Special Issue on “Technologies for Production, Processing, and Extractions of Nature Product Compounds”

Natural bioactive compounds include a plethora of structures and functionalities providing a consistent pool of molecules to produce nutraceuticals, functional foods, and food additives. Moreover, they have also shown great market potential for industrial applications in the pharmaceutic and cosmetic sectors. These compounds, which are produced and recovered from various biological sources (such as fruits, vegetables, medicinal plants, wastes, and byproducts), can be found in nature either at high concentrations (i.e., polyphenols) or at very low levels (i.e., carotenoids) so that massive harvesting is needed to obtain enough [1,2]. Indeed, their structural diversity and complexity make chemical synthesis unprofitable.

Developing advanced technologies has been fundamental for overcoming the inherent difficulties in screening and producing these compounds. Traditionally, they are extracted by conventional liquid–liquid or solid–liquid extraction techniques, but this approach implies negative thermal influences on the extraction yield and quality with a large expenditure of organic solvents and energy. Moreover, with the growing consumer demands for greener alternatives that do not involve toxic chemicals as well as the industry concerns of sustainable, nontoxic routes of extraction, the applications of novel extraction technologies (including, for instance, ultrasound-assisted extraction, microwave-assisted extraction, and enzyme-assisted extraction, as well as their combination) finely optimized by suitable statistical approaches are becoming more and more diffused [3,4].

This Special Issue of the journal Processes on “Technologies for Production, Processing, and Extractions of Nature Product Compounds” (available online https://www.mdpi.com/journal/processes/special_issues/nature_product_compounds—accessed on 7 September 2023) showcases current insights about the setting and optimization of production and processing strategies as well as conventional and innovative extraction technologies of natural compounds. In addition to three reviews, it features seven research articles, covering a range of topics that highlight the versatility of the research area.

The increasing market demand for ectoine, a very interesting substance with several applications in the cosmetic field and food industries, has favored the development of cost-effective and sustainable large-scale production of this carboxylic acid derivative from microbial sources (i.e., halobacteria). Ng et al. [5] reviewed the existing and potential microbial sources of ectoine and its derivatives, as well as conventional methods and emerging technologies for enhancing the production and recovery of ectoine from microbial fermentation. The aqueous biphasic system (ABS), which is a practically feasible approach for the integration of fermentation, cell disruption, bioconversion, and clarification of various biomolecules in a single-step operation, was particularly focused. Nonetheless, implementing the ABS on an industrial-scale basis for the enhanced production and recovery of ectoine has yet to be exploited.

Considering the growing interest in the integration of insect-derived extracts in feed and food products, El Hajj et al. [6] have grouped different studies carried out on edible insects’ transformation processes and focused on the various treatment operations, extraction technologies, and solvents used in different processing steps. They also included an overview of current insights into the different steps of the transformation process from the insect reception to the protein, chitin, and chitosan extraction. Finally, the authors reflected on the most important future challenges of this sector.

In the 21st century, the market interest in hemp and its products has notably increased because seed portions can be utilized in the agri-food business, the woody component of the stem can be used in green buildings, the outer layer of the stems can be used in the textile industry, and the extraction of bioactive components from roots can play a vital role in the pharmacological industries. Thus, Naeem et al. [7] have gathered recent advancements in the use of hemp as a viable alternative for economies built on synthetic materials by the food, pharmaceutical, textiles, paper, building, and energy industries, among others, for both producing a green environment and profit.

The use of the ultrasound-assisted extraction (UAE) method can effectively improve the extraction or modify the properties of bioactives from plants and agri-food waste, often saving energy and time with respect to conventional methods by optimizing the processing parameters through response surface methodology (RSM) tools. For instance, Bagale et al. [8], by coupling Box–Behnken design to RSM, optimized sonochemical parameters such as temperature (33 °C), sonication time (40 min), and power (102.5 W/cm²) at fixed fucoidan concentrations compared to a normal process for assessing the impact of ultrasound process on the molecular weight, structure, and antioxidant activity of fucoidan. The outcomes demonstrated that sonochemical treatment significantly decreased molecular weight (318 kDa) and increased antioxidant activity (88.9%) compared to the control process (815 kDa and 65.3%, respectively). Clodoveo et al. [9] proposed the optimization of the ultrasound bath extraction of polyphenols from sweet cherry pulp by monitoring cyanidin-3O-rutinoside, quercetin-3O-rutinoside, and trans-3-O-coumaroylquinic acid, representing the main anthocyanin, flavonol, and hydroxycinnamate, respectively, identified in the extracts through chromatographic analyses (HPLC-DAD), as output variables. The optimization was performed following a two-level central composite design and the influence of the selected independent variables (i.e., extraction time and solid-to-solvent ratio) was checked through RSM. The maximum recovery of the phenolic compounds was obtained at 3 min and 0.25 g/mL in water/ethanol (1:1, v/v) at a set temperature (25 °C), sonication power (100 W), and sonication frequency (37 kHz). Subsequent validation experiments proved the effectiveness and reliability of the gathered mathematical models in defining the best UAE conditions. Moreover, Clodoveo et al. [10] set an alternative and less drastic method based on ultrasound technology to produce carob syrup with respect to the traditional very time-consuming process, involving solid–liquid extraction in boiling water and concentration at a high temperature (>100 °C). Processing conditions (i.e., time, temperature, and liquid–solid ratio) influencing the extraction of total soluble solids and total phenolic compounds were optimized using a central composite design coupled to RSM. Reliable mathematical models allowed us to predict the highest TSS (24 ± 2° Brix) and TPC (1.7 ± 0.5 mg/mL) values that could be obtained at 15 min, 35 °C, and 2 mL/g. Different HPLC-DAD phenolic patterns were determined between syrups produced by traditional and ultrasound methods; epicatechin, 4-hydroxycoumaric acid, and ferulic acid were more concentrated in the former, while procyanidin B₂, myricitrin, and quercitrin were prevalent in the latter one.

Process optimization and the study of extraction kinetics of natural compounds from vegetable sources were faced in two other articles. Specifically, Hebert et al. [11] studied the purification of sinigrin and gluconapin (two glucosinolates that are very suitable for the development of phytosanitary products due to their fungicidal, bactericidal, and insecticidal effects) extracted from defatted mustard seeds by using macroporous anion exchange resins. A strong and a weak anion-exchange resin purification was first tested in a static (batch) mode to determine the optimal operating conditions and then in a dynamic (continuous) mode (column) to validate the process. The results showed that the strongly basic resin PA312LOH ensures better adsorption of glucosinolates and selective purification towards the proteins; furthermore, the experimental data fit well with the Freundlich isotherm. The desorption of glucosinolates was then investigated. Firstly, the operating conditions were optimized by studying the effects of salt concentration and the eluate–resin ratio. This preliminary optimization allowed the recovery of 72.9% of intact sinigrin and the juice purity was increased from 43.05% to 79.63%. Secondly, dynamic (continuous mode) experiments allowed the recovery of 64.5% of sinigrin and 28% of gluconapin by varying the eluent ionic strength and the flow rate. While Lomovskiy et al. [12] examined the kinetics of melanin extraction from the fungus Ganoderma applanatum and buckwheat husk. They showed how particle size was one of the main factors to consider; indeed, ultra-fine grinding of plant raw materials (to achieve a particle size of less than 300 µm) is a very appealing method for increasing the extraction rate. In particular, the kinetics of melanin extraction were described by some kinetic models that include the first-order equation, the Baker and Lonsdale model, the Axelrud equation, and the Ritger–Peppas model.

Finally, the last two works dealt with the production of especially relevant metabolites; Dovale-Rosabal et al. [13] optimized the synthesis of structured acylglycerols (SAcyl) from commercially refined salmon oil to enhance the positioning of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the sn-2 location of the glycerol moiety, which is a fundamental step for the bioavailability of n-3 long-chain polyunsaturated fatty acids. For this purpose, they applied an acidolysis process under the CO₂ supercritical condition on the immobilized lipase B from Candida Antarctica and, following a Draper-Lin composite design through the RSM of 18 experiments, they optimized the extraction including SAcyl compounds. Mass spectrometry (MALDI-TOF) analysis was employed to identify the EPA/DHA location at the sn-2 position in the resulting glycerol moiety. In the fraction obtained, an increase in the EPA and DHA content at the sn-2 position was detected. Remarkably, the optimized SAcyl obtained after 6 h, 82 bar, and 60 °C led to the highest EPA/DHA yield at the sn-2 position in the resulting molecule. Haituk et al. [14] collected, from different tropical host plants, 11 sooty mold isolates identified under Capnodium, Leptoxyphium, and Trichomerium, based on morphology and phylogeny. For the secondary metabolite analysis, the isolates were grown on Potato Dextrose Broth, filtered, and extracted in methanol. The extracts were enriched with proteins and specifically inhibited Curvularia sp. The total phenolic content and ABTS activity were largely recovered from the filtrate corresponding to the inhibition of Alternaria sp.; while the flavonoid and free radical reduction suggested a relative induction of growth of the Fusarium sp., Colletotrichum sp., and Pestalotiopsis sp. Hence, this study revealed the diversity of sooty molds in Thailand via a modern phylogenetic approach.