Marinating Meat, Proteins, acidicy, and the isoelectric point.

Marinating Meat, Proteins, acidicy, and the isoelectric point.

The science
Today we will seek an answer to the Question: Why does acid make meat tough and dry?
On page 132 of Cook’s Illustrated, “The Science of Good Cooking“ we find not only a great recipe for Chicken Fajitas but also an interesting paragraph on the isoelectric point of meat and the influences of acidity on protein.

   “A protein molecule normally contains many positive and negative charges. These electrical charges determine how proteins interact with each other. Often there are more positive or more negative charges, which cause the protein molecules to repel each other, keeping them far apart. But sometimes these electrical charges completely balance each other out. This moment of balance is called the isoelectric point. And while being in balance sounds like a good thing, when proteins hit the isoelectric point, they pack closer together squeezing out any excess liquid. And this squeezing, as we know, can make the meat tough and dry. But what causes proteins to reach to isoelectric point? You guessed it: Acid. Most chicken proteins, for example, are slightly acidic, with a pH between 6.0 and 6.5. The isoelectric point of all muscle proteins however occurs at a pH of 5.2”. (132) 

The ingredients for the marinate

   

The experiment that I will conduct today will look at the effects of an acidic marinade on the chicken proteins.

“If there are a lot of acidic ingredients in a marinade, and it is left on a piece of chicken for a long time the risk is high that the pH of the chicken will drop to the isoelectric point, causing the proteins to pack together and the meat to become tough and dry” (132).

Experiment and Recipe
If exposure to acidity for a longer period of time makes chicken tough and dry than we should obviously examine this in a mini-experiment. The experiment will involve the chicken and the marinade, but the dish that I will be making is chicken fajitas as found on page 132 of “The Science of Good Cooking”.

The recipe for the Fajitas – Serves 4 to 6 (ingredients for marinate are in bold)
6 table spoons vegetable oil
1/3 cup lime juice
1 Jalapeño chile, stemmed, seeded, and minced
1 ½ table spoons minced fresh cilantro

Grilling the chicken and vegetables

3 garlic cloves, minced
1 tablespoon Worcestershire sauce
1 ½ teaspoons packed brown sugar
salt and pepper
1 ½ pounds boneless, skinless chicken breasts, trimmed
1 large red onion
2 large bell peppers, quartered, stemmed and seeded.
8-12 flour tortillas

I marinated part of the chicken 3 hours prior to cooking. The original recipe only asked to only marinate the chicken for only 15 quick minutes. The key to this experiment is the difference in time of marinating the chicken. Will the 3 hour marinated chicken reach its isoelectric point due to exposure to acidic conditions and become dry and tough? A blind taste test will give us the answer.

My hypothesis for this experiment is that the chicken that is marinated 3 hours in an acidic marinate will be tougher and dryer than the chicken that is only marinated 15 minutes in the same marinate.  The independent variable is the length of exposure to marinate and the dependent variable is the dryness and toughness of the meat. I will standardize the meat, as I used pieces from the same chicken breast. I also will grill the two chicken groups (3 hour marinated and 15 minute marinated) on the same grill at the same time with the same temperature. 

The marinate affects the chicken 

The two test groups 

The data I collected was based on a blind taste test. I used 5 subjects who all blind tasted 4 pieces of chicken in pairs of 1 from each group. The subjects then determined which piece of chicken they found dryer and tougher.

Results
The blind taste test showed that out of 10 comparisons the subjects chose the longer marinated chicken to be dryer than the shorter marinated chicken. 8 times the 3 hour marinated chicken was picked to be dryer than the 15 minute marinated chicken and only 2 times it was the other way around. The table below shows the results and the subjects who participated in the blind taste test.

A = 3 hour marinated chicken
B = 15 minute marinated chicken

	1st taste	2nd taste
Tom	A	A
Jack	A	B
Madeline	A	A
Patrick	A	A
Tim	A	B

The 3 hour marinated chicken is picked 8 times,
while the 15 minute marinated chicken is only picked 2 times.

Discussion
The results support my hypothesis, and I can safely say that the chicken that was marinated for 3 hours did taste dryer and tougher than the chicken that was only marinated for 15 minutes. We can only speculate however if the isoelectric point in the chicken  was actually reached.
Future research could take a closer look at how the isoelectric point can maybe be influenced without acid. It would also be interesting to compare the isoelectric point of other types of meat and other animals. It is good to note however that not all marinates are supposed to be used several hours before cooking. acidic conditions denature proteins and sometimes these influences actually makes the meat less juicy and tender.

Dinner time
The chicken fajitas with grilled vegetables and guacamole was delicious.
 

Chicken Fajitas for dinner

Sources: 

• Cook's illustrated, The Science of Good Cooking