Among cooking hobbyists, canning is so cool, it’s hot. Making up batches of pickled cucumbers, tomatoes, and peaches purloined from an overripe August garden or the prizes of an exuberant shopping spree at the local farmer’s market, younger home cooks are turning to grandmothers and Web sites for advice on how to preserve favorite summer foods safely when a craving strikes in mid-January. Up until recently, practicing the art of jarring and canning has all…