So, of course, I did an experiment!
I took two beef fast-fry steaks and marinated one in crushed kiwi and the other in some soy sauce and vinegar ('cause I like that marinade)
I let the meat marinade for a couple of hours - although some recipes say that might be too long.
The steak in the kiwi marinade felt a lot floppier than the soy sauce marinade:
I fried up some onions and garlic in olive oil and then placed the steaks in the pan. I used a trick I had recently seen on a cooking show - to brown the meat well, since fast-fry steaks often curl a bit in the pan, fill a pot half full with water and place it on top of the steaks to press them down. Choose a pot that is slightly smaller at the base than the frying pan so most of the meat is covered. The increased contact with the bottom of the pan will brown the meat better. Leave it for a few minutes and then turn the meat. I just threw in the marinade from both the steaks so the gravy it produced was both sweet, salty and tart - a lovely combo. Served with egg noodles it was delicious.
I had cut each steak in half and served my son and I one of each marinade for the taste test. The verdict was that the kiwi marinade was a bit more tender (although with fast-fry steaks it is a bit hard to differentiate) but we liked the marinade of the soysauce better - need to work on adding other flavours to the kiwi marinade. But the gravy I made with the pan drippings were delicious!
So why did the meat tenderize more?
Turns out that kiwis, pineapples, mango and papaya contain an enzyme that will break down the protein in meats. The enzyme, actinidin, is called a protease enzyme. Enzymes are very large molecules - so large that scientists need a shorthand to describe them - they make them look like ribbons:
|Sequence - each letter represents part of the molecule and the squiggles and arrows indicate how the molecule folds and twists|
"protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein." http://en.wikipedia.org/wiki/Protease
One protein involved is Actin (which is involved with the muscular tissues)
|From Wikicommons: http://en.wikipedia.org/wiki/File:Actin_with_ADP_highlighted.png|
As you can see proteins and enzymes are similar since they are both macromolecules. The actinidine reacts with the amino acid parts of the protein to break the big chains and therefore break down the strength of the muscle meat and tenderize it...
By the way, our bodies also contain proteases such as pepsin and serine to help us break down and digest the rest of the protein we eat.
Addendum (Sept 7, 2011): Just found this M.Sc.(Food Science) thesis by Jin Han on tenderizing lamb with kiwi juice: