## 4 Discussion
Xylitol is a sugar substitute that is used as a natural sweetener in sugar-free chewing gums. There is a lot of evidence supporting the use of chewing gum with xylitol in caries' prevention via different mechanisms such as increasing the salivary flow, increasing salivary pH, and enhancement of the enamel surface remineralization [8; 9]. [PERSON] and [PERSON] [10] reported a significant increase in salivary pH and flow rate after use of xylitol chewing gum. There is also a report of an increase in salivary pH with xylitol chewing gum when compared to other sugar-free chewing gums [11]. In addition, xylitol can decrease acid production in plaque, resulting in a higher plaque pH, which is a less cariogenic environment [12]. It should be noted here that many of the abovementioned effects of xylitol are indisputable and proven in clinical studies. However, when it comes to the pH, salivary pH, dental plaque pH, and overall intraoral pH are very often mentioned in the same context. These are completely different parameters from the point of view of caries' pathogenesis. Thus, when it is said that xylitol contributes to an increase in the pH of dental plaque over a long period of time, its immediate effect on the pH of saliva is neglected, which, according to recent reports, may be just the opposite. Instead of raising the salivary pH, it can be lowered. This is exactly what the results of our research show. Immediately after the start of chewing gum with xylitol, the salivary pH decreases, which is most pronounced between the 5 th and 10 th minutes, and then the values slowly return to the initial ones. This is important from a clinical point of view, because patients are advised to use chewing gum in the first half-hour after a meal, when the most significant acid attack is already present in the mouth, and if the saliva is initially slightly acidic, salivary pH can easily drop below the critical pH value of 5.5. This finding supports the need for some additional caution when choosing caries' preventive measures and advice on consuming xylitol chewing gum in people whose salivary pH is constitutionally lower. Similar studies, where pH of saliva was determined during chewing xylitol gum, used a commercial pH indicator [34] or a benchtop pH meter for measurements of pH [35; 36; 37; 38; 39; 40]. The measurements of the amount of O\({}_{2}\) and CO\({}_{2}\) in saliva samples are rare in the literature and the proposed solutions are based on the chemical and electrochemical reactions and calculations [41]. Our solution uses optical detection and provides a lower number of needed samples for measurements, due to the small surface of PreSens sensors that are in direct contact with the sample. The proposed solution enables the realization of multifunctional MF chips with simultaneous monitoring of different parameters in saliva and uses less than 2 mL of saliva for measurements. On the other hand, the measuring system can be additionally miniaturized and measurement of the collected sample can be performed in a totally closed system to prevent sample contamination. The roles of gases dissolved in saliva and present in dental plaque are undoubtedly important for the development of the caries' process, but their absolute values are surprisingly rarely documented in the literature. Good oxygenation is certainly important for maintaining the integrity of soft tissues and tooth surfaces, shifting the balance from anaerobic to aerobic environment. However, this is a very complex process that is difficult to quantify in clinical settings and even more difficult to simulate in laboratory investigations. Moreover, the amount of carbon dioxide dissolved in saliva is very significant, because the bicarbonate buffer is the most important salivary buffer, on which its resistance to pH changes directly depends. Also, some previous studies have reported that changes in the amount of carbon dioxide in saliva may be associated with an increased risk of caries.