The Science of Yogurt Marinades

A few years ago, I went on a radio show and the host asked me about marinating meat overnight. I admitted that I indeed left meat to marinate overnight, with the caveat that it depended on the type of marinade and the type of protein. Tossing a bag of meat with a marinade into the refrigerator the night before is convenient, but there’s a strong likelihood that the surface of the meat will turn mushy due to acids in the marinade. That's why the general consensus is to avoid marinating meat for lengthy periods of time. But I’ve never been convinced that this rule applies to all acids in all marinades. After all, we tend to marinate fish, poultry, pork, lamb, and beef in different ways, so why shouldn’t we think about acids in marinades with similar nuance?

Generalizations make it easier to remember important rules in the kitchen, but they often run the risk of letting significant details slide. Consider the way we often think of acids in the kitchen. We’re taught to remember a few basic properties: they taste sour, they have a low pH, they react with baking soda, and they denature proteins. But different cooking acids possess different properties and, consequently, behave quite differently from one another. When I was researching and writing my new cookbook, The Flavor Equation, I learned how these differences in behavior can be applied to great effect in our kitchens. For example, citric acid is much more efficient at extracting pectin from potatoes when compared to other acids.

In some Indian cookbooks and recipes for yogurt-marinated foods, you might notice this detail in the instructions: “marinate the meat for 4 to 6 hours, preferably overnight.” When I marinate beef, chicken, or lamb at home with yogurt-based marinades, I often marinate them overnight in the refrigerator and I’ve never noticed any mushy textures or loss in quality, running counter to conventional wisdom. In fact, I find that tougher cuts of meat, especially beef and lamb, become fully tender during cooking a little quicker than if I marinated them for a shorter time in the same marinade.

Besides my experiences in the kitchen, I also knew from my past work as a molecular biologist that the various properties of acids are capitalized on in labs. When preparing tissue samples for microscopes and staining with various dyes, we often used different types and combinations of acids to prepare our samples, and this played an important role in the quality of staining. Based on these observations, I thought there was a pretty good chance that using different acids in marinades would produce different results, so I conducted a few easy experiments in my kitchen to see whether and just how much changing the acid type might change the texture and flavor of marinated meats.

The Role of Acids in Marinades

In the kitchen most of us work with three common acids—vinegar, which contains acetic acid produced by fermentation of sugars; citric acid, which comes from citrus fruit like lemons and limes; and lactic acid, which is produced by the fermentation of sugars (such as lactose in milk) by lactic acid–eating bacteria. These three acids share properties common to all acids—the sourness, the low pH, and the ability to denature proteins—but their chemical structures are drastically different. In addition, of the three, acetic acid is the only one that animal cells cannot synthesize on their own: during anaerobic respiration, muscles produce lactic acid, and citric acid is produced inside the mitochondrion (the energy house of the cell) during metabolism.

Since cells can produce certain acids, it’s not surprising that they've evolved to develop mechanisms to deal with the harsher effects of those acids. Cells are made up of many different molecules, including proteins, and they should be able to protect themselves on a daily basis from acids in their environment because, if left unchecked, these acids can unleash havoc on the cells’ structure and functions. Special proteins called transporter proteins help shuttle and move acids like citric and lactic acid within cells to maintain the pH balance and equilibrium. (While under normal conditions, acetic acid isn’t produced or synthesized by animals, there are some transporter proteins that it can interact with.)

Based on this knowledge, it would make sense that lactic acid would have the mildest effect on proteins in meat, since muscles must regularly regulate the amount of lactic acid they contain; acetic acid would have the harshest effect on proteins in meat, because muscles haven’t developed robust mechanisms to deal with it; and citric acid’s effect on meat would fall somewhere between those two extremes, since it is present in cells in more limited quantities. Another important point to mention: fresh meat might come from dead animals, but the cells aren’t "dead"; they are still biochemically active, which means the processes described above still occur.

The Basics of Marinating Meat

Before we get to the experiments, let’s go over some of the basic science behind marinating meat and how a marinade affects texture and water content.

A marinade is typically a water-based mixture of various ingredients, each of which is meant to improve either the flavor or the texture (or both) of whatever is being marinated. But marinades also improve the “juiciness” of cooked meat by encouraging the absorption of water, which is another way of saying a marinade can improve meat’s water-holding capacity, that is, its ability to bind water.

About 5% of the water in animal flesh is bound to proteins; the remaining 95% is held between the filaments that make up the muscle. The proteins and amino acids present in meat determine how much water it can absorb during marination. For example, the muscle protein myosin can bind a large quantity of water because it’s rich in aspartic and glutamic acids, amino acids that have molecules that can bind at least four to seven molecules of water.

But a given piece of meat’s water-holding capacity can also be altered by raising or lowering it through chemical processes. The most obvious example of this is applying heat: During cooking, a large quantity of water is lost as proteins in the muscle—like collagen, myosin, and actin—shrink during denaturation and push water out, similar to the way water is released from a squeezed sponge.

Here are two of the primary ways marinades can alter a meat's water-holding capacity:

• Table salt (sodium chloride), in addition to seasoning the meat, can help dissolve some meat proteins like the muscle protein myosin, which can increase tenderness. Salts of phosphate can increase the amount of water absorbed by chicken breasts (this is true of table salt as well, although to a lesser degree).
• Acids help denature proteins by changing their shape, which can, in beneficial instances, tenderize the meat; as mentioned above, this effect can be so pronounced as to make the meat mushy and unpalatable. When proteins change their shape, amino acids that were once hidden might be exposed, and they provide new possibilities for water to interact and bind to the proteins in meat. How acids affect protein also depends on the type of protein; for example, acids also help solubilize the collagen present in meat and as a result collagen-rich meat will turn very tender.

Setting Up the Experiments

When I first started to design my experiments for this study, I thought I’d do a direct comparison of how meat fared when exposed to marinades that incorporate three different sources of acid: yogurt, vinegar, and citrus juice. However, the problem with all of those experimental setups was that including yogurt made any comparison unreliable because it’s too complex: yogurts contain so many different ingredients and substances besides acids, like fats, proteins, phosphates, etc., all of which can affect the meat. I needed a simpler experimental system, so I decided to compare the effects of pure cooking acids on meat.

For my later experiments, I prepared solutions of the cooking acids (my concentrations were based on what they would be, on average, in different marinades) in distilled water. Since salt plays a very important role in how much water is retained by proteins in meat and dissolves some muscle proteins, I also set up a separate set of experiments where the cooking acid solutions contained salt. The experiments were primarily focused on measuring and comparing the amount of water absorbed and lost by meat that has been marinated and cooked. Some amount of water loss is to be expected in each case since cooking (with heat) inevitably causes water loss from proteins.

My overall experimental set up involved taking pieces of skinless, boneless chicken breast or boneless leg of lamb and marinating them in different marinades, with and without salt. The weight of the meat was measured three times to determine the percentage increase or loss in weight: at the start, after marination (overnight), and after cooking. The chicken was cooked sous vide at 149°F (65°C) for 1 hour while the lamb was cooked at 165°F (74°C) for 4 hours. I used sous vide because this method provides higher precision, the meat heats uniformly from all sides in the water bath, and it’s kept in an enclosed chamber, which effectively eliminates the effects of evaporation.

Effects of Different Acid-Based Marinades on Meat

For the first round of experiments using yogurt marinades, I didn’t notice any statistically significant differences between cooked chicken marinated in just yogurt overnight and cooked chicken that wasn’t. Adding salt seemed to reduce the amount of water lost after cooking in both yogurt-marinated and non-yogurt-marinated chicken.

But, as I mentioned earlier, it’s not that easy to make any sense of the results when using yogurt using my simplistic experimental set up, so in the next round, I set up two sets of separate experiments, one using boneless, skinless chicken breasts and another with the boneless leg of lamb. In each instance, the meat was marinated in a solution of an acid (either lactic, citric, or acetic acids) with and without salt overnight in the refrigerator and cooked sous vide.

Effects of Different Acid-Based Marinades on Chicken

In chicken, I observed some minor differences in terms of the percentage increase in weight after marination and percentage loss in weight after cooking. But the only statistically significant quantitative difference I noted was between the groups marinated only with lactic acid or acetic acid (in both cases, with no added salt)—the lactic acid treatment showed the lowest amount of weight loss after cooking.

However, I did observe a qualitative difference, which had to do with the cloudy precipitate of proteins that would accumulate in the trapped juices in the sous vide bags as the chicken cooked. That cloudy protein mass is similar to something you’ve probably seen when you’ve (over)cooked meat or fish—an egg white-like substance stuck on the surface. The mushy or pasty texture of some cooked, marinated meat is produced by the denaturation of surface proteins by the marinade, but also by that cloudy precipitate drying out and becoming caked to the surface. Because the meats cooked in these experiments were contained in a sealed bag and kept at a lower temperature, instead of drying out, that precipitate appears like a cloudy gel in the bag.

In my tests, the chicken marinated with lactic acid didn’t produce much, if any, of this cloudy gel-like mass, but the chicken that was marinated in either citric or acetic acid produced a large quantity of that cloudy precipitate, and it was also very thick.

The textures of the cooked chicken breasts were also quite different: The chicken marinated with lactic acid was much more tender and flaked like cooked fish when pressed with a fork or knife. The citric acid marinade produced a slightly firmer texture in the chicken, while the acetic acid marinade produced the toughest and crumbliest texture.

Effects of Different Acid-Based Marinades on Lamb

In the same experimental setup, lamb appeared to be much more responsive to being marinated in acid. Lactic acid-marinated lamb produced a statistically significant increase in weight after marination, compared with lamb marinated in acetic acid. It also lost less weight after cooking when compared to lamb marinated in citric or acetic acid (this finding held up in the presence or absence of salt). Citric acid was also much better at improving weight gain in marinated pieces of lamb, and also showed a smaller loss in weight after cooking when compared to acetic acid.

In terms of protein texture, the groups treated with lactic and citric acid produced the least amount of cloudy precipitate, while acetic acid produced the cloudiest specimens. In each case, the meat was tender, very soft, and cut easily.

When developing my lamb biryani recipe, I tested another question: Does marinating lamb in yogurt overnight reduce cooking time? I first cooked lamb in yogurt without marinating it and on average it took me about one and a half hours to get a tender texture with meat that was almost falling apart. When I repeated the same recipe but marinated the lamb with yogurt overnight, it took around 40 minutes to get the same texture.

Conclusions

Based on my results in chicken and lamb, I’ve come to a few conclusions.

Of all the cooking acids, lactic acid is the gentlest acid when used in meat marinades, followed by citric acid, then acetic acid. Lactic acid’s effect on the texture of chicken and lamb was not as drastic as the effect of citric acid or vinegar, and it improved the outcomes in terms of weight gained during marination (more weight was gained) and weight lost during cooking (less weight was lost).

It’s not surprising that meat is better equipped at dealing with lactic acid and citric acid than it is acetic acid, since animal tissues have evolved mechanisms to deal with organic acids they frequently encounter in their environment. But the fact that lactic acid increases the amount of water absorbed by meat, and helps the meat hold onto it through the cooking process, means that there are few downsides to longer marinades with yogurt and significant benefits: more juicy, nicely textured meat, and, in the case of tougher meats like lamb leg, a significant reduction in the amount of time it takes to cook them until tender.

So, after all these experiments, am I going to continue to marinate my chicken and lamb in yogurt overnight? The answer is most certainly, “Yes!”

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