This Is Why Your Homemade Fudge Turned Out Grainy - Tasting Table
Briefly

"Cooking sugar is inherently a tricky and challenging process to avoid crystallization which results in a grainy product. Table sugar is composed of two sugar molecules..."
"To prevent this, Stewart says to employ an "invert sugar" like corn syrup, which you'll find in many fudge and caramel recipes."
"One of the most important is to follow the temperature rule of making fudge. A candy thermometer is quite helpful for this process..."
Read at Tasting Table
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