Question

what is the nature of substance in saliva before meal

Answer 1

neutral is its pH scale

Answer 2

Generally, the pH of saliva is 7.4, which is equal to the pH of blood that is 7.4, & pH of spinal fluid that is also 7.4. 
The pH of saliva is almost constant but it varies depending on the measurement and various Investigators determine the pH of saliva in range of pH 5.0 to pH 8.0 
This varies depending on the type of meal, if you have taken acidic meal then pH is slightly low, if you have taken alkaline meal then pH is slightly high. 
Therefore, it is advisable to wash your mouth before the measurement of pH of saliva. 

Additionally, the pH test of saliva represents the most consistent and most definitive physical sign of the ionic calcium deficiency syndrome. The pH of the non-deficient and healthy person is in the 7.5 (dark blue) to 7.1 (blue) slightly alkaline range. The range from 6.5 (blue-green) which is weakly acidic to 4.5 (light yellow) which is strongly acidic represents states from mildly deficient to strongly deficient, respectively. Most children are dark blue, a pH of 7.5. Over half of adults are green-yellow, a pH of 6.5 or lower, reflecting the calcium deficiency of aging and lifestyle defects. Cancer patients are usually a bright yellow, a pH of 4.5.