**1. 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 invivo 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.
