Citric acid-induced changes in the structure of the mineral component of enamel stored in artificial saliva were studied by attenuated total reflectance infrared spectroscopy as well as complementary electron probe microanalysis and scanning electron microscopy. The results indicate that the application of artificial saliva for several hours (the minimum time period proved is 4？h) leads to slight, partial recovering of the local structure of eroded enamel apatite. However, artificial saliva surrounding cannot stop the process of loosening and breaking of P–O–Ca atomic linkages in enamel subjected to multiple citric acid treatments. Irreversible changes in the atomic bonding within 700？nm thick enamel surface layer are observed after three times exposure for 1？min to aqueous solution of citric acid having a pH value of 2.23, with a 24-hour interval between the individual treatments. The additional treatment with basic fluoride-containing solutions (1.0% NaF) did not demonstrate a protective effect on the enamel apatite structure per se. 1. Introduction Acidic foods or beverages tend to cause tooth erosion, a chemical dissolution of surface hard tissues resulting from a tooth exposure to a variety of acids [1, 2]. Salivary flow plays an important role in minimizing enamel erosion by acid attack [2, 3]. To elucidate how saliva counteracts erosive demineralization, several characteristics of enamel, for example, hardness, surface morphology, mineral loss, and lesion depth, have been investigated [4–7]. It is speculated that saliva buffering capacity, content of calcium and phosphate groups, pellicle acquired on tooth surface and so forth, can act either solely or in combination to inhibit the tooth erosive procedures [8–10]. In vitro and in vivo studies proved that natural saliva and its synthetic substitutes reduce enamel mineral loss [4, 5], enhance enamel rehardening [7, 11], and decrease erosive lesion depth [4, 5]. Sodium fluoride solutions have been aimed at minimizing erosive processes. Recently, it has been reported that the in vitro application of NaF solutions on enamel reduces the acidic-induced mineral loss [12, 13]. Hughes et al. [14] also found that fluoride application in acidic solutions or as a pretreatment reduced enamel erosion although the benefit observed was small and probably not clinically relevant. Hove et al. [15, 16] came to a similar conclusion when investigating enamel exposed to HCl (simulating a gastric reflux situation); while the in vitro study demonstrated a reduction of etching depth by 25% due to a protective effect of
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