promising technique for different applications in the food industry because they can improve extraction capacity and recovery of nutritionally valuable compounds as well as the bioavailability of micronutrients and compounds in a wide range of foods [59].

Several studies on the extraction of antioxidant compounds in agri-foods show enhanced yields with pulsed electric fields. For instance, a comparison study between a heat treatment at 90°C for 60 or 30 seconds and high intensity pulsed electric field in strawberries juice, showed that strawberry juice treated with high intensity pulsed electric field maintained greater amount of phenolic acids and total anthocyanins than thermally treated juices [63, 64]. Likewise, the recovery of phenols from the shell of the pomegranate by pulsed electric field has been assayed, resulting in a similar antioxidant extraction yields and an energy saving of 50% compared to an ultrasound extraction technique [65].
