Keywords

10.1 Introduction

Consumers demand fresh and minimally processed foods with natural ingredients that enhance health or prevent disease. This trend raises industries’ and researchers’ interest in developing processing techniques that result in higher quality foods free of chemical additives. Thermal treatment, commonly used to increase the shelf life of foods through the inactivation of microorganisms and enzymes, has detrimental effects on processed foods‘nutritional and sensory attributes, including the loss of antioxidant activity, phenolics, and discoloration. Nonthermal technologies have been highly recommended in the food industry as an alternative to conventional processes to prevent quality losses in food products. High-pressure processing (HHP), ultrasound (Us), pulsed light (PL), UV-light, cold plasma (CP), pulsed electric field (PEF), and radio frequency (RF) are some nonthermal techniques of the emerging research that can improve, maintain or change properties of compounds related to natural additives in food manufacturing.

A suitable nonthermal technology may promote several modifications in natural food additives (NFAs), improving sensory and texture properties, digestibility, and antimicrobial and antioxidant activities. An increase efficiency when extracting intracellular compounds such as phenolics, pigments, starches, and proteins has been the main effect reported in using nonthermal technologies in foods [1,2,3,4,5,6]. In contrast, structural changes such as depolarization and crosslink are reported mainly in macromolecules (e.g., starch and protein) [7,8,9]. Furthermore, technologies such as CP and PEF can act as abiotic stressors, inducing reactions and the formation of bioactive compounds [4, 5, 10, 11].

In food processing, aiming at improving natural compounds, nonthermal technologies have advantages over traditional processes due to the possibility of reducing the use of solvents, energy efficiency, and shorter processing time, in addition to the quality of the final product [1]. Knowing the impact of these emerging technologies on changing the food compounds allows the development of strategies to improve food properties, reduce the number and amount of additives in a given product, or improve the quality of natural compounds that can be used as food additives. The following sections will present the impact of nonthermal techniques benefiting NFAs and discuss the mechanisms associated with them.

10.2 High-Pressure Processing

High-pressure processing (HPP) – also known as high isostatic pressure (HIP) or high hydrostatic pressure (HHP) – is a nonthermal treatment using pressures up to 1000 MPa into a product in controlled time and temperature conditions [12]. The observed HPP effects converges to increase surface hydrophobicity, change the structure of the non-covalent bonds, and cause molecules denaturation and aggregation (e.g., proteins) [13]. HPP can also improve protein functionality and digestibility of cereals and legumes [14] while reducing microorganisms for juice preservation [15].

High-pressure homogenization (HPH) – also called dynamic high-pressure (DHP) – imposes high-pressure conditions by pumping liquid food through a tiny gap in a valve, which results in a high velocity that causes high shear stresses. Consequently, it causes changes in food rheological properties [8]. HPH combines the effectiveness of high-frequency vibration, high-velocity impact, quick pressure drop, cavitation, and intense shear stress in a short time [16]. Typical HPH pressures are moderate and usually up to 100 MPa [17], while HPP can reach ten times more. HPH was also recently applied to food products aiming at microbial inactivation and changes in the protein’s techno-functional properties [12].

High pressures favor extracting bioactive compounds from plants (e.g., carotenoids, chlorophylls, sterols, fibers, and phenolics) [8] by decreasing solvent consumption, increasing extraction yields, and shortening the extraction time. Studies demonstrated that HPP could be applied to foods and increase their antioxidant capacity by maintaining/improving the concentration of anthocyanins [18, 19], ascorbic acid/vitamin C [20], tocopherols/vitamin E [21], which are natural antioxidants in foods. Likewise, the carotenoids (lutein, α-carotene, and β-carotene), which can be used as colorings and antioxidants, were evaluated after HPP treatment (600 MPa) and no differences between the untreated and treated pumpkin purées was observed [22]. Table 10.1 shows more examples of high-pressure technology applied to foods aiming their role as natural additives.

Table 10.1 Examples of high-pressure processing (HPP) applied to foods aiming their role as natural additives

10.3 Ultrasound

Ultrasound (Us) technique uses low-frequency and high-intensity soundwaves, ranging from 20 to 100 kHz [36]. As consequence, it leads to the cavitation phenomenon, forming gas bubbles within the liquid phase and causing local microexplosions and volume increase [37]. The Us provides high shear forces in the extractive agent, accelerates the mass transfer of bioactive compounds [38], and improves solubility due to cellular structure’s high stress and deformation [37]. The increased temperature, turbulence, and cavitation caused by the Us treatment also increase extraction efficiency [8] and reduce extraction time and protein aggregates [39]. Additionally, the cavitation bubbles result in micro-jetting and particle breakdown, improve solvent permeation into the food matrix, and enhance protein functionality [36, 39].

High-intensity ultrasound is a quick and cost-effective technology to modify proteins’ structural and functional properties [40] while recovering valuable bioactive compounds, such as natural additives from plants (e.g., carotenoids, chlorophylls, and phenolics) [41]. Table 10.2 displays examples of Us technology applied to foods aiming their role as natural additives.

Table 10.2 Examples of ultrasound (Us) technology applied to foods aiming their role as natural food additives

10.4 Cold Plasma

Plasma is described as ionized gas containing reactive species (e.g., in air it forms oxygen reactive species, ROS: atomic oxygen (O), superoxide anion (O2), ozone (O3), singlet oxygen (1O2), and hydroxyl radical (OH•), and reactive nitrogen species, RNS: atomic nitrogen (N), nitric oxide (NO•), and nitric dioxide (NO2•)), ultraviolet radiation (UV), free radicals, electrons, and charged particles [6, 8]. Usually, the plasma is generated by applying a high electrical potential difference between two electrodes that causes gas ionization due to free electrons colliding with the gas molecules. The plasma is classified as thermal and nonthermal. There is a local thermal equilibrium in thermal plasma, and all the species are at the same temperature. Conversely, in the nonthermal or cold plasma (CP), there is no local thermal equilibrium, characterized by an electron temperature much above that of the ions and neutral molecules [60].

CP technology has a great diversity of applications in various industry sectors. Specifically, agency regulators have not yet approved the CP application in food [6]. However, a wide range of studies demonstrates the application of CP for nutritional improvements. For example, for a natural food addictive present in food products, CP can be used to alter the physicochemical properties of starches and proteins, the bioactive content and properties, modulate aromas, and change the pigment’s color. In addition, CP effectively inactivates microorganisms [61] and enzymes [62], enhances antioxidant activity [63], and degrades mycotoxin [64], pesticides [65], and allergenic [66]. Table 10.3 displays examples of CP technology applied to foods aiming their role as natural additives.

Table 10.3 Examples of cold plasma (CP) technology applied to foods aiming their role as natural additives

The design aspects of each CP generating system and operational parametric setup lead to different CP properties and, consequently, different food product properties after the treatment. Among others, the most impacting characteristics of a CP system are the source (piece of equipment design), feed gas, electrode material, and operating humidity, frequency, and voltage [6]. Therefore, plasma induces numerous reactions, and the synergistic contributions of them make plasma chemistry rather complex. Besides, multiple reaction pathways are plausible, including activating complex metabolic pathways in fruits and vegetables [10, 67], in special when CP interacts with foods matrix, that are complex and multicomponent systems. The interactions of reactive plasma particles with each food component lead to specific changes in chemical composition, generation of new products, and altering the component characteristics [68].

The phenolic compounds, responsible for the natural antioxidant activity, antimicrobial activity, flavor, and color in fruits and vegetables, can be altered by CP treatment by different mechanisms. The UV radiation and reactive species active cell defense mechanisms, acting as an abiotic stressor and inducing the biosynthesis of the phenolic [63], cause structural tissue damage, enhancing the extractability of bioactive compounds from the vacuoles [69, 70]. CP promotes the tannins depolymerization by breaking covalent bonds and forming smaller phenolic molecules (e.g., tannins to gallic acid) [71]. Also, the chemical transformations in the volatile compound profile are obtained [72, 73]. However, degradation of the compounds may occur depending on the treatment conditions, such as treatment time and feed gas [74, 75].

In response to the abiotic stress caused by CP, there is a higher consumption of sugars as a source of energy for the biosynthesis of phenolic compounds [63]. In contrast, the increase in sugar content is related to the depolymerization of starch, sucrose, and oligosaccharides, forming glucose, fructose, and other small-chain sugars [76]. The depolarization can also form molecules of small-chain of starch and oligosaccharide [9, 77].

CP also results in alterations of starch’s chemical, physical, and mechanical properties. Crosslinking induced by CP occurs due to the cleavage at the extremity of two polymeric starch chains (C–OH) and forming of a new C–O–C linkage [78], resulting in a decrease in the viscosity and retrogradation, which increases the stability of the paste at high temperatures and on cooling [79].

The etching caused by CP treatment on the starch granule surface indicates surface-structural disorganization, easing water permeation into the granules, decreasing long-range crystallites and short-range orders, gelatinization temperature, and melting enthalpy [79]. The starch granules are oxidized by CP reactive oxygen species, decreasing the pH due to forming chemical groups with acidic characters, such as the carbonyl group [80].

Concerning natural pigments, generally phenolics, the main effect caused by CP treatment is their extraction from the vacuoles by the cell membrane degradation. Also, the anthocyanin chromophores (responsible for coloring) can undergo oxidative cleavage and conjugated double bonds break by the reactive species, resulting in product color loss [81]. Besides, anthocyanins show different conformations at different pH, and as the CP treatment tends to decrease the medium pH, a color change can occur. Whereas chlorophyll degradation can occur by several routes, resulting in different colors. Colored intermediate compounds formed in the porphyrin ring’s pathways remain unchanged. The additional oxidation cleaves these intermediates’ porphyrin ring, producing fluorescent catabolites and, subsequently, colorless compounds [82, 83]. Groups can be changed or removed from the chlorophyll molecule periphery. This pathway can remove the phytol, forming green derivatives [83]. The acid environment also can change the chlorophyll color, where two hydrogen ions replace the Mg-atom of the porphyrin ring, converting chlorophylls into pheophytins, an olive-brown pigmentation [84, 85].

The main changes triggered by CP in proteins are ROS and RNS. The reactive species interact with the side chain of amino acid residue and protein polypeptide backbone, resulting in unfolding, crosslinking, fragmentation, and conformational changes [86]. The protein oxidation changes its functionalities, increasing the emulsifying and foaming capacity and foam stability at CP long exposure [87]. The CP in wheat flour promotes polymerization, solubility alteration, and forming of a gluten network, resulting in a stronger dough [88]. Also, CP can inactivate enzymes involved in undesirable reactions, such as peroxidase, polyphenol oxidase, and lipoxygenase, as well as the inactivation of the allergenic protein [6].

10.5 Pulsed Electric Field

The pulsed electric field (PEF) applies high voltage pulses for a very short time (from several nanoseconds to milliseconds) to a food product placed between two electrodes [8]. PEF induces the formation of irreversible or reversible pores in biological cell membranes, known as the electroporation phenomenon (cell electrical breakdown) [90, 91]. This nonthermal technology can improve food products through microbial inactivation due to the dielectric breakdown of the cell membrane and enzyme inactivation. The PEF pretreatment of food products is effective for osmotic dehydration [92], improvement of freezing and thawing processes [93], and reducing drying process time [94]. The main effect of PEF on bioactive compounds relays on the disruption of cells which increases the mass transfer of intracellular compounds, making them more available. As a result, bioactive extraction efficiency increases, shorting extraction time, reducing solvent consumption, and maintaining the quality of these compounds [5]. Table 10.4 displays examples of PEF technology applied to foods aiming their role as NFAs.

Table 10.4 Examples of pulsed electric field (PEF) technology applied to foods aiming their role as natural food additives

The PEF treatment efficacy depends, among other factors, on the electric field intensity, temperature, treatment time, pulse wave, and physical properties of food, such as electrical conductivity, size, and shape of cells. For example, foods with more current-conducting compounds within their cells (e.g., ions of dissociated salts and charged molecules of proteins) are more susceptible to electroporation [95].

The PEF treatment increased the content of phenolic compounds and antioxidant activity of fruits and vegetables and improved the extraction of natural pigments (e.g., carotenoids and anthocyanins) [96, 97]. However, depending on the PEF operating parameters, it may decrease the bioactive compounds content [11, 95]. Also, different PEF parameters can modulate the chemical composition of bioactive extracts [98]. The co-pigmentation and pigments formation may be favored by PEF pretreatment, as observed for winemaking before the macerating fermentation step, in which the polyphenols extraction increased 48%, and the wine color attributes increased 56% [99].

The PEF voltage can cause dissociation of water and other molecules, producing free radicals and hydrogen peroxide [100]. Thus, PEF treatment can act as an abiotic stressor for the biosynthesis of secondary metabolites, increasing the phenolic content in the food products [97]. Also, the pigments chlorophyll are affected by free radicals, mainly the chemical bonds between the pyrrole ring and central magnesium ions of the molecule that can form the chlorophyll aggregated structures and increase the stability [101].

PEF treatment can influence biomacromolecules’ physicochemical and functional properties [102]. In addition to improving the extraction of proteins [103] and starch [104] from the cell tissue, treating proteins with PEF can induce structural and functional changes. The ionization of various chemical groups or breaking of electrostatic interactions alters the secondary and tertiary structure of proteins, consecutively in the loss of α-helix and β-sheet, resulting in modifications such as unfolding, crosslinking, and aggregation of proteins [105,106,107]. Also, PEF alters starch properties by disintegrating amylopectin linkages and damaging starch granules, allowing water molecules to ingress into the crystalline region, decreasing crystallinity, gelatinization temperatures, and increasing water holding capacity [108]. These damages facilitate enzymes attack to the granules, increasing the digestibility. On the other hand, starch acetylation can increase the content of slowly digestible starch fractions [109].

10.6 Pulsed Light/UV-Light

Ultraviolet (UV) and pulsed light (PL) treatments are used as alternatives to chemical and thermal processing to inactivate microorganisms on surfaces, liquid foods, beverages, ingredients, and packaging, producing foods with better quality, extended shelf-life, and often with enhanced health benefits [111,112,113]. However, both technologies are based on irradiation, so the products can suffer photoreactions, depending on the food‘s optical properties, such as absorption, transmission, reflection, and the light spectrum emissions and irradiation doses. Once foodstuffs are exposed for too long and high doses of light energy, secondary products can be produced and cause matter changes, such as discoloration, off-flavors, loss of vitamins, and other essential nutrients [114,115,116].

The efficacy of both treatments is generally related to the absorption of UV-C light by microbial nucleic acids, causing photochemical changes, but for PL treatment, photothermal and photophysical changes on microorganisms are also related [111]. Therefore, UV-light technology is often found as monochromatic light in the UV-C spectrum (λ = 254 nm). UV-C devices work with low power; thus, long times are needed to be effective against microorganisms, which can cause the degradation of some other compounds, such as carotenoids, chlorophylls, flavonoids, and lipids [114]. In contrast, PL is found as a polychromatic light that includes ultraviolet (200–400 nm), visible light (380–780 nm), and infrared radiation (700–1100 nm) [111]. A capacitor stores high-intensity power energy and is released in short-intense pulses no longer than 2 ms. US Food and Drug Administration (FDA) approved PL to treat food surfaces with fluence levels not higher than 12 J cm−2 [117]. Together with other technologies and even mild temperatures, these technologies can enhance results further.

The literature is scarce on NFAs as ingredients in foodstuffs treated by light. However, natural compounds extracted from raw foods can be added to improve nutritional or functional properties of other products, Table 10.5. Plants exposed to light with different wavelengths have shown synthesized pigments, such as anthocyanins, associated with enzymes changes [116], which is often related to the visible blue light [118]. However, the exposition of these compounds to UV-C light causes their degradation [116]. Curcumin is natural pigment extracted from plants that have been discussed in the literature because of its photosensitizer property, which generates reactive oxygen species (ROS) after excitation, causing microbial inactivation [119].

Table 10.5 Examples of UV and Pulse Light (PL) technology applied to foods aiming their role as natural food additives

Vitamins are crucial for human metabolism, and many foods can be fortified with these additives, which can be natural or synthetic. The literature reports UV-C light transforming ergosterol into vitamin D2 [120], and to break chemical bonds between vitamin B3 and nucleotides, and vitamin B5 and coenzyme A, turning into more bioavailable vitamins [115]. Nevertheless, light radiation also degrades vitamins C and B6 [115, 121].

Light control is essential in the flavor stability of vegetable oils and other unsaturated fats. The literature reports many cases of light-induced oxidation, as rapeseed, corn, soybean, and coconut oils and milk fat subjected to light in the wavelength ranging from 350 to 750 nm [122]. Fishes are rich in unsaturated fatty acids such as Omega-3 polyunsaturated fatty acids (PUFAs), a natural health additive used in supplements. Lipid oxidation increased by 6% when a sample was submitted to low UV-light doses and 13% for high doses [123]. A study on the addition of essential oils to foods aiming to improve the lethality of a UV-light treatment showed a synergistic effect on inactivating biofilms of S. Typhimurium [124].

Proteins are functional molecules that, after exposure to PL, aggregate with lipids and carbohydrates, reducing their solubility [125]. In milk proteins, it was reported that the only conformational modification was the aggregation of disulfide bonds [126]. However, when whey protein isolated (WPI) was exposed to PL treatment, its solubility and foaming ability were improved due to the dissociation and partial unfolding of WPI [127]. In addition, egg white proteins treated by PL showed structural changes, resulting in different functional properties, such as increased immunoreactivity, decreased gelling temperature, and higher foam stability [128].

UV-light and PL treatment have shown the potential to inactivate bacteria in clear and transparent liquids. However, their efficiency is compromised as turbidity increases, and for solid foods; low absorption and shadowing effects are challenging to be solved for the application of PL in the food industry. On the other hand, photodegradation products are restricted to the product’s surface, often having low impact on the sensory and nutritional properties of the products.

10.7 Conclusions

The technologies reported herein are attractive due to their capacity to produce and preserve natural additives in foods, superior to the food quality when conventional thermal processes are applied for inactivating pathogenic and spoilage micro-organisms and enzymes. In addition, those nonthermal methods cause structural changes in cell membranes and structural modifications in some (macro)molecules, favoring extraction, digestibility, and desirable functional/nutritional properties.

Some of these emerging nonthermal technologies are on a small scale (laboratory or pilot level) and need to be scaled up before industrial use. On the other hand, the development of industrial equipment after scientific development has been intense and fast, as is the case of high-pressure systems. It is crucial to consider governmental regulations in each country and the safety aspects of each pair of technology-food product. Furthermore, costs, cultural changes, and consumer awareness are challenges in implementing a nonthermal process to obtain and modify natural compounds. Based on the state-of-the-art, these emerging nonthermal methods will keep evolving and reaching the food industry since they require fewer chemical additives, favoring natural additives usage.