Novel Non-Thermal Processing Technologies: Impact on Food Phenolic Compounds during Processing

In recent times, food consumption has advanced beyond simply meeting growth and development needs to include the supply of ingredients that can protect against diseases. Among such non-nutritive ingredients are phenolic compounds. These are benzene-ringed secondary metabolites produced in plants upon exposure to environmental stress. Previous studies have linked phenolic compounds to bioactive benefits (e.g., antioxidative, anti-inflammatory, and anti-cancer) with these bioactivities dependent on their biochemical structure and concentrations of individual phenolic compounds present in the food system. However, majority of plant foods are thermally processed into ready-to-eat forms, with these processing methods potentially altering the structure and subsequent bioactivities of endogenous phenolic compounds. Thus, the aim of this chapter is to highlight on emerging nonthermal novel technologies (such as pulsed electric field, radiation, ultrasonication, high hydrostatic pressure processing and high pressure carbon dioxide processing) that can be exploited by the food industry to preserve/enhance bioactivities of phenolic compounds during processing.


Introduction
In recent times, food consumption has advanced beyond simply meeting growth and development needs to include the supply of ingredients that can offer protection against diseases. The demand for such foods can be attributed to proven research data and advocacy by nutrition regulating bodies of the direct relationship between food composition and risks of diseases [1]. Food components that offer protection against disease development are known as bioactive compounds, with majority of these compounds reported as secondary metabolites. Secondary metabolites are non-nutritive compounds produced in plants as protection agents against oxidative stress upon exposure to above-threshold environmental conditions [2]. Among such non-nutritive secondary metabolites are phenolic compounds.
Phenolic compounds are benzene-ringed metabolites, with at least one phenol unit and one or more hydroxyl substituents [3]. Literature has reported about 10, 000 different classes of phenolic compounds, with these classes presented within three main groups including phenolic acids (e.g., hydroxycinnamic and hydroxybenzoic acids), flavonoids (e.g., anthocyanin, proanthocyanidins, flavonols etc.,), stilbenes (e.g., resveratrol and piceatannol etc.,), tannins (e.g., hydrolysable and condensed tannins), lignin, lignans and coumarins [4]. Different in-vitro and in-vivo studies have demonstrated bioactive capacity of phenolic compounds through their antioxidant, anti-inflammatory, anticancer, antidiabetic, cardiovascular protection and anti-cholesterol health effects. However, these reported bioactive properties are dependent on the type, concentration and biochemical structure of phenolic compounds present in a food system. Structurally, the bioactive capacity of phenolic compounds is dependent on factors such as the number and position of hydroxyl groups on the aromatic ring, hydrogen atoms of the adjacent hydroxyl groups (o-diphenol) present in the A, B and C rings of flavonoids, and the presence of double bonds in the benzene ring and oxo functional group (C=O) [5].
Nevertheless, majority of plant foods are subjected to different processing methods prior to consumption, with these processing methods causing changes in the biochemical stability and subsequent bioactivity of phenolic compounds present in the food. Naturally, phenolic compounds are present in foods as free or glycosylated (i.e., bound to protein and carbohydrate molecules), and are released from the food matrix during processing [6]. It is no doubt that, majority of traditional and industrial food processing methods are thermal intensive, with literature reporting a decrease in their concentrations of phenolic compounds and their subsequent bioactivities during transformation into ready-to-eat food products. Therefore, the food industry is continuously searching for alternative non-thermal techniques that can help retain/increase concentrations and bioactivities of phenolic compounds during processing. In this chapter, we focused on current non-thermal food processing technologies such as ultrasonication, high hydrostatic pressure processing, radiation, high pressure carbon dioxide processing and pulsed electric field. We seek to bring to light our understanding of their principles of operation, as well as how these novel non-thermal technologies influence yield and bioactivities of phenolic compounds during processing of some plant-based food products.

Thermal food processing and its effects on food phenolic compounds
Majority of plant foods are thermally processed into their ready-to-eat and stable forms. Notable examples of thermal processing techniques include roasting, microwaving, boiling, steaming and drying. However, research has demonstrated changes in food bioactive compositions such as phenolics during thermal processing. For instance, boiled broccoli showed reduced concentrations of caffeic acid (2.2 mg/100 g), quercetin (10 mg/100 g) and antioxidant capacity compared to their raw forms (caffeic aicd-6.6 mg/100 g and quercetin -23.5 mg/100 g) [7]. Similarly, roasted coffee beans showed reduced phenolic levels and antioxidant capacity. These changes were attributed to oxidation by the triggering of non-enzymatic trans-glycosylation of phenolics into melanoidin as a result of Maillard reaction. Additionally, oxidation of phenolic compounds into their less bioactive forms such as quinones, and (−)-catechin into (+)-catechin has been reported as a result of thermal processes such as roasting [8]. Due to these bioactive reductions, there is a collective effort by researchers to develop non or medium thermal techniques that can help enhance/maintain phenolic levels along food processing.
It is no doubt that, the key purpose of agro processing is to convert raw food materials into their stable ready-to-eat forms through the inactivation of endogenous enzymes and microorganisms. Compared to thermal processing, non-thermal processing involves the application of no or minimal heat to a food system in order to inactivate spoilage enzymes and microbes, without compromising its nutrients, bioactive and organoleptic characteristics [9]. At household and industrial levels, the main advantage of non-thermal processing over thermal processing, is its capacity to exhibit thermal processing benefits while conserving the freshness and phytochemical composition of the food system under low energy consumption. Thus, non-thermal techniques can be considered as green alternatives for production of consumer-acceptable foods with rich composition of bioactive compounds such as phenolics. Examples of novel non-thermal processing techniques currently being applied in the food industry include high hydrostatic pressure processing, ultrasonication (acoustic emissions), radiation, pulsed electric field and high-pressure carbon dioxide processing. Therefore, the objective of this section is to discuss the underlining principles of key non-thermal processing techniques, with the main focus on how they influence concentrations and bioactivities of phenolic compounds during the conversion of food systems into ready-to-eat forms.

High hydrostatic pressure (HHP) processing
Compared to thermal food processing techniques, HHP processing is advantageous to the food industry due to its limited effects on nutritional and organoleptic qualities, as well as its negating effect on food size and geometry [10]. Basically, HHP involves the exposure of food systems to 100-1,000 MPa through a pressure transmitting medium (i.e., water or any other food grade solvent) at room or mild temperatures [11]. Depending on the density of the solvent and food components, temperature during HHP processing can increase by 3°C at every 100 MPa [12]. Therefore, the efficiency of HHP is dependent on parameters such as temperature, pressure level, pressure holding time, liquid/solid ratio, solvent type and solvent concentration [13]. When a food system is exposed to HHP, covalent bonds linking tightly packed cellular structural walls and membranes are broken, leading to cellular increment in porosity and permeability. Based on this principle, it can be postulated that, HHP processing can lead to (1) structural defragmentation for enhanced extraction of free and bound phenolic compounds (2) liberation of endogenous enzymes such as polyphenol oxidase and peroxidase which can cause degradation of phenolic compounds into o-quinones with subsequent reduction in their bioactivities. Thus, the pressure level and temperature applied during HHP are very crucial in order to avoid the generation of increasing temperatures and excess structural damage, which can negatively influence the yield and stability of phenolic compounds during processing. However, according to Heinz and Buckow [14] the low compressibility of covalent bonds compared to weak energy bonds, enables the native structures of low molecular weight molecules (such as phenolic compounds, vitamins and minerals) to be preserved during HHP processing, compared to macromolecules such as starch and protein.
Over the years, the food industry has mainly exploited HHP on commercial scale for preservation of plant foods (e.g., vegetables, juice and beverages) and muscle foods (e.g., meat, seafood and fish). Literature has reported mixed data on impacts of HHP on food phenolic compounds, with some reporting incremental changes and others showing an opposite trend. Patras et al. [15] studied the effect Phenolic Compounds 4 of HHP (i.e., 400 and 600 MPa for 15 min) on phenolic composition of strawberry puree. According to their study, strawberry puree treated with 400 MPa showed higher concentrations of phenolic compounds compared to the puree treated with 600 MPa and the control, with phenolic composition of the 600 MPa treated strawberry puree insignificantly different from the control. In this same study, the concentrations of anthocyanins and other phenolic compounds in blackberry pressure treated puree were increased by 108% with 200 MPa for 5 to 5 min, whereas blackberry puree treated with 400 MPa for 5 to 15 min showed phenolic increments by 92%, compared to their control counterparts. A similar trend was observed with apple treated HHP samples, where a pressure of 600 MPa induced a 75% loss of phenolic compounds after pasteurization, compared to their 400 MPa treated samples which showed significant retention of phenolic compounds [16]. Irrespective of this initial trend, Keenan and his colleagues interestingly observed an opposite trend when the HPP treated apple samples were stored for 14 and 21 days, where the apples treated with 600 MPa significantly retained their composition of phenolic compounds compared to their 400 MPa alternatives. In another study, Huang et al. [17] observed increased antioxidant activity and levels of phenolic compounds (i.e., +/− catechin, chlorogenic acid, neochlorogenic acid, epicatechin, ferulic acid and p-coumaric acid) with HHP treated apricot nectar. Similarly, manuka honey treated with HPP at 600 MPa (10 min, 25°C) showed increased concentration of phenolic compounds after 12 weeks of storage [18]. In another research investigated by Shen et al. [18], HHP (400, 500 and 600 MPa) treated jujube (Ziziphus jujuba Mill.) showed improved concentrations of phenolic compounds with increasing pressure, with the highest concentrations of total phenolic content (7.9%) and flavonoids (18.4%) obtained with 600 MPa.
Overall, reports showing increased concentrations of phenolic compounds can be linked to the structural damage of cell walls and membranes by HHP, thus increasing porosity and facilitating the release of bound phenolic compounds from structural carbohydrates and proteins. Also, reports showing decreased concentrations of phenolic compounds at high pressure levels were attributed by authors to the presence and activation rates of endogenous polyphenol oxidases and peroxidases during HHP as discussed earlier in this section. Thus, in order to retain/enhance the composition of food phenolic compounds and their accompanied bioactivities, HHP should be regulated between room temperature or slightly above, in order to avoid or limit the formation of oxidative undesirable products accompanied with high thermal treatments. Further data on how phenolic composition of foods changed upon exposure to HPP are displayed in Table 1.

Pulsed electric field (PEF) processing
PEF has gained much attention in the food industry due to its appreciable preservation capacity in comparison with high thermal pasteurization. According to Picart and Cheftel [24], PEF is advantageous over high thermal processing due to its capacity in inactivating microorganisms, while still preserving the nutritional and sensory quality attributes (e.g., colour, flavour and texture) of the food. Majority of the literature on PEF has been reported with food preservation, with dearth reports on its effects with phenolic compounds. Primarily, the principle of PEF involves the application of short pulses of high electric fields through a product placed between a set of electrodes [24]. When a food material is exposed to PEF, the generated electrical fields are able to create electroporation across the cell membranes, thus leading to cell membrane porosity as a result of structural disintegration [25]. The efficiency of PEF depends on factors such as electrical field strength, exposure time, applied energy density, pulse width and shape, pulse frequency and applied Novel Non-Thermal Processing Technologies: Impact on Food Phenolic Compounds… DOI: http://dx.doi.org /10.5772/intechopen.98688 food characteristics (e.g., pH and electrical conductivity) as discussed by Mañas and Pagán [26].
With respect to phenolic compounds, PEF can be used to exploit their concentrations and bioactivities from two approaches: (1) since phenolic compounds are mainly stored in the tightly packed cell wall and membranes, structural damage can enhance extraction and subsequent concentrations of phenolic compounds from storage cells compared to their native forms; (2) creation of structural porosity can induce oxidative stress in the food system, thus leading to the stimulation of biosynthesis pathways responsible for the production of antioxidants such as phenolic compounds [2]. For example, red cabbage treated with PEF showed a 2.15 times enhanced anthocyanin yield compared to the non-PEF treatment [27]. Luengo et al. [28] also reported an increased yield of total phenolic compounds with PEF treated tomatoes and grapes. Similarly, concentrations of phenolic compounds after treating borage (Borago officinalis) leaves with PEF were significantly enhanced, according to the work of Segovia et al. [29], with the authors also observing enhanced antioxidant capacities. In another study reported by Liu et al. [30][31], PEF treated onions showed 2.2 and 2.7 times increased total phenolics and flavonoid levels, compared to their control forms. Results from this study also showed enhanced antioxidant capacities with PEF treated onions. Their observation with enhanced antioxidant capacities can be attributed to the increased yield of phenolic compounds and the possibility of extracting other groups of bound phenolic compounds that were otherwise trapped in the cellular walls of the control samples. Regarding extraction efficiency and yield of phenolic compounds from plant foods, lots of positive literature has been reported with PEF applications and shown in Table 2.
PEF has also been associated with increased yield of fruit juice phenolic compounds and bioactivity. For juice application, the most exploited potential effects of PEF include colour, pH, acidity, soluble solids, concentration of phenolic compounds and activity of polyphenol oxidase (i.e., the enzyme responsible for juice browning through the degradation of phenolic compounds into o-quinones) [1]. PEF treated tomato juice showed higher concentrations of chlorogenic acid and quercetin, compared to their thermal treated alternatives [33]. In another study by Agcam et al. [34], concentrations of caffeic acid were enhanced with PEF treated tomato juice over storage time, whereas the concentrations of p-coumaric acid reduced over storage. The authors attributed their findings to the increased activity of hydroxylase, leading to the conversion of p-coumaric acid into caffeic acid along the storage period. Simultaneous to the tomato juice, the authors further reported the highest concentrations of total phenolic compounds (443.42 mg/GAE) with PEF treated orange juice, compared to their thermal treated forms (439.07 mg/GAE). Other authors such as Boussetta et al. [35] also postulated positive correlations between PEF and maximization of phenolic compounds during winemaking. Confirming their postulation is the work of Puértolas et al. [36], where an electric field of 5 kV/cm increased the yield of total phenolic compounds of wines developed from treated Cabernet Sauvignon grapes. A similar interesting observation that proved the relationship between pulse type and exposure time is the study of Delsart et al. [37], where PEF (4 kV/cm at 1 ms and 0.7 kV/cm at 200 ms) exposed Cabernet Sauvignon grapes showed improved extraction kinetic effects on vacuolar and parietal tannins, respectively. Besides extraction and fruit juices, PEF has also been investigated with whole foods. Wiktor et al. [38] investigated different PEF intensities (1.85, 3 and 5 kV/cm) and pulse numbers (10, 50 and 100 exponential shaped pulses) on accumulation of phenolic compounds and antioxidant capacity in apple (var. Ligol). After their research, Wiktor and his colleagues [38] observed the highest total phenolic content and antioxidant capacity with apple tissues treated with 1.85 kV/cm at 10 pulse. Similar to their study, Soliva-Fortuny et al. [39] observed the highest yield of total phenolic content (13%) and flavone-3-ol (92%) with PEF treated apple, compared to control. Despites these positive results, it's also important to highlight that, some studies have reported reduced levels of phenolic compounds in foods treated with PEF. For instance, the findings of Odriozola-serrano et al. [33] and Aguilar-Rosas et al. [40][41] demonstrated significant reductions of phenolic compounds in strawberry and apple juices, respectively, upon treatment with PEF, with these authors correlating their observations to oxidative activities of polyphenol oxidase. In a normal unfractured cell, endogenous enzymes such as polyphenol oxidase are tightly held in packed membranes. However, when the cell is exposed to PEF, the induction of cellular porosity will lead to their release from tightly packed membranes, thus initiating oxidative reactions in the presence of oxygen and substrates such as phenolic compounds. However, these explanations are based on theoretical assumptions. Deeper studies investigating the presence and catalysis of polyphenol oxidase and peroxidase in PEF treated food systems should be conducted, in order to provide scientific proof of their activities, as well as threshold levels of PEF

Ultrasound (US)
In recent years, ultrasound (US) has gained attention in the food industry for pasteurization and preservation purposes. Unlike HHP, US is not only limited to inactivation of microorganism's, but also includes the deactivation of enzymes [1]. For food processing purposes, US can be divided into two categories (1) low intensity US (lower than 1 W/cm 2 ), which is a non-destructive approach used to measure the structure, composition and flow rate of a food (2) high intensity US (between 10 and 1000 W/cm 2 ), which involves the use of high frequencies to cause structural damage to the tissues and membranes of the exposed food system [42]. Among these two approaches, high intensity US is the most applied in the food industry with respect to phenolic compounds for (a) enhancing extraction yield of phenolic compounds from food materials (b) induction of oxidative stress in a food system by causing tissue cavitation and porosity, towards the stimulation of biosynthetic pathways (i.e., phenylpropanoid and shikimate pathways) responsible for the production of phenolic compounds as defense agents against induced oxidative stress [2].
According to Toma et al. [42] when US is applied to a food system, there is the formation of cavitation bubbles which creates a pressure zone change up to 400 km/h, leading to increased porosity, rupturing or removal of cell membranes for enhanced mass transfer from the cells interior upon collapsing. Yu et al. [43] treated Romaine lettuce (Lactuca sativa var. longifolia) with US (25 kHz, 2 kW, 1-3 min) and observed increased total phenolic content, compared to the control. From this same research, storage studies showed 22.5% increased phenolic concentrations after 60 h of storage with US treated Romaine lettuce (1 min treatment time) than their control forms. In another study involving black cumin (Nigella sativa), pretreatment with US (30, 60 and 90 W; 25 kHz; 30, 45 and 60 min) showed increasing total phenolic concentrations (ranges of 93.21 to 106.6 ppm) with increasing US conditions. The authors also observed enhanced antioxidant capacity with US treated black cumin and attributed this trend to the 5% increased total phenolic content in US treated samples compared to the control  Table 3.

Effect of ultrasound (US) on phenolic composition of selected food systems.
trend of phenolic compounds and antioxidant capacities with increasing US treatment parameters [2]. From this study, the highest level of total phenolic acids (216.7 mg/100 g), total flavonoids (203.5 mg/100 g), total anthocyanins (30.35 mg/100 g) and total antioxidant capacities (98%) were observed with 360 W (60 min) US treated common bean sprouts at 9 h of sprouting compared to the control. The authors further observed increased levels of oxidative stress markers (i.e., hydrogen peroxide, catalase and peroxidase) and activities of phenolic triggering enzymes (i.e., phenylalanine ammonia-lyse and tyrosine ammonia-lyase) with increasing US conditions. Thus, confirming with literature that US can improve the yield and bioactivity of food phenolic compounds through the induction of oxidative stress and triggering of the phenylpropanoid pathway. In another study, grape juice treated with ultrasound showed increased concentrations of phenolic compounds by 114.3%, compared to the control [48]. Naturally, phenolic compounds occur in foods as free or bound forms. According to Lieu and Le [48] the capacity of ultrasound to breakdown covalent bonds linking phenolic compounds bounded to cell wall components (i.e., carbohydrates and proteins) led to their release and observed increased yield. These explanations explain the report of Khan et al. [49], where orange peels treated with US (150 W) showed the highest yield of naringin (70.3 mg GAE/100 g), hesperidin (205.2 mg GAE/100 g) and total phenolic composition (275.8 mg GAE/100 g). Besides the principle of cavitation discussed previously in this section, another explanation for this increasing trend has been postulated by Khan et al. [1] to the addition of hydroxyl functional group to aromatic compounds such as phenolics during ultrasonication.

Radiation
Light is one of the most essential factors for plant growth and development. Plant foods grown in the field use sunlight for photosynthesis and production of bioactive metabolites such as phenolic compounds, whereas those grown commercially under controlled conditions use artificial light sources to meet their needs for photosynthesis and production of secondary metabolites such as phenolic compounds. Thus, changes in light quality, quantity, intensity and duration can be exploited to influence the final yield and bioactivities of phenolic compounds in different food systems [50]. According to Bantis et al. [51], five wavelength ranges including red (660-700 nm), far-red (700-750 nm), blue (495-400 nm), UV-A (400-315 nm) and UV-R8 (315-280 nm) has been described with respect to plant radiation. Among these wavelengths, blue, green, red and white are the most reported, with respect to accumulation and bioactivities of food phenolic compounds. Upon exposure to sunlight or artificial light, changes in the light parameters can induce oxidative stress, which are sensed by proteinaceous receptors on cell membranes, for subsequent triggering of metabolic pathways responsible for the production of secondary metabolites such as phenolic compounds [52]. For example, blue light was demonstrated by Qian et al. [52] to improve the biosynthesis of phenolic compounds through enhanced stimulation of malonyl CoA and coumaroyl (key substrates associated with the phenylpropanoid pathway for biosynthesis of phenolic compounds).
For controlled processing, artificial lighting technology such as light emitting diode (LED) has been proven by previous studies to be effective for accumulation of phenolic compounds in diverse human food crops, among other artificial light technologies such as high sodium pressure (HSP) and high-intensity discharge (HID). LED is the most preferred radiation technology among agroprocessors due to their efficient use of energy and capacity to produce food products rich in nutrients and bioactive secondary metabolites comparable to blue, green, red and far-red treated Novel Non-Thermal Processing Technologies: Impact on Food Phenolic Compounds… DOI: http://dx.doi.org /10.5772/intechopen.98688 foods [53]. In the study of Alothman et al. [54], effect of UV-C (dose: 2.158 J/m 2 ) on flavonoid, total phenolics and vitamin C compositions were evaluated. From their result, total phenolic content and flavonoid levels were significantly increased with UV-C treated banana and guava, whereas no significant increments were observed with UV-C treated pineapple fruits and control. In another study, UV-C treated freshly cut mangoes showed increased concentrations of flavonoids and total phenolic compounds with increasing UV-C exposure time (10, 20 and 30 min) [55].
Also, black, red and white rice varieties were treated with gamma irradiation (10 kGy) and its influence on free and bound phenolic composites were investigated by Shao et al. [56]. According to their study, gamma irradiated white, red and black rice varieties produced the highest yield of bound phenolic compounds at 19.7, 40.2 and 59.5 mg GAE/100 g, respectively. The authors observed significant changes in bound phenolic compounds with all varieties of gamma irradiated rice, compared to free phenolic compounds. Bound phenolic compounds in cereal grains are covalently bonded to other cell wall fragments, thus the observation of Shao et al. [56] can be attributed to the capacity of gamma irradiation to break the covalent bonds, cause depolymerization of higher molecular weight phenolic compounds into smaller molecular weight phenolic compounds such as gallic and ferulic acids. Azad et al. [57] studied the impact of different LEDs such as blue (450-495 nm), green (510-550 nm) and fluorescent lamps on accumulation of phenolic compounds in soybean (Glycine max L.) sprouts at different sprouting stages (3, 4, 5, 6 and 7 days). According to their results, total phenolic content and isoflavones were maximum with blue light (at days 5 and 6), compared to the green and fluorescent light treated sprouts. Furthermore, the antioxidant capacity (DPPH) observed with blue light treated soybean sprouts were significantly higher than the other investigated LED treatments. Blue light treated soybean sprouts exhibited DPPH capacity of 75%, whereas the green and fluorescent treated sprouts showed 69 and 58%, respectively. Further literature demonstrating the effect of light on different food systems are displayed in Table 4.

High pressure carbon dioxide (HPCD) processing
Due to the connection between thermal pasteurization and degradation of organoleptic and nutritional qualities, the food industry is continuously searching for alternative techniques that can provide the advantages of thermal processing without compromise on nutritional and sensory attributes. High pressure carbon dioxide (HPCD) processing is a nonthermal mechanism applied for food pasteurization by using pressurized CO 2 between 0.1 MPa and 50 MPa [62]. Some advantages of HPCD include low cost, non-toxicity, non-inflammability and renewability of CO 2 [63]. Factors that influence the efficiency of HPCD processing include food structure, CO 2 physical state, time, pressure and temperature. Until now, research has provided data on HPCD with temperature ranges between 25 and 100°C, it is important to mention that pressure and temperature ranges of >28 MPa and > 60°C are recommended for food applications and fractionation of bioactive metabolites such as phenolics compounds [64]. Although previous authors have tried to explain the principle of HPCD, there still exist a non-well-defined theory. However, a common explanation among all reviewers is a reflection of the potential disintegration of structural cellular membranes and subsequent leakage of cytoplasmic composites during HPCD exposure [65]. Based on this explanation, it can be postulated that HPCD can modify phenolic composition of plant food systems through (1) cell wall and membrane deformation, thus enhancing the release of bound phenolic compounds from structural molecules (2) combination of CO 2 and low temperatures will help prevent oxidation of phenolic compounds through inactivation of polyphenol oxidase (PPO).
Gasperi et al. [66] investigated HPCD (10 MPa, 36°C and 10 min) effects on phenolic composition of carrot juice, with their results showing retention of phenolic compounds. In another study by Del Pozo-Insfran et al. [67], although phenolic composition of HPCD treated muscadine grape juice remained unchanged after processing, their antioxidant capacity was significantly increased after 1 week of storage whereas their control form exhibited reduced antioxidant capacity. This observation could be attributed to the groups of phenolic compounds released upon HPCD processing and the capacity of HPCD to inactivate endogenous PPO, than the control. Thus, helping retain the concentrations and bioactivity of native phenolic compounds in muscadine juice during HPCD processing. Ferrentino et al. [68] reported no changes in total phenolic levels with HPCD treated red grapes. Similarly, apple juice subjected to high thermal processing and HPCD were evaluated for changes in phenolic compositions by Noci et al. [69]. According to the authors, high thermal treated apple juice showed significant reductions in concentrations of total phenolic compounds, whereas the HPCD treatments exhibited insignificant phenolic reductions compared to the control and thermal treatments. In synchrony with these studies are the works of Guo et al. [70] and Agcam et al. [34], which investigated the influence of ultra-high temperature and HPCD processing on phenolic compositions of lychee and litchi juices, respectively. According to the findings of Guo et al. [70], ultra-high temperature processing significantly reduced concentrations of caffeic acid, epicatechin, 4-methylcatechol and rutin in treated lychee juice, whereas HPCD retained concentrations of the aforementioned phenolic compounds after treatment. Comparable to their findings, HPCD treated litchi juice showed preserved concentrations of total phenolic content, rutin, (−)-epicatechin, chlorogenic acid, and antioxidant capacity, compared to  [60] Date palm mazafati (Phoenix dactylifera L.) Gamma radiation; 0, 0.5, 1 and 2.5 kGy Optimum concentration of total phenolic compounds and antioxidant capacity were observed with 2.5 kGy than control and other treatments. [61]  the control whereas their ultra-high temperature treated counterparts exhibited significant losses of these investigated phenolic compounds [34].
Dearth research data is available on the influence of HPCD on phenolic composition of food products during processing. Thus, in-depth studies are crucial in order to stimulate their efficient applications in the food industry. However, available literature on HPCD processing seems to provide a trend of phenolics preservation rather than increments or degradation. This trend may be attributed to the low solubility of phenolic compounds in CO 2 . Therefore, posing a challenge for their efficient extraction from food matrices.

Conclusion and future perspectives
Consumption of phenolic-dense foods has been attributed to positive health benefits such as antioxidative, anticancer and inflammatory effects. It is no doubt that, food processing conditions can either increase or decrease their phenolic compositions and subsequent bioactivities. The wide application of thermal techniques along plant food processing has been associated with high cost, reduced concentrations of phenolic compounds and their subsequent bioactivities. Thus, to encourage the food industry to produce phenolic-dense functional foods, there is an increasing demand to exploit alternative non-thermal processing techniques that can help enhance/maintain phenolic levels and bioactivities. In this quest, literature has identified pulsed electric field, high hydrostatic pressure (HHP) processing, ultrasound, radiation and high-pressure carbon dioxide (HPCD) processing as novel non-thermal techniques that can be exploited to enhance/retain bioactivities of food phenolic compounds. Arguably, these novel non-thermal processing methods will not only help enhance/retain food phenolic compositions but will also help the industry to improve food nutritional value, shelf-life and sensory attributes. However, in order to scale-up their applications, key technical queries such as (a) besides sustainability, will the non-thermal method oxidize phenolics and other bioactive compounds (b) what key parameters of the non-thermal technique should be optimized in order to maximize the yield and bioactivities of phenolic compounds, case by case (c) how does the bioactive composition of different food groups respond upon exposure to each non-thermal method case by case (d) are there any groups of hazardous compounds liberated during non-thermal processing? If yes, what are the health effects of these compounds and how do they influence the nutritional and sensory attributes of the treated food.