## 4 Impacts of HTST and UHT on Food Quality
### Bioactive Compounds
#### 4.3.2. Phenolics  
Phenolic compounds are secondary metabolites of plants that are natural antioxidants. In their structure, they have at least one aromatic ring with one or more -OH groups. Phenolics can be divided into flavonoids (anthocyanins, flavanols, flavanones, flavonoids, flavones, isoflav and chalcones) and nonflavonoid molecules (phenolic acids, stilbenes, lignans, tannins and curcuminoids). They are most often combined with other molecules, such as sugars or organic acids, making glycosides [45]. Phenolics have health-promoting properties, such as antioxidant, anti-diabetic, antihypertensive, anti-inflammatory, anti-allergenic, antiatherogenic, antimicrobial, antithrombic, cardioprotective, vasodilatory, neuroprotective, anti-platelet-aggregation, gene-encoding antioxidant enzyme-stimulating, epigenetic regulation and cytotoxic [45; 79; 80]. Phenolics are reactive and thermally sensitive [81]. The literature reports that quantitative and qualitative changes in phenolics of food matrices are associated with numerous processes, such as plant cell disruption, degradation, hydroxylation, isomerization, oxidative polymerization, methylation, iso-prenylation, glycosylation and dimerization. This can lead to changes in biological activity, destruction, modification or formation of new phenolic compounds [35; 82; 83; 84]. [PERSON] et al. [84], in their study, pointed out the following four possible reasons for the increase in the contents of selected phenolic groups following heating at 110 \({}^{\circ}\)C for 8.6 s of red raspberry juice: (1) release of phenols due to partial hydrolysis; (2) release of phenolic compounds from cells that ruptured because of heating; (3) thermal inactivation of oxidases; and (4) change in the numbers and positions of hydroxyl groups in phenolics due to thermal treatment. Confirmations of the presented possible transformations of phenolic compounds are provided in tables in this manuscript, where it can be seen that with the same preservation parameters, the degree of change in the content of individual phenolic compounds can vary significantly. One of the most important groups of phenolic compounds are anthocyanins. They are glycosides or acylglycos of anthocyanin (or aglycones). They have a C6-C3-C6 skeleton. The main anthocyanins in food are cyanidin, delphinidin, pelargonidin, petunidin, peonidin and malvidin. Their color and stability depend on the pH of the environment, ranging from red (methylation, the most stable), through blue (hydroxylation, the least stable) to colorless [85]. During pasteurization, significant degradation usually occurs, resulting in changes in their colors and nutritional values (Table 2). Their stability and degradation depend on the preservation parameters used (temperature, time, heat dose, presence of oxygen and light) and on the properties of the heated product (pH, presence of enzymes, copigments, proteins, sugars and metal ions, chemical structures and concentrations of anthocyanin compounds). Copigmentation also plays a significant role in stabilizing anthocyanins [81; 85]. The literature reports that anthocyanin retention can be positively affected by a mild heating step (\(\sim\)50 \({}^{\circ}\)C) [85].