Hullin 119: Join Together (without a band)

A pod that has beans that come in contact with a creepy-crawly will not impart impurity (the beans are protected well enough on their own). In contrast to a pod with smaller legumes. What kind of "protections" of the food can come together to make the food large enough to impart impurity? For example, the 3 peels of an onion have different practical implications with regard to this kind of protection.