Solution

milk-solution.js

setInterval ( getMilk, 1000 );
	var old = "";

	function getMilk() {
	    jQuery.getJSON("/api/milk/single_milk/?dev=1",function(data) {
	        
	            milkAmount = data['result']['milk-amount'];
	        	

	            if(old != milkAmount) { 
	            	if (milkAmount != undefined){
	            		// console.log('not the same');

	            		jQuery('#list').html('<h1>'+milkAmount+' ml</h1>');
	            	}
	            }
	                
	            old = milkAmount;
	            
	        
	    });

}