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07/14/2004
Creamy Technology
If you’re expecting a column on the hydrogen economy and fuel
cells, sorry, but I don’t feel up to such a weighty subject.
Yesterday, my wife and I drove into New York to Columbia
Presbyterian for an unusual case of both of us having
cystoscopies. If you’re unfamiliar with a cystoscopy, consider
yourself lucky and I leave it to you to figure out the obvious path
for the doctor to take to peer into your bladder. Being
thoroughly drained from the procedure, I searched for a light and
frothy subject and have settled on Edy’s Grand Light ice cream.
Lest you think that Bortrum is turning his column into a
commercial, I stress that I’ve never tasted this variety of Edy’s
ice cream. In order to maintain my objectivity, I shall defer any
taste testing of the product until after I post this column.
Dreyer’s Grand Light is presumably the same ice cream, Edy’s
and Dreyer’s being the same company. If you happened to have
read my column of 2/20/2001 (see archives), you know that I
literally owe my life to Breyer’s (not Dreyer’s) ice cream. One
day in 1938, when I was 10 years old living in Mechanicsburg,
Pennsylvania, I was sent to pick up the hand-dipped ice cream
for dessert. The choice was between a local brand, Rakestraw’s,
with its ice cream factory only a block down the alley from our
house, and Breyer’s, sold at a small store a few blocks away. My
family chose Breyer’s. The ammonia plant at Rakestraw’s blew
up, killing two children, at the very time I would have been in the
plant had we not chosen Breyer’s.
Ice cream having played such an important role in my life, I was
naturally attracted to an article by Robert Kunzig titled “The
Physics of…Ice Cream” in the June issue of Discover magazine.
I was surprised to learn that, on many Sundays in my youth, we
in my family often made use of “scraped-surface heat exchange”
technology. Let me describe what we did. During the week, we
saved the cream off the top of the unpasteurized, unhomogenized
raw (yes, I said “raw”!) milk delivered by Konhaus Dairy. On
Sunday, my mother mixed the cream with raw (again, they were
“raw’!) eggs, sugar and vanilla. We then put the mixture in a
two-quart cylindrical metal container into which was placed a
paddle-like device known as the dasher. The capped assemblage
was then placed in a hand-cranked ice cream freezer containing a
mixture of ice and salt.
Either my brother or I was assigned to turn the handle until the
mix was frozen, which took roughly half an hour. One of us
usually was lucky enough to get to lick the dasher. What
happened during the process of hand cranking the freezer? The
addition of salt to ice lowered the melting point of the ice,
extracting the heat from the metal container. Ice crystals formed
on the inside wall of the container. The rotating dasher scraped
the ice crystals off the wall into the mix. In other words, our ice
cream freezer was a scraped-surface heat exchanger – who
knew? It all seemed so simple then.
Ice cream is far from a simple food, as I learned from the
Discover article and visits to the Web sites of the Swiss Federal
Institute of Technology in Zurich (Einstein’s old stomping
ground) and of the University of Guelph in Ontario. Erich
Windhab in Zurich and Douglas Goff at Guelph are two ice
cream researchers mentioned in the article. Windhab is the
inventor of ULTICE, Ultra Low Temperature Ice Cream
Extrusion, an innovation in ice cream technology. Where can
you judge for yourself the merits of ULTICE? Edy’s/Dreyer’s
Grand Light (disclaimer: again, I’ve not tasted it and am not
touting it), is cited in the Discover article as an ice cream
produced using Windhab’s invention.
Commercial ice cream manufacturers have employed the same
scraped-surface heat exchange technology as in the hand-cranked
freezer (invented here in New Jersey in 1846 by one Nancy
Johnson). One commercial scraped surface version is the
“barrel” freezer in which the ice cream mix is pumped
continuously into one end of a tubular container and emerges
from the other end only 30 seconds later as a soft ice cream. A
rotating assembly of scraper blades disperses the ice crystals and
beats air into the mix. The process takes only 30 seconds
because the refrigerant isn’t a salt-ice mixture but much colder
liquid ammonia or Freon.
Typically, the commercial product leaves the “barrel” for
packaging at a temperature of about 21 to 23 degrees Fahrenheit
and only half the water in the mix is frozen. The packaged ice
cream is quite soft and it’s necessary to further harden it by
placing it in a “hardening tunnel” for cooling down to about 4
degrees F. This hardening step adds to the expense of producing
the ice cream and also can lead to an undesirable iciness. During
the hardening, more water freezes and where will it go most
likely? On the ice crystals already formed in the barrel. These
crystals grow larger and you get a “crunchy” ice cream.
To avoid this iciness, manufacturers may use additives such as
gums to slow down the water so it forms new crystals instead of
depositing on existing crystals. Rapid freezing also encourages
the growth of more, finer crystals. The texture and
microstructure of ice cream involves the sizes and distributions
of not only the ice crystals but the air cells and the aggregates of
fat. There’s much more than you probably would ever want to
know about the science of ice cream on Goff’s Guelph Web site.
Goff is a physical chemist, as am I, but I won’t burden you with
the fascinating kinetics and thermodynamics of ice cream.
So, what about Windhab and his ULTICE? In the barrel process,
the paddle scrapers may be rotating at speeds up to about 200
revolutions a minute. I can assure you that, when that ice cream
was getting hard, I was cranking more like 20-30 rpm! The high
speed in the commercial process promotes finer crystals and
smaller fat aggregates and beats in the air, which is 20-50 percent
of the content of commercial ice creams. But, there’s a problem
with the high speed turning. It generates heat and there’s a point
at which the heat generated matches the heat extracted. This
limits the exit temperature of the mix, which leads to less frozen
water and a softer ice cream and the need for the expensive
hardening step.
Windhab decided to ditch the dasher and the scraped-surface
approach! Instead, he came up with a tubular arrangement in
which two parallel screws rotate at 15 rpm or less. As it freezes,
the ice cream mix is kneaded and churned between the threads of
the screws as it heads towards the outlet. The slowly rotating
screws generate much less heat than the 200 rpm dashers and the
shearing of the mix between the two screws leads to smaller size
ice crystals and air cells and fat aggregates. When the ice cream
comes out of the tube, it is at only about –0.4 to 7 degrees F
compared to the 20 plus degrees for the barrel method.
As a result, more of the water is frozen and the ice cream doesn’t
need the hardening. In fact, Windhab says you can take the ice
cream out of the outlet and form pretzels with it! With his
process, the smaller sizes of ice crystals etc. makes the ice cream
taste creamy, “creaminess” being more of a function of the
microstructure of the ice cream and its feel in the mouth than of
its fat content. Here’s where Edy’s/Dreyer’s Grand Light comes
in. It’s being marketed as a slow-churned low-fat ice cream
that’s just as creamy as the old fashioned fatty stuff. According
to the Discover article, the ULTICE process also eliminates the
need for additives.
Perversely, I tend to like my ice cream on the icy side. Perhaps
it’s because to me Breyer’s seems on the icy side and I’m forever
psychologically tied to that brand. However, I’ll certainly give
low-fat Grand Light a try.
Allen F. Bortrum
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