A randomized study in healthy adults showed rinsing with tap water or apple juice for 1 minute increased salivary friction compared with unstimulated saliva, with water producing the greater effect. Friction levels in both groups returned to baseline within 10 minutes.
In tribology testing, the mean friction coefficient for unstimulated saliva was 0.011. Following a 1-minute water rinse, the coefficient increased to 0.112, while after apple juice it rose to 0.045. At 10 minutes, values declined toward baseline, measuring 0.049 for water and 0.022 for juice. Statistically significant differences were observed at 1 minute but diminished by 10 minutes.
Protein concentration analysis showed the highest mean for unstimulated saliva at 1.67 mg/mL and the lowest 10 minutes after juice at 1.08 mg/mL. At 1 minute, water produced a lower protein concentration compared with juice.
SDS-PAGE analysis showed no changes in mucin or amylase expression. However, cystatins decreased 10 minutes after juice, and carbonic anhydrase and immunoglobulins decreased immediately after juice exposure, with carbonic anhydrase returning toward baseline at 10 minutes.
Quartz crystal microbalance with dissipation monitoring demonstrated that juice alone formed a sparse, rigid film that persisted following buffer rinse. When saliva was present first, juice compacted the layer before it expanded again after buffer, indicating reversible changes.
The study included 32 adults aged 18 to 56 years, with a mean age of 38 years; 78% were women. Each participant attended two visits at least 3 days apart, rinsing with water at one visit and apple juice at the other. Saliva was collected prior to rinsing, 1 minute after, and 10 minutes after. The water had a pH of 7.5 with chloride, sodium, and manganese content. The apple juice had a pH of 3.5 and contained 111 g/L of sugar.
The researchers noted several limitations. Protein analysis was conducted in a subset of participants. The study population was limited to healthy adults, reducing generalizability. SDS-PAGE measured expression but not enzyme activity, and processing could have affected loosely bound proteins. Results from apple juice may not apply to other acidic beverages.
The findings showed that both water and apple juice caused short-term changes in saliva lubrication and protein composition, which resolved within 10 minutes. Water produced the stronger immediate effect on friction, while juice altered specific protein levels.
“These properties are crucial in understanding saliva’s resilience and ability to revert to low friction (if at all) after being subjected to varying oral (feeding) conditions,” said lead author Samina Zaheer, PhD, of the University of Portsmouth, and colleagues.
The researchers reported no external funding and no competing interests.
Source: PLOS One
