Elisheva wonders why it may be preferable to chill each sample of expressed
milk before adding it to already chilled milk.

This is reasoning based on what we know about bacterial growth in other
media (media here means bacteria food, not newspapers/TV/internet).  If you
really want food to spoil fast, then heating it up, and cooling it partly,
and heating it up a little again, is a good way to do it.  Of course, fresh
milk is not quite the same because of the antibacterial qualities in it, but
even human milk is not totally immune to spoilage.  It is quicker to chill a
small volume than a larger volume, and one avoids the pitfalls of letting
all the milk get warmed, then cooled, then warmed, then cooled again.

It may be unnecessary, but it's safer to err on the side of caution, and
follow a procedure which should minimize opportunity for bacterial growth.

Another poster would advise holding milk from one pumping, at room
temperature until baby was ready for the next feed.  I am not sure what
advantage this gives.  Is it to avoid needing to warm it up from fridge
temperature?  Milk loses nothing from being refrigerated fresh.  Freezing
and pasteurizing both change the quality, in specific ways.  Refrigeration
only keeps it cold and prolongs the safe storage time.  If warming it is a
problem then I can understand it.  If the mother lives in a house with hot
running water then getting the milk up to a nice temperature takes a very
short time.

Along the same lines, I would feel more comfortable advising mothers to
thoroughly rinse their pumps as soon as possible after use.  If milk is
allowed to dry on them, there is a possibility of some of that sticky stuff
being very difficult to get off later, and there could be small crevices
where there are traces of 'old' milk.  Rancidity and bacterial growth are
two possible concerns.

Please note I am not making sweeping claims or predicting disaster if my
suggestions are not followed, merely pointing out possible consequences
which are easily avoided with a small amount of effort.  For a baby getting
an occasional meal of expressed milk it probably makes no difference, and
the nitrosamines if a rubber bottle teat is used are likely of more concern.
For a baby subsisting on expressed milk alone I think scrupulous hygiene in
the handling of this very valuable food is a virtue to be striven for.

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