When you test a cracker for sugar, it does not test positive. However if you chew up the cracker and test it again, it does test positive. What are some reasons that this might occur?
A cracker was made with some kind of flour, which contains a carbohydrate known as starch--a polysaccharide. This means that many glucose monomers are joined to form a complex polymer which is in fact, starch. Therefore, once starch is digested by hydrolytic enzymes in the body, it breaks down to the simple monosaccharide glucose.
When you perform the sugar test, you need to add a few drops of the chemical indicator Benedict's solution. This chemical is a deep blue color and will change colors depending on the amount and presence of sugar. You must heat the test--tube for the color change to occur in the presence of sugar.
If you do not get a positive result for sugar in the food sample, you can try to chew it for a few minutes. The enzyme ptyalin is present in saliva which begins the process of starch digestion. It is an amylase which acts specifically upon starch and hydrolyzes it into maltose and dextrin. That is why bread or a cracker which is on your tongue for a minute or two, begins to taste sweet--maltose is a sugar.
Place the chewed cracker in your test--tube with the chemical indicator and heat it for a minute. If sugar is present, it turns green first, then it turns orange or even brick red if a lot of sugar is present.