Green Extraction Techniques in Food Analysis

Food analysis demands are mandatory from quality, safety, and authentication point of view, and there is an increase in analytical activity in both the control laboratory and research and development. This chapter presents the current state-of-the-art of Green Analytical Chemistry and its main strategies for improving the sustainability of analytical methods, reducing their environmental impact, and offering solutions to the needs that arise from food analysis. Direct analysis is presented as the ideal method that avoids the use of solvents or reagents and the generation of waste. Miniaturization, automation, and the use of sustainable solvents, in addition to reducing energy consumption, are the basic strategies that allow us to achieve the objectives of Green Analytical Chemistry. The reduction of single-use plastic laboratory material and their waste has also been considered an objective for analytical method greenness.

Green extraction of natural products was and will always remain an important research subject in various fields. It is based on developing techniques that meet the six principles of eco-extraction. This concept responds to the challenges of the 21st century, aiming to protect the environment, the operator, and the consumer by reducing hazardous solvent consumption and by favoring the use of more environmentally friendly methods. In this chapter, we review the principles of eco-extraction in detail, followed by an overview of four methods widely used in extraction, namely ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), subcritical water extraction (SWE), and supercritical fluid extraction (SFE).

The present chapter aims to provide a brief overview of the environmentally friendly solvents most commonly used in food analysis, including water, carbon dioxide, ethanol, ionic liquids, (natural) deep eutectic solvents (NA)DES, surfactants, and switchable solvents. A general outlook of their properties, production sources, and classification is provided. The advantages and limitations of the use of these solvents in food analysis are evaluated from the point of view of Green Analytical Chemistry. Some recent applications have been selected to illustrate the potential of environmentally friendly solvents in combination with assisted extraction techniques and miniaturized techniques for the development of green extraction methods in food analysis.

Among the different strategies applied in recent years for the development of green extraction techniques in food analysis, the design and use of deep eutectic solvents (DESs) have aroused the utmost attention due to the advantages provided by these materials in terms of sustainability and versatility. Different types of DESs have been applied in this field including hydrophilic and hydrophobic mixtures, natural DESs, or polymeric-DESs. In this sense, the great availability of components and the wide range of possible combinations constitute potential tools to increase the selectivity and enhance the extraction capacity of the procedures, which is an important concern when complex food samples are tackled. This broad spectrum of possibilities has allowed the extraction of diverse compounds including not only contaminants such as pesticides, plastic migrants, heavy metals, or pharmaceuticals, among others, but also the extraction of biomolecules from food and food by-products. However, despite the advantages of these materials, there are important drawbacks like their high viscosity and low volatility that limit their application. In this context, an important effort has been carried out by the study of different combinations and the development of numerous approaches. In this chapter, the most relevant applications of DESs in the last five years in food analysis have been compiled and discussed in order to provide a global view of the advantages and limitations of the application of these green extraction solvents in the field. Additionally, the current trends and future perspectives in the use of DESs in food analysis are also pointed out.

The significant potential of ionic liquids (ILs) in the extraction and separation of valuable products from food samples is deeply discussed in this chapter, where the main studies on the application of ionic liquids to food analysis are presented. The novel extraction strategies reviewed in this chapter have the potential to significantly enhance the extraction yield, in particular when the combination of ionic liquids with accelerated and green extraction techniques, such as microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE) or subcritical water extraction (SBWE) are used. ILs are considered environmentally-friendly solvents and they offer some advantageous properties which are particularly relevant in extraction systems in food matrices, such as their low toxicity and volatility and different polarity, hydrophobicity and selectivity. A particular section is devoted to microextraction techniques with ionic liquids, which have shown great performance in the extraction of valuable compounds for a variety of food samples. This chapter summarizes and gives an overview of the latest developments and applications of ILs in the extraction of bioactive compounds from food. 

Supramolecular solvents (SUPRASs) are becoming more and more demanded for sample preparation in food analysis. Their inherent properties (e.g. different polarity microenvironments, multiple binding sites, discontinuous nature, easy tailoring of their properties, etc.) make them highly efficient for the extraction of single- and multi-class contaminants in food matrices. Likewise, they offer numerous opportunities for the development of innovative sample treatment platforms not attainable by conventional solvents. In this chapter, the fundamentals underlying the production of SUPRASs and their more relevant properties regarding their application to the extraction of food contaminants are discussed. An overview of representative developments in this field is given based on the different types of SUPRASs applied so far in food analysis. Major achievements attained, mainly related to the extraction of single- and multi-components prior to their quantification by liquid chromatography coupled to different detection systems, are critically presented. The main challenges to be faced in order to get SUPRAS-based methodologies that meet European requirements for screening/quantification of contaminants in food and promote their use in food control labs are discussed.

In this chapter, we highlight the basic concepts behind the use of SFE to select molecules present in food matrices, e.g., carotenoids, essential oils, waxes, and phenolic compounds. Also, we highlight the SFE equipment setup, the methods for process intensification, and mass transfer mechanisms involved in the process, besides the advantages and drawbacks. Supercritical fluids have been suggested as a powerful tool to improve the performance of analytical methods in terms of reduced steps for sample preparation and waste generation, besides enhanced precision and recovery of analytes detected. The offline association of SFE with analytical detection has been elucidated for decades. Currently, many efforts have been made to reach the miniaturization of equipment as well as the online hyphenation between extraction and analytical detection with supercritical fluids as a novel method for sample preparation to detect food analytes in real time with accuracy and robustness. 

Gas Expanded Liquids (GXL) are mixtures of liquid solvents (organic, water) and gases or supercritical fluids with diverse physicochemical properties halfway between pure liquids and supercritical fluids. The possibility of changing their properties by introducing small changes in pressure, temperature, and/or solvent/gas ratio, makes these solvents a very interesting and appropriate option for developing green extraction protocols for food analysis. In general, GXLs have similar densities as the solvent used in their composition, while having improved mass transfer through reduced viscosity, increased solute diffusivity, and decreased interfacial tension. Some other advantages are related to the wide range of polarities that can be obtained, depending on the liquid selected. Moreover, the substitution of a liquid fraction for a gas reduces the final use of organic solvent, thus improving the green character of GXLs. In the present chapter, the physicochemical properties of GXL are addressed along together with the description of applications in the food science and technology area.

 Pressurized liquid extraction (PLE) is regarded as an emergent extraction technique; it is an appropriate tool to obtain green extracts from foods or related samples. Studies on the content of contaminants in foods or food raw materials can be carried out by PLE. In the same way, studies on the obtention of bioactive extracts from classic and emerging foods and their by-products can be carried out by PLE too. Besides sequential process combinations of PLE with other innovative extraction techniques could generate benefits for the food industry. The objective of this chapter is to clearly define the role that this technique plays in food analysis, as well as the updated spectrum of some of its applications during the last lustrum.

 Microwave assistance is an optimum strategy to shorten time, solvent, and energy consumption during the extraction of target solutes from different sources. This intensification strategy has been successfully applied to laboratory methods to enhance the extraction performance of a number of bioactive compounds of interest for food, cosmetic and pharmaceutical applications. This chapter presents an overview of the fundamentals, equipment configurations, combinations with other techniques, and some representative applications for the extraction of compounds from food products and byproducts.

The transition to a circular bioeconomic model that incorporates sustainable extraction processes such as enzyme-assisted extraction (EAE) is motivated by climate change, population growth, and changing diets to address food security and safety, and preserve natural resources (land, and water) and biodiversity. EAE can be applied to extract nutrients and bioactive molecules for food analysis and profiling, and for industrial exploitation of bioactive compounds from novel feedstocks. Commercial extraction processes require high recovery of the targeted compounds and must guarantee the preservation of the biological activity of the products, which is difficult to achieve using conventional methods. EAE is a possible alternative to preserve the quality of final products while reducing the industrial footprint in the food sector at a larger scale. This chapter describes the parameters that impact the extraction yield obtained in the EAE process and provides recent examples of its successful application for the extraction of polymers and bioactive compounds of very diverse matrices (plant, animal, mushrooms, yeast, food waste, and insects), with emphasis on process conditions. This chapter also identifies the challenges and opportunities of EAE and the emerging areas of research to facilitate the economic feasibility of the enzymatic extraction of bioactive molecules. Costs related to enzyme production and its use are one of the main impediments to the industrial application of the EAE process. Recent research progress suggests that reduction of EAE costs can be achieved by a holistic approach considering all steps: enzyme production (by using cheap enzyme production media, in-house enzyme production), selection of feedstock (i.e., food byproducts), enzyme recycling (enzyme immobilization, nano-biocatalysts), the search of novel enzymes (marine degrading polysaccharides), more robust enzymes (i.e., extremozymes) and/or enzyme improvement (bioengineering), and EAE process optimization (minimum optimal enzyme dosage). EAE technology for food analysis and production of bioactive molecules keeps building momentum as it is sustainable, environmentally friendly, and innovative

This chapter reviews the fundamentals of the Pulsed Electric Field (PEF) and its applications to the extraction of high-added value substances from food matrices. The electroporation process on the cell membrane is explained and the most recent works dealing with the use of PEF for extracting essential molecules for the human body such as lipids, phenolic compounds, carotenoids, proteins, carbohydrates, and vitamins, from food and plant matrices, and food waste, are described in detail. The combination of PEF with other extraction techniques is a common practice and improves the extractability of specific compounds to increase the recovery yields. 

 High-Voltage Electrical Discharges (HVED) are considered an emergent extraction technique based on the application of high-pulsed voltages. The aim of this chapter was to review its fundamentals for applications at laboratory and industrial scales. The configuration of devices and employed electrodes is described. Moreover, main steps required for using HVED and most important factors affecting this technique are also highlighted. Extraction of high added-value compounds from food waste and plant matrices using batch HVED has been the most usual application in last five years. In many cases, the low selectivity of the technique has made the use of a solid-liquid extraction step after HVED application necessary.

High Hydrostatic Pressure (HHP) is a green extraction method, which finds several uses in different branches of science. HHP is a novel non-thermal technique mostly used in food processing. The “high pressure” in HHP states an ultra-high cold isostatic hydraulic pressure, which processes basically at low or mild process temperatures (<45 °C) ranging between 100 and 800 MPa. In some applications, this pressure can extend up to 1000 MPa. In food processing, there are several purposes for using HHP, such as sterilizing, coagulating, and gelatinizing food samples. Alternatively, HHP has many remarkable uses in some branches of science besides food processing. This chapter aims to present the capabilities of HHP as a green extraction technique in the food and pharmaceutical industries.