We food-lovers often eat with our eyes. This is evidenced by the internet phenomenon known as “food porn” (or #foodporm, as it’s often tagged), which salutes mouth-watering (and G-rated!) photos of beautiful food. Color is an important factor in the appearance of a dish or ingredient. Color is how we judge how, ripe, fresh, or well-cooked our food is. In addition to being an important quality indicator, the color of food can also be visually stunning and contribute to the overall appeal and flavor of food.
Including lots of beautiful colors on a plate is a sure way to impress, but understanding the science behind these pigments can make the difference between bright, beautiful veggies, and dull brown mush. Hundreds of different food pigments exist in fruits and vegetables, but today we’ll focus on a few of the pigments that are most sensitive to cooking conditions—anthocyanins and chlorophylls.
Anthocyanins are a family of pigments that provide red, purple, blue and white foods with their color. They are water soluble and have beneficial antioxidant properties. They are commonly found in apples, berries, radishes, and red cabbage. In addition to being beautiful, anthocyanins are also one of the most sensitive pigments we encounter in food. They are susceptible to (sometimes extreme) color changes based on varying pH or certain reactive metals such as iron, aluminum, and tin.
Under neutral conditions, anthocyanins are usually a beautiful purple color. When they’re exposed to acidic conditions, they usually become more red or pink as flavylium ions form and absorb and reflect different wavelengths of light. When they’re exposed to alkaline (basic) conditions, they usually become more blue or green as quinone-type anhydro bases form.
One of the coolest features of this reaction is that it’s reversible. Meaning if you find that your food has changed color due to the addition of too much of an acidic or alkaline ingredient, you can bring the color back to its neutral state by adding more acid or base to balance it out. Although this isn’t likely to happen in food, exposure to extremely alkaline conditions (like bleach) can cause the pigment to break down permanently into colorless molecules.
The unique reactivity of anthocyanins makes for a cool food science experiment you can do at home to see first-hand the impact of pH on foods. Scroll down to see a suggested science experiment using red cabbage (anthocyanins) and green beans (chlorophylls).
Another observation: In addition to the color change, the cabbage cooked under alkaline conditions looks quite mushy. This is because the hemicellulose that makes up much of plant structure is soluble in alkali. Meaning that structure literally dissolves away when you cook at a high pH. This will happen eventually to any vegetable cooked in alkali, not just cabbage.
Chlorophylls are pigments that give green foods their beautiful color. They are also essential in the process of photosynthesis, which is how plants derive energy from light. Since many of the vegetables we eat are naturally green due to chlorophyll, the preservation of it is of great importance to food manufacturers.
Luckily, chlorophylls are not as reactive as anthocyanins; however, they have a tendency to turn dull and brown during extended cooking and storage. For proof of this, take for example green beans. Canned green beans tend to be a dull olive-brown color, whereas frozen green beans are intense green. As it turns out, pH has a lot to do with this color difference. Canned green beans are brown because the acids naturally contained in the bean are released into the cooking water, but are unable to escape the can, causing canned green beans to be cooked and stored under slightly acidic conditions. Frozen green beans are blanched in neutral water then frozen to preserve their color and freshness. Neutral cooking water causes far fewer changes in chlorophyll than acidic cooking water.
Under neutral conditions, chlorophyll is a pigment that’s insoluble in water and gives food a pleasant green color. However, when exposed to heat and acidic conditions, chlorophyll becomes a compound known as pheophytin, which gives food a dull olive brown color. Alternately, when chlorophyll is exposed to heat and alkaline conditions (not pictured), it becomes a different compound, known as chlorophyllin, which is water soluble and gives food a bright green color. Since this change causes the pigment to become water soluble, the cooking water may turn bright green as well.
You may notice, from the image above, that the frozen (blanched) green beans are greener than both the canned and fresh (raw) green beans. This is due to the fact that blanching (brief boiling) causes the air pockets between cell walls to rupture, allowing the true intensity of the green pigment to shine through unclouded. This effect is independent of any effects pH may have on color.
Experiment: The effect of pH on pigment and texture
|Control||Acid Test||Alkali Test|
|Red cabbage, sliced or chopped
Green beans, fresh
|50 g||50 g||50 g|
|Tap water (close to neutral)||300 g||300 g||300 g|
|Citric Acid (acidic)||0 g||1/2 tsp||0 g|
|Baking Soda (alkaline)||0 g||0 g||1 tsp|
Test each batch (control, acid test, base test) in its own saucepan using identical conditions whenever possible. Label your pans before starting so that you don’t lose track of which test your doing! If you’re using the same pan for multiple batches, be sure to wash and rinse it thoroughly between each batch to make sure that no pigment, acid, alkali, or soap residue carries over from the previous batch.
Add the vegetable (either cabbage or green beans) to a small saucepan with the water. Add either acid or alkali agent, depending on which test batch you’re making. If you’re making the control, add nothing.
Stir and bring to a boil over high heat. Once boiling, cover and reduce heat to medium. Cook 5 minutes, then remove from heat and pour into a clear container.
Observe and record differences in color and texture (do not taste).
Ideas for further testing:
Because the green beans are a bit less reactive and slower-cooking than the cabbage, there are some additional steps which may help emphasize the contrast between those samples.
1. Try boiling the green beans, covered, for an additional 10 minutes to see if any additional changes are seen in color or texture.
2. Or try doubling the amount of acid or alkali agent and cooking for the same amount of time. Do you see any difference from the original testing levels?
3. After cooking, pour the vegetables and cooking liquid into a bowl or clear glass and allow them to rest for several hours. Are any additional changes in color or texture observed due to the extra storage time? Comparing before and after pictures can be helpful.