Hi there, Joyce! You can experiment with adding a touch more xanthan gum but we'd recommend adding no more than 1/8 teaspoon, too much xanthan gum can make for a  gummy texture. Other reasons why the muffins might have been a little crumbly include over-mixing and too much flour in the batter — to ensure you have the proper amount we recommend either using a kitchen scale to weigh your ingredients or using the fluff and sprinkle technique for measuring flour in volume. We hope this helps for future baking adventures! 

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