11/26/2021

New Cold Storage Method Solves Freezer Burn—And Saves Energy

12:10 minutes

a plate of pale green beans with ice all over it
Freeze-burned green beans, like these, may be a thing of the past with a new freezing method. Credit: Shutterstock

Have you ever pulled a long-anticipated pint of ice cream out of the freezer, only to find the strawberries crunchy and the normally creamy substance chalky and caked with ice? Freezer burn, a phenomenon caused by water in food crystallizing into ice inside the ice cream or fruit or meat during freezing, is a menace to taste buds, a driver of food waste, and even damages some of the nutritional benefits of food. And it’s always a risk as long as food preservation relies on very cold temperatures. Even flash-freezing, which works much faster, can still create small ice crystals.

But United States Department of Agriculture (USDA) food scientists, working with a team at the University of California-Berkeley, have a method that could help solve this problem. Normal food freezing, called isobaric, keeps food at whatever pressure the surrounding air is. But what if you change that? Isochoric freezing, the new method, adds pressure to the food while lowering temperature, so the food becomes cold enough to preserve without its moisture turning into ice. No ice means no freezer burn. And, potentially, a much lower energy footprint for the commercial food industry: up to billions fewer kilowatt-hours, according to recent research.

Ira talks to USDA food technologist Cristina Bilbao-Sainz and mechanical engineer Matthew Powell-Palm about how pressure and temperature can be manipulated to make food last longer, and hopefully taste better. Plus, the challenges of turning a good idea into a widespread technology.


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Segment Guests

Cristina Bilbao-Sainz

Cristina Bilbao-Sainz is a food technologist at the USDA’s Agricultural Research Service.

Matthew Powell-Palm

Matthew Powell-Palm is a mechanical engineer and postdoctoral scholar at UC Berkeley.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. We’ve all had that moment. You’ve got that pint of ice cream in the freezer– strawberry, maybe? I’ll go with coffee. You’ve been looking forward to this treat all day. You open the carton, and it’s covered in ice crystals– freezer burn.

Oh, your ice cream is going to taste bad and feel chalky and strange in your mouth. Those little chunks of strawberry– weirdly crunchy. Bleh, as they say. That dreaded freezer burn happens when the water in your food forms ice crystals that destroy the cellular structure of the food itself. It’s a common risk when we preserve food by exposing it to very cold air, right? And the result is bad taste, of course, but also some loss of the food’s nutritional value. That’s the bad news.

The good news is USDA food scientists, working with a team at the University of California Berkeley, have something that could solve that problem, a whole new method of chilling food to preserve it. This new method, called isochoric freezing, actually plays with pressure. So the food becomes cold without its moisture turning into ice– no ice, no freezer burn, and potentially a much lower energy footprint for the commercial food industry. We’re talking billions fewer kilowatt hours.

Here first to explain more about why this is so exciting– Dr. Cristina Bilbao, a food technologist with the USDA’s Agricultural Research Service in Albany, California. Welcome, Cristina.

CRISTINA BILBAO: Hi. Hi, Ira. Thank you. Thank you for inviting me to the show.

IRA FLATOW: Always a pleasure. Maybe you were a little frozen there because we’re going to talk about freezing, right?

CRISTINA BILBAO: Yeah, yeah.

IRA FLATOW: I had no idea that we needed a new way to freeze food. Why is conventional food freezing flawed?

CRISTINA BILBAO: So conventional freezing, it has the advantage that the temperature is very low. It can slow down the deterioration processes, such as respiration, oxidation, and microbial growth. However, ice formation during freezing can cause cellular damages that results in the product with poor quality in terms of maybe texture, color. They might lose some nutrients. That’s the reason we need to find out a new technology that can be used to preserve food as freezing temperatures but without any ice formation inside the food product.

Isochoric freezing, what it does, it takes advantage of preserving the food at the freezing temperatures. And therefore, it is slowing down the deterioration reactions but without any ice formation inside the food product.

IRA FLATOW: So what happens to foods you froze with the new method? What were they like after being frozen?

CRISTINA BILBAO: We were pleasantly surprised at how similar these food products are to the fresh products. In terms of the appearance, they don’t lose volume. They don’t lose mass– the texture, the color, even the nutrient content. So it’s very, very similar to the fresh products.

IRA FLATOW: Huh. So it even tastes the same way?

CRISTINA BILBAO: We have only tested tomatoes and raw potatoes, and they tasted the same. Yes.

IRA FLATOW: Not every fruit and vegetable is a good fit, right? What kinds of food would this method be best for?

CRISTINA BILBAO: I think this technology can be used to extend the shelf life of fresh produce. Good candidates would be those fruit and vegetables that are difficult to freeze using conventional freezing processes, like tomatoes. We have also found out that minimally processed foods, such as cut potatoes, can be also a good candidate. Cut potatoes, when they are packed in vacuum or in a modified atmosphere, the shelf life is only five to seven days in the refrigerator. So this technology can be used to extend the shelf life of these cut product.

IRA FLATOW: Wow. Does food frozen this way retain its nutritional value also?

CRISTINA BILBAO: Yes. We can do it in a way that it retains the nutritional value.

IRA FLATOW: So is this exciting to you, I mean, to have a new way to freeze food? This seems like something that doesn’t come along very often.

CRISTINA BILBAO: Yeah. We have also found out that this technology can improve the food safety and reduce the energy cost of refrigeration. Now we are going to investigate the potential use of isochoric freezing for cold pasteurization and preservation of fluid foods such as milk and fruit juices and vegetable juices.

There is an increased consumer demand for more fresh, authentic, fruity foods. The industry is offering pasteurized fruit fluids. The shelf life of these products is very short. So we are now trying to investigate the potential of using isochoric freezing to pasteurize and preserve these fluid foods in just one step.

IRA FLATOW: Wow. That sounds exciting. I want to thank you for taking time to be with us today.

CRISTINA BILBAO: You’re welcome. Thank you to you.

IRA FLATOW: Dr. Cristina Bilbao is a food technologist with the USDA’s Agricultural Research Service in Albany, California. I want to turn now to another researcher on the team, Dr. Matthew Powell-Palm, a mechanical engineer and postdoctoral scholar at UC Berkeley. He is based in Bozeman, Montana. Welcome, Matt.

MATTHEW POWELL-PALM: Hi, Ira. Good to be here.

IRA FLATOW: Nice to have you. First of all, let’s geek out a bit about this freezing method because I am a geek and I got to know all the details. I mentioned it involves keeping foods at a higher pressure than just sticking it in a freezer. So explain what we’re doing exactly, what the effects are of that.

MATTHEW POWELL-PALM: Yeah, so isochoric– the word means constant volume. So what we’re doing is we’re taking foods and we’re trapping them in a constant-volume box, a constant-volume container. And what this does, thermodynamically speaking, is it cuts them off from the atmosphere.

And so what we find is that if we take food products that are mostly water and we cut them off from the atmosphere and we start to freeze them, cool them down in a confined volume, then ice– because it wants to expand relative to liquid water, ice will try and expand. But the container will push back against it, and this sort of tug of war will create an internal pressure in the system.

And essentially the confinement of the system prevents the ice from freezing the food. So just a little bit of ice grows at the sort of periphery of the container. And it drives this hydrostatic pressure that stabilizes the whole system at sub-zero temperatures without allowing the foods to freeze solid. That was maybe a little more info than you were looking for, Ira.

IRA FLATOW: So we need a little bit of water inside the little freezing chamber in order for this to work, then?

MATTHEW POWELL-PALM: Indeed, indeed. So we’re swapping– if you think of conventional food freezing as happening in air, this mode of food freezing happens in water and takes advantage of the, quote, unquote, “incompressibility” of liquids.

IRA FLATOW: You know, this seems like such a simple principle in some ways. It’s sort of the opposite of the pressure cooker. Instead of heating it up, you’re keeping it cold, right?

MATTHEW POWELL-PALM: Exactly.

IRA FLATOW: Quite interesting. One of the things that Cristina just mentioned was its potential for energy savings. Why is this a more efficient process than just sticking your strawberries in a cold box?

MATTHEW POWELL-PALM: A tomato is over 90% water, right? So a lot of the foods that we consume are mostly water, and the freezing of water is incredibly energy intensive, right? So just the process of converting the liquid state of water into its crystalline icy state requires a huge input of energy.

So at the core of this isochoric energy savings premise is the fact that we simply aren’t allowing the food products themselves to freeze, to solidify. We’re keeping them at subfreezing temperatures, but we’re using this interesting constant volume, high pressure relationship to keep the foods themselves from freezing. And it saves us from having to pay, let’s call it, the energetic toll of converting the water inside the food into ice.

IRA FLATOW: Yeah, that phase shift really draws a lot of energy.

MATTHEW POWELL-PALM: Food preservation in isochoric systems requires much milder cold. So we can hold, let’s say, tomatoes for length scales of months at only minus 2 or minus 3 Celsius instead of minus 20 Celsius. And so asking your freezer to operate at minus 2 requires of it much less energy than it operating at minus 20 or below.

IRA FLATOW: And this doesn’t have to be a system for food. We can keep all kinds of things at cold temperatures. And you’ve been doing work on finding applications for human tissue preservation and transplants. Tell us more about that.

MATTHEW POWELL-PALM: Yeah, I think one of– transplant medicine is one of the absolutely most fascinating applications here because– like, for instance, if we look at heart transplant, which is particularly relevant in the US where heart disease is one of the major annual killers, we get thousands and thousands of donor hearts that are made available every year. But we end up transplanting only about 30% of them.

And the reason that so many go to waste is our simple inability to hold those hearts outside the body for sufficiently long periods of time to get them into someone who needs them. So you imagine how complicated a heart transplant is. You only have four to six hours after the death of the donor to get that heart into a recipient.

So it’s an unbelievable logistical hurdle. And so we’re looking at using the same fundamental thermodynamic premise– isochoric freezing– to enable preserving hearts outside the body for, let’s say, 24 hours or two days instead of four to six hours. So really anywhere that the shelf life of biological matter is a problem, we can apply this technology.

IRA FLATOW: Wow. Wow. What are some of the challenges you have to overcome? What about the container? We’re talking about– you have a very small experimental container, right? Don’t you have to scale that up?

MATTHEW POWELL-PALM: I think of problems as a whole– technological problems– in two categories, science problems and engineering problems. So we’ve been working for the last several years to settle the science of isochoric freezing, the food science and the thermodynamics, you name it. And so the next sort of phase is tackling the engineering.

And the nice thing about it is that the building of large pressure-bearing containers is not new. It’s something that the industry has already figured out for oil and gas, for the storage of various compressed liquids and gases, you name it. So now what we’re looking at is taking knowledge that already exists out there in the engineering and mechanics world and using it to scale our– yes– our 2-liter prototype systems up to 50 liters, 100 liters, 500 liters. So that’s something that will happen likely outside of the university but is top of mind here for the next couple of years.

IRA FLATOW: Well, I want to thank you for taking time to be with us today. I think I’ve learned a whole lot about food.

MATTHEW POWELL-PALM: Well, thank you, Ira.

IRA FLATOW: Dr. Matthew Powell-Palm, postdoctoral scholar and mechanical engineer with the University of California at Berkeley.

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