High-pressure processing (HPP) is a non-thermal operation, which is useful to preserve foods containing heat-sensitive components due to its higher extraction yields, minimum thermal degradation, and shorter time. HPP helps to bring faster diffusion and cell disruption, improving solvent accessibility and leading to better extraction [26]. For example, Zhou et al. [24] applied a non-thermal treatment, ultra-high pressure (UHP), to enhance the content of free, esterified, and IBP fractions in oil palm (Elaeis guineensis Jacq.) fruits, thus leading to a significantly increased TPC and total flavonoid content (TFC), especially those of the insoluble-bound phenolic fraction. Moreover, the antioxidant activity and the content of individual phenolic compounds, mainly caffeic acid, increased in all three fractions by about 30% upon UHP. This could be due to the ability of UHP to destroy the cell walls of oil palm fruits, resulting in their enhanced bioaccessibility [24]. Similarly, free, esterified, and IBPs were extracted using UHP from mango leaves, and results suggested that UHP significantly influenced TPC, TFC, the contents of individual compounds, and antioxidative and cytoprotective properties, mainly those related to IBPs [25]. The positive effect of UHP on the yield of phenolics could be linked to the disruption of the cell wall or the chemical bonds between the cellular components, such as proteins, cellulose, hemicellulose, lignin, and phytochemicals [25]. On the other hand, the free, esterified, and IBPs of the sea cucumber (C. frondosa) body wall was investigated using HPP pre-treatment (200, 400, and 600 MPa for 5, 10, and 15 min) [26]. Treatment of 600 MPa for 10 min improved the TPC, TFC, antioxidant activity, and the contents and numbers of phenolic compounds. HPP could enhance solvent penetration into the sea cucumber body wall through the interference of the cellular matrices, which may improve mass transfer and permeability, causing a better release of phenolics, including IBPs. Likewise, the contents and number of individual phenolic compounds and their bioactivities of sea cucumber processing discards increased upon HPP [27].

Fermentation is a traditional non-thermal food processing method where sugar molecules are converted to lactic acid, ethanol, and gas via microbial action. Microorganisms secrete a variety of extracellular enzymes, including proteases, carbohydrases, and lipases, during fermentation to break down macromolecules such as starch, cellulose, proteins, and phenolic polymers into smaller components (e.g., glucose, peptides, free amino acids, and phenolic derivatives). Moreover, fermentation could release IBPs from the cell wall substances through the degradation of cell walls by cellulase, hemicellulase, esterase, amylase, pectinase, and glucanase [2]. Fermentation improves the antioxidant activity of fermented foods, which could be related to the liberation of IBPs by cell-wall-disintegrating enzymes. For instance, Shumoy et al. [29] determined the soluble and bound phenolics from a traditional fermented pancake (Injera) and found that fermentation increased the contents of soluble phenolics by 92-150% after 72 h and bound phenolics by 13–55%, as fermentation progressed from 0 to 120 h. The improvement of bound phenolics could be related to the break down of ester linkages via enzymes such as xylanases, esterases, and phenol oxidases. However, the percentage of bound phenolic improvement was lower compared to the soluble fraction, which could be linked to the conversion of soluble phenolics from bound phenolics upon fermentation. Furthermore, the organic acids produced during the lactic acid bacteria (LAB) fermentation could hydrolyze bound phenolics from the cellular substances, leading to the release of bound phenolic compounds [29]. In contrast, Yeo et al. [30] suggested that the fermentation of lentil hull significantly decreased the content of IBPs, indicating their liberation upon fermentation. However, the efficiency of bioconversion from IBPs to soluble phenolics was low, suggesting the loss of the released bound phenolics during fermentation. Moreover, all IBPs were not converted into bioavailable soluble phenolics, and this could be due to their structural variations during fermentation.

On the other hand, germination stimulates the rupturing of the dormancy of seeds by sprouting and growth, activating cell metabolism. Therefore, structural macromolecules such as starch and proteins can be converted into smaller molecules by hydrolytic enzymes released from activated cells, affecting the content of bound phenolics and their formation [2]. The effect of germination on the free phenolics and IBPs of mustard grains (Brassica nigraand and Sinapsis alba) was investigated and found to positively affect the content as well as the antioxidant activity of S. alba [31]. However, the opposite scenario was found for the B. nigra, suggesting the conversion of IBPs to soluble phenolics. The liberation of IBPs could be associated with the increased total volume of the cell wall associated with cell division (biosynthesis) during germination [2]. On the other hand, the ratio of IBPs to soluble phenolics of lentils was investigated to monitor changes in antioxidant activity upon germination [32]. Results indicated that the overall ratio of IBPs to soluble phenolics improved during germination, and this could possibly be due to the conversion of phenolics from soluble into insoluble-bound form. The decrease in soluble phenolics could be related to the transportation from the intracellular space to cell walls or the degradation of flavonoids by reactive oxygen species (ROS) [32].

Thermal treatments such as roasting, extrusion cooking, boiling, hot drying, steam explosion, and infrared and microwave heating not only help to improve the flavor, texture, and taste of foods but also release biomolecules. For example, Li et al. [11] stated that thermal (dried, lightly cooked, and well-cooked) processing of hawthorn significantly increased IBPs, but decreased soluble phenolics. Boiling can disrupt the covalent and hydrogen bonds between phenolics and cellular components, releasing more IBPs, while soluble phenolics could degrade under heat treatment or convert into IBPs upon condensation reactions with sugars and proteins via hydrogen bonds [11]. In contrast, hydrothermal (boiling) processing of lentils increased the content of soluble phenolics and decreased IBPs, suggesting their possible release from cellular components [12]. However, the reduction in IBPs was around four times higher than the increase in the content of soluble phenolics. This could be due to the conversion of IBPs into soluble phenolics and/or the loss of bound phenolics upon heat treatment. The loss of bound phenolics could be linked to the formation of irreversible covalent bonds with other macromolecules, including starch, cellulose, and proteins, that cannot be liberated via regular IBP extraction procedures [12]. Similarly, thermal pre-treatment decreased the overall bound phenolics of virgin Camellia oleifera seed oil and increased free phenolics [33]. On the other hand, Peng et al. [34] applied microwave and enzymatic treatments and their combination to release IBPs from grapefruit peel and found that the combination of these treatments afforded the highest content of IBPs. Moreover, the combination of these treatments resulted in a weakening of the dietary fiber, which was confirmed using scanning electron microscopy (SEM), and in removing lignin, which was checked via X-ray diffraction and FT-IR.