SUBJECT: Starch digestion/solutions

DATE: 3/97

 

 

For years, I have been doing the classic starch digestion experiment in

lab. The students test the 1% starch stock solution (which has been cooked

to get the starch "disolved") for both starch and maltose. But sometimes,

they get a weak positive maltose in their starch. Is there one kind of

starch that works better in this type of experiment over another? In

addition, sometimes the starch comes out relatively clear and at other

times, quite cloudy.

 

From: eb04124@tiptoe.fhda.edu (Ed Burling)

 

 

Ed Burling asked about starch solutions for use in labs, and it so

happens that a colleague of mine just gave me some info on that subject.

I don't know the exact source of this tip (looks like it was photocopied

out of a catalogue), but it's credited to Rod Sharka at Lyons Township

HS in Illinois. He says that starch doesn't dissolve in hot water well

directly. You can suspend the starch in cold water and then slowly bring

it to a boil, but this can be time consuming. He recommends bringing

about 90% of the water to a boil, and then mixing the starch in about 10% of

the total volume of cold water. Then you can just dump the cold water

suspension into the boiling water and it should go into solution

immediately. Should stay in solution after cooling too. I haven't tried

it yet, but I plan to have my TA's try it soon. Good luck!

 

Guy Farish

Biology Department

Adams State College

Alamosa, CO 81102

(719) 587-7969 FAX (719) 587-7242

 

 

 

This is the standard way to make gravy or white sauce and I would say

starch solution qualifies as a synonym to either. The kitchen truly is a

great laboratory!

 

Chris

 

From: Chris Minor <vcmahaff@iastate.edu>

 

 

 

I think that whenever orders are packed in those cornstarch peanuts, a pouch of

powdered milk and chocolate flavoring should be included. Then, instead of

saving the peanuts with the already existing piles of peanuts or dissolving them

down the drain (which is only fun the first time), we could make chocolate

pudding and have a tasty snack.

Perhaps the organic chem lab could pitch in and make some vanillin, also.

 

On a more serious note....I'm not entirely sure what is happening with the

starch when we dissolve and heat it, and my chemistry is certainly lacking in

this area (TRANSLATION: PLEASE CORRECT ME IF I'M WRONG!).

HOWEVER, the process is definitely not as simple as the posts here imply.

Before heating, a starch/water solution is classified as a non-Newtonian fluid,

and it has some interesting properties. I believe blood plasma is also a non-

Newtonian fluid. If I remember correctly, when we heat the starch solution it

becomes a colloid, like whipped cream or vulcanized rubber, and it is

technically not a liquid. This colloid becomes even thicker and more solid

after cooling, making cold gravy a nice substitute for butter and mayo on roast

turkey sandwiches.

 

And now...off to lunch.

 

Doug Jensen

Berea College

 

 

As one who investigates starch-degrading enzymes I should step up to the

plate on this one! The solubility of starch after boiling depends in part

on its structure. Plant starch is a mixture of amylose (alpha-1,4 linkages

only) and amylopectin (alpha-1,4 and -1,6 linkages). The number of

branches (-1,6) in amylopectin can also vary from plant to plant. These

parameters can affect the solubility of starch. Sigma sells 12 different

kinds (!) and they are not that expensive so I recommend trying several

different kinds to see what works best.

 

Regarding the background "maltose" observed in some starch preps, it might

or might not be avoidable. Presumable you are using some measure of

reducing sugar to detect maltose such as Benedict's reagent. Even starch

has some reducing ends so depending on the size of the starch molecules

(affected by the plant source and the method of isolation) one could get

variable amounts of reducing activity from starch alone. Some starch preps

might also be contaminated with glucose or maltose. Washing the starch

with cold water should eliminate these water soluble sugars. Another

problem to be aware of is that saliva is a rich source of alpha-amylase. If

one talks while preparing solutions, and if one spits while talking

(unavoidable), tiny droplets of spit can land in your starch and initiate

degradation!

 

Hope this helps.

 

Jon

-----------------------------------------------------------------

Jonathan Monroe voice: 540-568-6649 (office)

Department of Biology 540-568-6045 (lab)

James Madison University fax: 540-568-3333

Harrisonburg, VA 22807-0001 e-mail: monroejd@jmu.edu

http://www.jmu.edu/biology/biofac/jmonroe/jmonroe.html

 

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