I need to write about this issue as it's potentially hazardous.

All powdered infant formula (PIF) products and additives can be contaminated [before opening] with pathogens which can grow when incubated overnight with anything, water or breastmilk, at temperatures the organism tolerates (and some have wide tolerance margins, which is why WHO says use water at 70degrees Celsius to make up formula, despite the increased nutritional risks this may entail.) Botulinim toxin and fungal spores are very hardy and almost ubiquitous in the natural environment: since formula is not made in a vacuum, it is able to be contaminated with dust in manufacture and preparation.  (And BTW,  powdered formula  in the unopened can IS a proven source of pathogens which can damage susceptible infants, and incubation times influence the infectious dose it provides.)

Perhaps I've misread the post. But I cannot believe anyone adds powders to breastmilk which is to be used many hours later, much less next day.. Under Australian law, by adding such things to women's milk the person has manufactured a new product that never existed before, and so is responsible legally for the harms it may cause. All PIF should be treated with the greatest caution, and used immediately once prepared. The UK government and WHO both have good instructions re formula prep and use; they didn't dream these up without good reason.

Bacteria can spoil any liquid food product; enzymes can break down fats and produce off odours and indigestible by-products, lactose can bind to proteins and become indigestible so that it ferments in the large bowel.... possible explanations are multiple. There are different smells: acid sour smells probably indicate rancidity and changes to fats due to oxidation, and rotten smells probably indicate putrefactive bacteria at work; but what ingredient is putrefying and what fats are going rancid is unknowable. And some bacteria don't cause changes in smell or taste but create tasteless toxins like botulinum. But I always tell parents to learn the normal smell and taste of any formula they use, because how else will they know when it is off? It may also not be the breastmilk spoiling, but breastmilk digesting fats in the spoiling powdered additive. I don't know the composition of the named product; I wouldn't hazard a guess. 

Freezing breastmilk also causes fats to break down and sour smells to develop, though such milk may not be contaminated at all, simply subject to normal lipase action and physical changes to fat globules . In the past I have used sour thawed breastmilk (as we often use sour bovine cream and soured bovine milk products) in cooking, but babies may object to the changed smell and flavour. And all breastmilk smells and tastes differently, the 700+species of bacteria in it vary from one mother to another (and perhaps one breast to another, explaining door differences there) and maternal diet is always changing.

Beneprotein: bene means good, does it not? what is bad protein? foreign protein babies react to, perhaps? 
I have just googled this product and discovered it to be a 100% bovine whey product with soy lecithin (America's most common allergens)  which is nowhere (that I can find) recommended by its maker for small sick babies. Whey contains more beta lactoglobulin, a potent allergen, and formula companies are lowering levels of BLG  by using more selected whey proteins such as alpha lactalbumin these days (there are hundreds of bovine milk proteins). So I assume follow up studies of exposed children are being done to assess allergy to cows milk protein and soy? When breastmilk contains less than 1gm of protein, how much of the 6gms protein per packet was used? Why? How often? How precise are the powder measurements and what studies of the mother's milk are done before that is adulterated with this cow/soy product? what of the potential renal damage and obesity programming documented from high protein/IgF1 levels? What studies are being done of the impact of altered plasma amino acid levels on brain development and programming? Are levels of tyrosine or other amino acids high enough to be concerned about actual brain insult? What are the effects on dopamine systems of the novel neurotransmitter mix produced by this adulteration? Why isn't Beneprotein listed among the 16 paediatric products of its maker? Does anyone using this stuff -talked about on the company site as a supplement for wound healing- know? Are rates of NEC being tracked if this is used in VLBW infants? Does the manufacturer of Beneprotein advise this use of this product? Major personal and institutional  liability issues arise if they don't!!!

No time for more, but I hope this makes it clear no one should take lightly the addition of possibly unsterile and almost always imprecise and certainly antigen-laden powders to breastmilk. In my view the 20th century NEC epidemic was iatrogenic and inexcusable. When will they ever learn, when will they ever learn......??

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