In biology, sugar is referred to as carbohydrate. carbohydrate can be divided into three groups which are monosaccharide, disaccharide and polysaccharide. Monosaccharide can be futher divided into glucose, fructose and galactose. These three monomer is the basic sugar that built up disaccharide and polysaccharide. Starch is classified into polysaccharide because it exists in two types of long chains which is known as amylose and amylopectin. The monomer of starch are a-glucose. a-glucose combines with each other by the formation of glycosidic bonds. According to my
knownledge, diet soda contained starch can be considered "sugar-free" because starch are the storage component of plants and it can be easier to hydrolyses into its monomer which is a-glucose compared to protein or other components.

Hey but in our body starch can be hydrolysed to set glucose free … that means it ultimately gives sugar to body…. then how its sugar free?

well, first of all a diet soda is sugar free, cause there is no sugar in it(by sugar biologists understand sucrose). But the fact that the starch can be hydrolysed to glucose doesn’t have any relevance here. Second, a drink cannot contain starch, cause starch is insoluble in water.

third, you cannot clasify monosacharides as glucose, fructose and galactose. That is like classifying cars into Ford, BMW and Mercedes. There are many other monosacharides except those, just like there are many other cars except those. examples include mannose, arabinose, ribose etc

Sugarfree gum often uses sugar alcohols…depends on how stringent you want to get I mean, if you had paper or cotton inside your soda, I guess you could still call it sugar free.

Thank you that you let me know more about "sugar free" soda and you make me understand more about the concept.