I get the spoiled/soapy/sour/metallic milk question fairly
frequently.  My understanding is that it is lipase in the milk which
breaks down the fats and gives it that unappetizing (to some)
pre-digested smell and taste.  After one mom asked me if the
nutritional value of her expressed milk was compromised by this, I
realized the need to tell moms that lipase is not a defect or fault
but a wonderfully functional component of mother's milk.

In _Breastfeeding: A Guide for the Medical Profession_ by Lawrence
and Lawrence, 5th ed, 1999, p. 146:
"Milk fat is almost completely digestible.  The emulsion of fat in
breast milk is greater than in cow's milk, resulting in smaller
globules.  Milk lipases play an active role in creating the emulsion,
which yields a finer curd and facilitates the digestion of
triacylglycerols (TGs).  The newborn easily digests and completely
uses the well-emulsified small fat globules of human milk.  Free
fatty acids are important sources of energy for the infant.

" . . . The lipases in human milk make the free fatty acids available
in a large proportion even before the digestive phase of the
intestine.  . . .

[Note: this may be one of the reasons why formula-fed babies have
been shown to require about 30% more calories for adequate growth
compared to thriving breastfed babies.  Digestive factors such as
lipase ensure that almost all the calories are made available.]

" . . . Additional lipases in the skim milk fraction are stimulated
by bile salts.  Bile salt-stimulated lipase (BSSL) has greater
activity and splits all three ester bonds of the triglyceride.  This
lipase is also stable in the duodenum and contributes to the
hydrolysis of the TGs in the presence of the bile salts.  . . .
BSSL activity is protective against infection by virtue of the
production of free fatty acids and monoglycerides, products of fat
digestion that have antiinfective properties.

"The enzyme activity of BSSL is remarkably stable during prolonged
storage up to 2 years at either -20C or -70C (-4F to -94F).  It has
also been noted to be stable at 15C, 25C, and 38C (59F, 70F, and
100F)."

[Note that freezing the milk does NOT inactivate or even reduce the
activity of BSSL.  The BSSL is in the skim milk fraction -- I wonder
if this is the explanation for Mary Jozwiak's earlier observation
that the 'off' smell and taste seem to develop more when the
expressed milk is vigorously shaken to re-mix the cream and skim
layers?]

And from _Breastfeeding and Human Lactation_ by Riordan and Auerbach,
2nd ed, 1998, p. 144:

"For human infants to digest fat, adequate lipase activity and bile
salt levels must be present.  BSSL and lipoprotein lipase present in
human milk compensate for immature pancreatic function and for the
absence of amylase [necessary for the digestion of starch] in
neonates . . .  When human milk is frozen or refrigerated, lipase is
not affected; however, heating severely reduces lipase activity.
Several protozoa - Giardia lamblia, Entamoeba histolytica, and
Trichomonas vaginalis - have been shown in vitro to be killed rapidly
by exposure to BSSL . . . "

A table on pages 144-145 states that "Lipid (unsaturated fatty acids
and monoglycerides) is shown in vitro to be active against: Herpes
simplex virus, Semliki Forest virus, influenza virus, dengue, Ross
River virus, Japanese B encephalitis virus, Sindbis virus, West Nile
virus."  And "Lipid (free) is shown in vitro to be active against
Giardia lamblia, Entomoeba histolytica, Trichomonas vaginalis."

So -- all this to state with confidence that lipase is a GOOD thing
to have in mom's milk!  It's just inconvenient for those times when
baby is getting expressed milk.  Mom can work around it by planning
ahead to have freshly expressed milk each day, and/or by
heat-treating any milk to be stored.  The length of time that
expressed milk can be stored before developing the 'off' smell and
taste seems to be variable, depending on the individual mother's
lipase levels.  Most women find that it takes at least a day or two
in the fridge before the milk becomes unappetizing, sometimes much
longer.

Margaret
mom of 2, LLLL, AAPL
Longmont, Colorado

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