These experiments investigated the reaction rate of lactase on milk lactose
by measuring milk osmolality; and explored the effect of formula reconstit
ution on milk osmolality. The investigations measured milk osmolality with
the Fiske Os, freezing-point osmometer, Lactase (Lactaid) incubated with pu
re lactose solutions established the validity of the method. Lactase was in
cubated for 24 hours with four reconstituted milk formulas (Milumil, and Co
w and Gate Nutrilon Plus, Farley's First Milk, SMA Gold). Milk osmolality i
ncreased most rapidly in the first 4 hours after the addition of lactase. T
he lactase enzyme completed over 90% of the reaction within 12 hours. The m
ilk osmolalities ranged from 487 to 591 mosm/kg after 24 hours with 2-4 dro
ps of lactase in 240 mi of formula. A clinical guideline osmolality of 400
mosm/kg was reached in 240 mi of formula at 1 to 12 hours depending on the
dose of lactase. High milk osmolalities due to prolonged enzyme incubation,
or high lactase doses could be reduced to around 400 mosm/kg by dilution o
f 240 mi of formula with an extra 60 ml of water. The initial osmolality of
formula after reconstitution by paediatric nurses varied widely and usuall
y exceeded the manufacturer's quoted osmolality. This initial osmolality wa
s a further influence on the final osmolality reached after the addition of
lactase. It is concluded that the recommended incubation time for Lactaid
of 24 hours is unnecessary as lactase exerts the majority of its effect in
less than 12 hours. Adjustment of Lactaid dose and incubation times will ma
intain milk formula osmolality within standard guidelines. Dilution with ex
tra water will correct inadvertent high enzyme doses and prolonged incubati
on times. The normal method of reconstituting milk formulas from powder may
be unreliable as the manufacturer's quoted osmolality was not reproduced w
hen milk formulas were reconstituted by paediatric nurses.