Re: What happens to animal cells when they are heated to a high temperature?

Hi Marie,

I will answer your question based on my knowledge of Food Science and 
information provided in the text "Foundations of Food Preparation" 1996. 
6th edition, J. Freeland-Graves & G. Peckham, Prentice Hall, Englewood 
Cliffs, NJ.
As the temperature increases during cooking, enzymes in the interior of the 
meat that degrade muscle proteins are activated. Between 104-122 F, protein 
chains begin to unfold and denature (proteins lose their quaternary and 
tertiary structure. These changes eventually cause the structure of the 
myofibrils to break and shorten. As the protein molecules aggregate, 
immobilized water is freed, decreasing the ability of the meat to hold 
water. You referred to this change in your question. Protien denaturation 
and water loss from the muscle contribute to increase in toughness of meat 
during cooking.
High temperatures results in the fat melting, and this fat can be absorbed 
by the meat or lost as drippings. The fat that covers the surface of the 
meat reduces water loss, which can aid in maintaining juiciness. 
Heat can also tenderize meat by breaking down connective tissue (collagen). 
The heat can denature the collagen resulting in the hydrolysis of the 
protien chains. The end product of this process is the formation of 
gelatin. Cooking at low temperatures for long periods of time causes two 
changes (1) Hydrolysis of collagen and (2) Breakdown of muscle fibers by 
proteolytic enzymes. These two processes result in meat that is more 
tender. This is why it is recommended that one use slow cooking methods for 
tougher cuts of meat (i.e. pot roast).
The color change that occurs to meat on cooking is a results of myoglobin 
breakdown (pigment that causes fresh hamburger to be red - oxymyoglobin). 
The surface of cooked meat browns as a result of the partial brekdown of 
its proteins, fat, pigments and other constituents. The browning that 
occurs on the surface of fried meat is the result of the Maillard reaction 
(reaction between the carbonyl group of a reducing sugar and and amino acid 
or amino group (i.e. that found on lysine)of a protein or peptide.
I hope this answers you question.

Sincerely,

Al Bushway
Professor of Food Science