$(".crFoodCatalog").live("keydown",function(e){
  var keycode = $.browser.msie ? e.keyCode : e.which;
  var flag=keycode >=48 && keycode <= 57;
  console.log(flag);
  if(!flag){return false;}
});

------解决方案--------------------
keyup试试

------解决方案--------------------
$('.crFoodCatalog).keyup(function() {
 var _this = $(this);
 _this.val(_this.val().replace(/\D/g, ''));
});