Lara comments:

"what I've gathered is that there is apparently no evidence for ordinary
milk-shaking causing measurable disruption of cells or proteins"

Actually, measuring this denaturation can be easily done under laboratory
conditions, but to what end?

The question we are really asking is, how can we minimize denaturation while
getting a sufficient mix of cream back into the milk? 

Remember that denaturation means loss of biochemical function, so this is an
important question. In other words, enzymes, hormones, immunoglobulins, and
on and on, lose their bioactive functions (but luckily, retain their
nutritive functions) when denatured. Now, there are vitamins which suffer
from oxidation (adding air to the milk by shaking) and so adversely affect
nutrition, but that is not our focus here.

The answer is, we can minimize denaturation by putting the least amount of
mechanical action into the mixing process. Simply turning the bottle gently
from end to end achieves a satisfactory mixing and retains more bioactivity
than an enthusiastic shaking. If you are shaking hard enough to see bubbles,
you have proven to yourself that you have caused some denaturation.

Since there are so many interesting and valuable bioactive components to
human milk, and since it is simple to go easy with mixing, common sense
suggests we should exercise reasonable restraint with mixing as we do with
heating the milk.

In general, if the baby is strong and healthy, and only received one bottle
a week, for example, I suppose one could heat and shake it to a
fare-thee-well without compromising the baby's health. But if a baby is
receiving several bottles every day while mom is at work, and the caregiver
is making every bottle a milkshake, this unnecessary loss could have a
cumulative effect.

It's not that any mixing causes a total loss of bioactivity. It's a matter
of degree, and a question of moderation. Just treat breastmilk as carefully
and tenderly as you would any other living human tissue, and that will be
good enough.

Arly Helm, MS, Food Science and Nutrition, IBCLC

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