JqueryUI sortable append variable to post

I am using JqueryUI and I need to post a variable along with the array that .sortable("serialize") provides.  I tried to append but had no luck.  The code works fine until I try to append my variable.  

$("ul#adult_list").sortable({ 
         opacity: 0.6, 
         cursor: 'move',
		 update: function(){
					
					var data = $("ul#adult_list").sortable("serialize");
					
					var srt_type = $("ul#adult_list").attr('data-srt-type');
					
					data.append("srtType", srt_type);
					
					$('#message').html('<img src="/loader_small.gif"/><span class="pad_l_2">Saving Changes</span>');
				
					$.post("sortable_ajax.php", data, function(theResponse){
															  
						$("#message").html(theResponse);
						
					}); //end post
				
                }//end update function
         });

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