The minimum specific gravity of potatoes recommended for making French fries is 1.08.
Specific gravity is a quick way to estimate a potato’s dry-matter and starch content. A value of about 1.080 or higher generally indicates a potato with enough solids to produce a crisp exterior and a fluffy, mealy interior after frying.
Potatoes with higher specific gravity also tend to absorb less oil and provide better processing yields. Russet-type potatoes are popular for fries partly because they commonly fall in this high-solids range. The ideal processing range is often given as roughly 1.080 to 1.095, rather than as one universal exact number.
Specific gravity is not the only quality test. Reducing sugars matter greatly because excess sugar can make fries turn dark during frying and contribute to acrylamide formation. A potato below about 1.070 is generally considered poorly suited to high-quality fries, often producing a waxier or soggier result.