According to Breastfeeding Answers made Simple by Nancy Mohrbacher
'Sour or rancid smelling milk is probably unrelated to milk lipase levels.  According to some milk storage experts, the most likely cause is chemical oxidation, rather than lipase-caused digestion of milk fat or bacterial contamination (Jones and Tully 2006).  Possible contributing factors are the mother's intake of polyunsaturated fats or free copper or iron ions in her water.  In this case, heating the milk can actually speed oxidation, making the problem worse.  While she is storing milk for her baby, suggest any mother whose expressed milk smells rancid or sour temporarily avoid her usual drinking water and any fish-oil or flaxseed supplements, as well as any foods like anchovies that contain rancid fats.  While handling her milk, suggest she also avoid exposing it to her local water.  It may also help to increase her antioxidant intake by taking beta-carotene and Vitamin E.'
( To be clear Soapy smell still thought to be due to lipase levels. ) 
A mum rang me last year with sour smelling milk.  We suspected it may have been due to the different water - she was staying with her mother at the time (can't remember what we knew about the mum's water).  Interesting issue and Nancy's book is great.
Best wishes,Sarah McCann.LLL Leader, IBCLC, BScHons.www.BreastfeedingResourcesNI.co.uk 		 	   		  
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