Measuring Accurately and Precisely

“Baking is a science.” We’ve all heard it before—in fact, when I get home from my science day job and bake, my brain is still in the same mode. In both cases, we combine specific ingredients in a certain way to make something exact happen. Baking relies on ingredients to interact just so to create a delicious treat. And as we’ve seen in the experiments I’ve shared, changes in the amounts of ingredients, the way they’re combined, or the temperature in the oven lead to changes at the molecular level that translate into a different baked good. Thus, accurate and precise measurement is important to recreate and write recipes. In this post, we’re going to discuss two concepts of measurement that are often conflated, accuracy and precision, and how they apply to measuring ingredients in the kitchen.

Accuracy versus precision

When I think about the difference between accuracy and precision, I still picture the figure my tenth-grade chemistry teacher used. I’m just going to put it here, and then we’ll put it into words.

Let’s say we want 250 grams of flour. The center of the board represents 250 grams exactly. The further from the center we get, the further we are from 250 grams. The dots represent separate, individual measurements, maybe from the same person measuring several times.

Accuracy refers to how close to the center we are. Are we pretty close, like in the first and second board? Or are we a little off, like in the third and fourth? Precision, on the other hand, has nothing to do with the target. It refers to how closely grouped the darts are. When we measured 250 grams of flour five times, was there a similar amount of flour each time, like in the first and third board? Or did we have a wider range, like in the second and fourth?

Both accuracy and precision are important in baking. When we read a recipe, we want to use the same amount of each ingredient the recipe developer used. In this case, our measurement needs to be close to the target. It needs to be accurate. Accuracy is often a function of the equipment itself. Are the measuring cups the right size? Is the scale calibrated? When we use this equipment to test a recipe over and over again, we also want our measurements to be precise so that we’re confident the amounts of the ingredients have not changed. As we’ll see, precision comes from a combination of the equipment’s design and our attention to detail. Compared to most cooking, baking requires high accuracy and precision for a recipe to work. But compared to science lab work, baking is pretty loose.

Measuring equipment and its limitations

Why do I say this? We can look at the equipment bakers use to measure versus the equipment scientists use. Lab equipment typically provides more precision. But in both the kitchen and the lab, measurements are only as precise as they need to be. Height charts don’t mark millimeters because the difference between 162.3 and 162.4 centimeters doesn’t have practical implications. By the same token, kitchen equipment measures only as finely as it needs to. Still, it’s helpful to understand its limitations so that we can make informed choices about how we use it.

Measuring volume

As we discussed in the last post, we can measure ingredients by volume, but that quickly introduces room for error for powders like flour and cocoa powder. These ingredients can pack too tightly into a measuring cup, leading to imprecise measurements. In a science lab, powdered solids are always measured by mass. But because precision is less crucial in the kitchen, many recipes include volume measurements. It does matter, however, how you fill the cup, especially if you’re measuring flour. In the next post, I’ll be sharing a comparison of muffins made with the “same amount” of flour measured in different ways. Even if you’re careful, scooping from the bag gets you too much!

Liquids are also measured more precisely in a lab. Scientists use graduated cylinders, which are marked with more gradations than glass measuring cups. If a scientist wants 100 milliliters of a liquid, for example, she would use a graduated cylinder that marks every 1–2 milliliters. A glass measuring cup usually marks every 20, so its hard to tell the difference between a couple milliliters. Furthermore, graduated cylinders are tall and narrow, and surface tension makes a little curve at the liquid’s surface called a meniscus. To get a precise measurement in the lab, you’re supposed to bend down to read the cylinder at eye level from the bottom of the meniscus. Glass measuring cups, however, are wide, which makes the meniscus less pronounced and harder to read. These factors limit the precision with which we can measure volume in the kitchen. Even the measuring cups you fill up all the way have room for error. Some people leave a little room at the top so that the liquid doesn’t spill, while others take advantage of surface tension and overfill the cup. While it’s ideal to fill measuring cups just up to the edge or to the line, differences this small don’t affect the baked good in a detectable way.

Measuring cups (left) are less precise than graduated cylinders (right).

Measuring mass

What about scales, then? In the last post, we said that mass was the most accurate way to measure powders and that it was accurate for liquids, too. But how precisely can our scales measure mass?

Most kitchen scales display values in grams (or tenths of ounces, which are larger—about 2.8 grams), so that is our limit. If the scale says 125, we can’t tell if we have 124.5 grams or 125.4. To account for this, scientists generally use more sensitive (and more expensive) scales to measure one more digit out from what the recipe says. So if she needed 125 grams of a chemical, a scientist would use a scale that reads tenths of grams so that she could measure 125.0 grams. In the kitchen, though, the difference of 0.5 grams is a mere 0.4%, which doesn’t produce an observable difference in the final baked good. For most baking purposes, then, kitchen scales are accurate enough. However, there is one caveat: small masses cannot be precisely measured on a typical kitchen scale. If we want 4 grams of baking powder, the difference of half a gram is a 12.5%. Unless your scale is sensitive enough to measure out tenths of grams, it is more accurate to use measuring spoons for masses less than 10 grams. In this case, we are limited by the precision of the available equipment.

Conclusions

As we’ve seen, baking doesn’t require high precision compared to science lab work. There’s wiggle room for error, and small inaccuracies due to limitations in equipment sensitivity will not noticeably change our baked goods. Even if we compound these inaccuracies with imprecision on our part, the result won’t necessarily be a “fail.” It might be a little different than what the recipe developer had written, but not a drastic enough change to be observable, especially without a comparison. Nevertheless, an understanding of accurate measurement is important if we want to recreate recipes or write our own for others to try. And it encourages consistency within our own baking to help us troubleshoot in the kitchen as well.



References

Wilbraham, A.; Staley, D. D.; Matta, M. S.; Waterman, E. L. Chemistry, Prentice Hall: Upper Saddle River, 2008.

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