To the Last Drop: Getting the Most Out of Liquid Containers

September 2, 2016

An unexpected downpour. A string of red traffic lights. Expired milk. The upside-down balancing act required to get the last bit of chocolate syrup out of the bottle. Life is full of little annoyances.

It turns out that the difficulty of getting the last drop of chocolate syrup out of the bottle is more than just annoying, it represents a serious waste issue. The syrup, honey, ketchup, oil and other liquids eventually tossed in the trash along with the container (or rinsed down the drain) adds up to a big waste of food and money. Replace the words “chocolate syrup” in the first paragraph with “costly, lifesaving medicine” and the problem takes on an even higher level of importance.

A new solution to this problem may be on the horizon, thanks to a team of Colorado State University scientists led by Arun Kota. In an article published in the American Chemical Society’s journal Applied Materials & Interfaces in July 2016, the team introduces two edible wax coatings they designed. Their work shows that you can easily pour chocolate syrup, Coca-Cola, honey, and several other liquids out of a container lined with one of these coatings. The coatings are easy to apply, nontoxic, and significantly reduce or completely eliminate the waste from liquid sticking to the container.

Zooming in on the Problem

Before exploring how the coatings work, it might help to consider why honey sticks to the honey jar (and to the measuring cup, the spoon, and the mixing bowl). If you could zoom way in to the molecular level at a place where honey meets jar, you would see that the honey molecules stick, or adhere, strongly to the molecules in the jar, thanks to electromagnetic forces. In order to get the last bit of honey out, you have to overcome this adhesive force by shaking, scraping, heating, or magic. Turning the jar upside-down helps because it gets the force of gravity on your side.

To put this more generally, the strength of the adhesion between a liquid and its container determines how easy it is to pour out the last few drops. In order to reduce waste, you have to reduce adhesion. One way to do this is to cover the inside of the container with a hydrophobic coating, a coating that doesn’t like water and therefore won’t bond as strongly to the liquid. An even better way is to use a superhydrophobic coating, one that really doesn’t like water.

Making a superhydrophobic coating requires the right chemistry and the right texture. To start with, you need a material that is hydrophobic (i.e. not very susceptible to the electromagnetic forces that cause adhesion) and whose properties make sense for your application (e.g. food packaging). Next, you need a process for turning the material into a thin, textured coating. Why does texture matter? Imagine a drop of water sitting on the bottom of a container. How well the drop sticks to the container is impacted by the amount of surface area that connects them. If the container is optimally textured at the micro- or nano-scale, air pockets will form between the drop and the container. This reduces the surface area over which they are connected and therefore their ability to stick together.

Superhydrophobic coatings cause water to bead up and roll off of a surface, kind of like rain on the waxed hood of a car. Manufacturers use superhydrophobic coatings to waterproof clothing, gear, and equipment for the outdoors. Nature uses one on the leaves of the lotus flower to remove dirt. These coatings have inspired a lot of research and new uses are being proposed all the time. They would be a logical choice for the inside of food containers, except for the fact that most superhydrophobic coatings are made from fluorocarbon materials. Fluorocarbons are safe in low doses, but research has indicated that they may be harmful to humans in the long term.

Kota’s team set out to create a superhydrophobic coating from materials approved by the US Food and Drug Administration (FDA) for direct contact with food using a simple, low-cost, scalable, single-step process. Ideally, they wanted to make it from edible materials just in case some of the coating seeped into the food it touched. Until now, no superhydrophobic coating has met all of these goals.

A stream of honey pours from a jar.
Honey Pour (cropped). Photo: Didriks (CC BY 2.0)

Waxy Solutions

Their innovation came in the form of beeswax. It has chemical properties similar to fluorocarbons, but beeswax is approved by the FDA for direct food contact, edible, and natural. By melting the wax and then applying it using a simple and inexpensive spray system, the team was able to create a thin layer of optimally textured beeswax on top of a surface.

The team created a similar coating out of carnauba wax. Although it might not sound familiar, you’ve probably encountered carnauba wax in dental floss, deodorant, lipstick, pharmaceutical tablets, polishing wax, sweets, or other products. It also goes by “Brazil wax” and “palm wax” and can give products a glossy finish.

The researchers studied and optimized their coatings in the lab. Their experimental results matched theory-based predictions well, and confirmed that the coatings are superhydrophobic and nontoxic. After an experiment indicated that these coatings would effectively repel Lipton Green Tea, Gatorade, pancake syrup, cola, orange juice, milk, coffee and a wide variety of other edible liquids, the team tested them out in a real life situation. They sprayed their coatings inside cups made of polystyrene, a plastic commonly used in food packaging. They filled the cups with different liquids and compared how easily the liquids poured out of coated and uncoated cups. As shown in the video above, the coated cups were much more efficient!

Moving from the Lab to the Kitchen

This is an exciting new technique, but it’s not ready for prime time quite yet. Unlike a cup that is coated in the lab and tested soon after, food packaging can be exposed to a variety of harsh and abrasive conditions along its journey to your kitchen table. The researchers are in the process of improving the durability of these coatings so that they stay intact and effective despite these rough environments. Another goal is to improve how well the coatings stick to different kinds surfaces so that they can be used in a variety of containers. Although they need to repel the liquid, the coatings also need to adhere tightly to the containers.

Other edible improvements to food packaging are underway. Just this week a team from the US Department of Agriculture led by Peggy Tomasula presented a new design for edible food packaging made from milk proteins at the 252nd National Meeting & Exposition of the American Chemical Society. Their design helps prevent food from spoiling and is biodegradable and sustainable. It could potentially be used as plastic wraps and pouches, as a spray that replaces the sugar coating on cereal that keeps flakes crunchy, and as a liner in pizza boxes to contain grease.

Many other research groups are working on creative ways of packaging foods that reduce waste, lessen the negative effects of food production on the environment, and reduce our exposure to potentially dangerous chemicals. If all goes well, the class of 2034 will never know how it feels to vigorously shake a ketchup bottle in frustration. In the meantime, be sure to add chocolate syrup to the grocery list even if it looks like there’s a little bit left.

—Kendra Redmond