## 4 Impacts of HTST and UHT on Food Quality
### Bioactive Compounds
#### 4.3.1 Vitamin C  
Vitamin C is a thermolabile, water-soluble antioxidant. It can scavenge singlet oxygen, superoxide radical, hydroxyl radical and hydrogen peroxide [19]. The kinetics of its decomposition depends on numerous factors, such as temperature, pH, light access and the presence of oxygen, metal catalysts or enzymes[71]. Vitamin C contents in productsdecrease during storage [72]. Degradation is most often associated with the effects of available oxygen (the increase in temperature determines the mobility of the molecules, thus increasing the contact of ascorbic acid with oxygen), causing oxidation of ascorbic acid to dehydroascorbic acid and further degradation to 2,3-diketogulonic acid or the formation of furfural and 3-hydroxy-2-pyrone. Conversion to dehydroascorbic acid is conducted without losses in vitamin C activity, but further degradation results in a loss of these desirable properties [73]. Vitamin C may be partially protected from degradation by the presence of phenolics [74]. Ascorbic acid degradation can reduce the nutritional and sensory quality of food products and cause changes in color and flavor due to nonenzymatic browning reactions [19]. Vitamin C has numerous health-promoting properties and is used in the prevention of numerous diseases, such as heart disease, cancer, the common cold, diabetes, atherosclerosis, macular degeneration, strokes, cataracts, glaucoma and COVID-19 [75]. The addition, it is involved in collagen synthesis and improves iron absorption [72]. UHT heating reduced the vitamin C content by 27.9% in red grapreruit juice [53], 13.4% in korla pear juice [54], and by 38.4% and 22.1% in clear and cloudy kiwifruit juices enriched with selenium, respectively [25] (Table 2). Much greater changes were observed by [PERSON] et al. [12] in UHT-processed lettuce juice--the vitamin C content dropped by 85.1%. Other researchers have also pointed out the degrading effect of the UHT method in this aspect [18; 22; 70]. On the other hand, [PERSON] et al. [44] indicated that UHT had no effect on L-ascorbic acid in mango nectar immediately after preservation but showed a significant decreases during 16 weeks of storage at 4 \({}^{\circ}\)C and 25 \({}^{\circ}\)C, respectively. [PERSON] et al. [36] also showed no significant changes in vitamin C content after preservation. On the other hand, researchers have indicated that by heating at 136 \({}^{\circ}\)C for 4.1 s, vitamin C can be extracted and significantly increase in content in acerola juice [76; 77]. Researchers highlight that the use of a closed system, which is tubular heat exchange, allows for the protection of this bioactive compound from the degrading effects of oxygen [76]. HTST conditions like 72 \({}^{\circ}\)C/15 s resulted in a significant reduction in the ascorbic acid content of peach juice immediately after preservation of 22%. During 28 days of storage, ascorbic acid decreased by 54.3% in HTST-treated juice, while in raw juice, this decrease was only 16.7% [31]. Increased temperatures significantly contribute to changes in the content of bioactive components [65; 31]. [PERSON] et al. [14], in modelling studies on orange juice, indicate that as the HTST preservation temperature increases while maintaining the same processing time, the vitamin C content decreases. Vitamin C retention in orange juice processed at 70, 80, 90 and 100 \({}^{\circ}\)C over 10 s were 0.92, 0.87, 0.85 and 0.82, respectively. Other researchers also report decreases in vitamin C contents after HTST preservation, as follows: [PERSON] et al. [57] of 12.9-59.6%, depending on the juice's composition; [PERSON] et al. [5] of 27.2%; [PERSON] et al. [56] of 14.3%; [PERSON] et al. [70] of 19.8%; and [PERSON] et al. [22] of about 9%. In lemonade, the ascorbic acid content decreased by 91.7%, while in citrus juice it only decreased by 12.1% under the same preservation conditions [64]. [PERSON] et al. [78] revealed that there was no significant change in vitamin C content as a result of heating orange juice at 76.8 \({}^{\circ}\)C/15 s. [PERSON] et al.