They are 2 very different compounds, both organic, but with differing effects on food and very different purposes.

Ascorbic is a food preserver - primarily color - while imparting no acidic taste and does not change the pH of the food nor does it have any antimicrobial properties.

Citric acid is a natural food acidifier in that it changes the actual pH of the food as well as the taste. So when drying food it is the color/appearance of the food you want to preserve. But citric is required when ever you need to increase the acid pH of the food. Citric acid is also a natural antioxident, antimicrobal, and germicidal which is why you often see it in cleaning solutions. Not that we want to scrub down the kitchen counters with our tomato juice. ;)

They cannot be used interchangeably in canning/home preserving.

Dave

Here is a link that might be useful: Are Citric Acid and Ascorbic Acid Twins?

Please read through the many many very recent posts recarding the MAIN differences between an ACIDIFIER and an ANTI-OXIDIZER. Both are covered many times even in just the last few days.