Soupy Science: Investigate How Dried Beans Absorb Water
Have you ever had black-eyed peas for New Year’s Day? They’re often used as part of a traditional meal in the South on this day. Dried beans themselves are a major ingredient in dishes served all over the world. In their dried form, they can be stored for years and then “brought back to life” by soaking them in water. In this science activity, you will explore how the temperature of the water used to rehydrate dried black-eyed peas (which are technically beans) affects how quickly they become rehydrated and ready to chow down on!
Beans come in dozens of shapes, sizes and colors and have been consumed throughout the world for thousands of years. They are low in fat and are good sources of protein, carbohydrates and fiber. The U.S. Department of Agriculture dietary guidelines suggest that beans, along with other foods that are low in fat, oils and sugars (including vegetables and grains) should make up the largest portion of our daily meals.
In addition to their nutritious qualities, beans are convenient because they can be dried and stored for years. Soaking the beans in water for a few hours (or less) softens the dried beans and prepares them for cooking. This rehydration process also occurs in nature. Beans are a form of seed, and they can sprout when exposed to water. Beans remain viable for long periods of time if they are kept dry, a feature that allows them to survive prolonged periods of drought in natural settings, and that allows us to store them for long periods of time prior to cooking. As beans soak in water, their volume increases. In this science activity, you’ll determine how soaking beans in hot versus cold water affects how quickly they absorb water.
- Ice cubes
- Cold tap water
- Hot tap water
- One cup of dried black-eyed peas. Alternatively, dried split peas, lentils or other legumes that do not require extensive soaking before cooking them could be used.
- Two plastic, transparent disposable cups, at least 12 oz in size
- Measuring cup
- Permanent marker
- Timer or clock
- Ruler (optional)
- Fill the bowl half full with ice cubes and then add cold tap water to the bowl, nearly filling it. You will use this as a source of ice water.
- Fill each plastic cup with ½ cup of dried black-eyed peas. Gently shake each cup so that the beans flatten out on the surface.
- Use a permanent marker to make a line on the side of each cup marking where the top of the black-eyed peas are. You will want to have the cups on the edge of a counter (or table) and look straight-on at the top of the beans to make this mark accurately. How do you think the bean level will change when water is added to the cups?
- At the same time, fill one plastic cup with ice cold water (from the bowl of ice cubes and water you prepared) and fill the other cup with very hot tap water. Fill both cups to the top with water.
- Set a timer, or note what time it is, and check on the cups every 10 minutes for the next 45 minutes. When you check on them, make a small line on each cup where the top of the beans are. Write the time next to the mark. How does the bean level in each cup change over time?
- After 45 minutes (or longer), which cup has the higher bean level? What does this tell you about whether cold or hot water is more quickly absorbed by the beans?
- When you are done with this activity, you could discard the beans, or you could include them in a bean-based recipe of your choice, such as black-eyed pea soup!
Extra: Another way to measure your results in this activity is to weigh the black-eyed peas. You could use multiple cups and weigh the beans from those cups (after straining out the water) as they are soaked in water for different amounts of time. How does the mass of the black-eyed peas change over time?
Extra: Try this activity using some different legumes (legumes include beans, peas and lentils). Do some legumes absorb water faster than others? What does this tell you about how long they might need to be soaked before cooking them?
Observations and Results
Did you find that the black-eyed peas had a higher level in the cup with the hot water over time compared to in the cup with the ice cold water?
When dried beans are soaked in water, their volume increases as they absorb the water. Beans absorb hot water more quickly than cold water, which is why the black-eyed peas used in this activity should have swelled faster in the hot water than in the cold water. For example, after only 10 minutes of soaking, you may have seen that the level of the black-eyed peas in the hot water cup was about half an inch higher than the level in the cold water cup, and that this difference remained consistent for the rest of the activity. This activity works well for dried black-eyed peas, split peas and lentils because they do not require extensive soaking before cooking; if you were to test other legumes that require more soaking, you would see that they absorb water much more slowly.
Ask an Expert
- Is the Soup Ready? Measure How Much Water is Absorbed by Dried Beans, from Science Buddies
- Science Activities for All Ages!, from Science Buddies