What chemical solution is the best to use for testing sugar in food plant?
The standard chemical test for sugar is Benedict's test. The reagent, a mixture of (mainly) copper sulfate and sodium hydroxide, is called Benedict's reagent. It can be purchased from many drug stores because it was once the standard test for sugar in the urine of diabetics.
Some of the solution to be tested (this could be the juice or extract of the fruit or vegetable in question) is mixed with Benedict's test reagent (by volume, usually about 4 solution to 1 reagent) and heated almost to boiling. A color change from the blue of the reagent to almost any other color -- green, yellow, orange, red, brown -- is an indication of the presence of what are called "reducing" sugars. For practical purposes, reducing sugars are monosaccharides (simple sugars) like glucose and fructose. It is important to note the most disaccharides (like sucrose, which is the sugar most often used at the table) will not give a positive Benedict's reaction.
The color sequence given -- green, yellow, orange, red, brown -- is in order of increasing concentration of Benedict's test for reducing sugar in the test solution.