Have you ever made a beautiful pesto blend, full of basil flavour, with a vibrant green colour, that didn’t look all that vibrant anymore the day after? Or left that peeled banana or sliced apple for a few hours to long on that plate, coming back to brown instead of fresh fruits? Or have you ever made your own cheese with a cheese packet with non-descript names?
You have seen enzymes at work in your food! Enzymes can be a very useful tool in food. But, they operate quite differently than a lot of other chemicals or foods. This post provides you with a basic understanding of enzymes to then understand their behaviour in a wide variety in foods!
What is an enzyme?
Enzymes may sound foreign and artificial, but actually enzymes are all around us. They are very common in nature. Without enzymes we humans wouldn’t be alive. We wouldn’t be able to digest a lot of our foods and a lot of chemical reactions wouldn’t take place anymore.
Enzymes are actually a specific sub class of proteins. Proteins are long molecules made up of chains of amino acids. These long chains of amino acids fold up and organize themselves into complex 3D structures. Enzymes, being proteins, do exactly that.
With these very complex 3D structures they can perform some highly specialized tasks. Their structure is such that a very specific molecule fits perfectly with that structure. The enzyme then makes it easier for that molecule to undergo a specific chemical reaction. In the case of those browning bananas it helps transform polyphenols into quinones.
The enzymes create an ideal environment for this reaction to occur. Enzymes serve as a ‘catalyst’ in these reactions. A catalyst doesn’t actually participate in the chemical reaction. It doesn’t give up or get atoms, electrons or protons. All that a catalyst does is to provide ideal conditions for the reaction to occur. As a result, the activation energy, which is the amount of energy that is required for reaction to even start, is lowered. This means it is a lot easier for the reaction to happen.
Names of enzymes almost always end with -ase. What goes before the -ase tends to describe that what reaction the enzyme catalyzes. For example, protease enzymes reaction with proteins. Oxidase enzymes catalyze oxidation reactions and amylases break down the amylose in starch.
Most enzymes then have a further descriptor of their behaviour in front of the generic group name. Polyphenol oxidase (PPO) oxidizes polyphenols specifically. Maltogenic amylase breaks down starches into maltose molecules. Unfortunately, not all names are that descriptive. Actinidain (present in a lot of fruits) breaks down proteins. Luckily its family name does describe what it does, it is an cysteine protease. The cysteine refers to the type of amino acid it acts upon.
Reactions catalyzed by enzymes are called enzymatic reactions. For an enzymatic reaction to occur you need at least the enzyme itself and a substrate. A substrate is the molecule that will react during the reaction.
Enzymes are highly specific, only one substrate or a group of substrates will ‘fit’ into the enzyme. The substrate is the key of the enzyme lock. Only specific keys fit into a lock. The get their specificity from their complex 3D structures.
During the enzymatic reaction, the substrate(s) react into a new molecule, the product. Sometimes other molecules might need to be present. For example, for the browning of bananas to occur, which is catalyzed by enzymes, there needs to be sufficient oxygen.
After the reaction the product releases from the enzyme. The enzyme itself hasn’t changed at all though. It is ready to catalyze yet another reaction.
Enzymatic reactions in food
Just about everyone who has eaten fruit (or baked an apple pie) has seen the polyphenol oxidase (PPO) enzyme in action. This enzyme catalyzes the browning of a lot of fruits and vegetables (bananas, apples, celeriac, etc.). This same reaction occurs when vegetables get frozen. The ice crystals break down the cells allowing the browning reaction to occur.
If you’ve ever tried to make a gelatin pudding containing fresh pineapples or kiwis you’re come across some proteases. These fruits contain enzymes that break down proteins. This is great if you’re cooking a piece of tough meat. The enzymes can soften the meat. However, in the case of gelatin it is less ideal. Gelatin is a protein itself, so the enzymes break down the gelatin resulting in a pudding that’s not set.
This is also why adding pineapple juice to your egg before frying makes for a strange, unappealing texture!
Enzymes have been used for hundreds, if not thousands of years, for making cheese. A mix of enzymes, including proteases, cause the proteins in milk to curdle. The resulting clumps can be made into cheese. This blend of enzymes comes from rennet which again can be harvested from a cow’s stomach (it can be made without animals now as well). Stomachs are a great source for enzymes since all animals use them to break down their food.
Influencing enzymatic reactions
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There are various ways to speed up, slow down or completely stop enzymatic reactions. When trying to do so you make use of the fact that enzymes don’t perform evenly under all conditions. Most enzymes have an optimal temperature at which they perform fastest. The same goes up for the pH-level of its environment.
By moving the conditions away from their optimum the enzymatic reactions slow down. The reverse is also true, ensuring the conditions are close to optimum speeds the reaction up.
To slow down browning of foods it is best to keep the food at cool temperatures. Also, be making the surface more acidic, e.g. by sprinkling on some lemon juice, browning can be slowed down.
Stopping enzymatic reactions – using heat
If you need to stop an enzymatic reaction completely, you need to deactivate the enzyme. In order for an enzyme to work it needs to be able to maintain that complex 3D structure. As soon as that is lost, the lock and key don’t fit together anymore and the catalysis won’t happen.
All enzymes, just like all other proteins, break down by intense heat. This process is called denaturation. It is also what causes an egg white to turn white upon cooking. The temperature and duration differ per enzyme. However, most enzymes in food are deactivated rapidly when placed into boiling water.
This is why shortly heating vegetables before freezing keeps them fresher in the freezer. The short heat treatment, also called blanching, deactivates all the enzymes. Even in the freezer the enzymes would have otherwise stayed active (be it at a slower rate).
It is also why for gelatin desserts you can use canned, but not fresh pineapple. The canned pineapple has been heated during canning. The heat treatment will have broken down the enzymes.
Inhibiting enzymatic reactions
Instead of changing the external environment of the enzyme, another effective strategy is to target the enzyme directly. One way to do so is by adding an ‘inhibitor’ to the enzyme. The inhibitor physically prevents the enzyme from catalyzing a reaction. It can do so in various ways.
First of all, the inhibitor could sit at the spot of the substrate. The inhibitor does need to be similar enough to a substrate to make this work. By taking in the spot of the substrate, it prevents any reaction from occurring. This is called competitive competition. Another type of inhibitor is one that doesn’t sit on the place of the substrate but does attach to the enzyme in another spot. By attaching to the enzyme it changes its configuration. As a result the ‘lock’ changes, which makes the enzyme inactive.