When you grab a carton of ice cream, take a look at the ingredients. You might see an ingredient named “ice structuring proteins” added to the ice cream to keep the tiny ice crystals small and prevent the formation of large ice layers that change the texture of the ice cream.
But where do these proteins come from and how do they stop ice growth?
Fish, plants and insects endogenous to cold environments rely on antifreeze proteins (AFPs) for survival and the prevention of freezing injuries. (The food industry understandably renamed them as “ice structuring proteins” since no one will eat ice cream with antifreeze in it.) These organisms contain tiny ice crystals in their bodily fluids that must remain small to prevent injury. That is where the antifreeze proteins come in: they bind to the ice surface and coat the ice crystals to stop further growth of the ice crystals.
But some antifreeze proteins from fish accelerate ice growth, which sounds counterproductive since their main function is to stop ice growth.
So researchers at Yeshiva University studied the unique ability of these proteins to both accelerate and inhibit the growth of ice. Their results were published in the Nov. 24, 2020, issue of Journal of Physical Chemistry B. Their advanced understanding of the way that different types of AFPs achieve their function could improve commercial use of these proteins both in the food industry and organ cryopreservation.
Dr. Ran Drori, assistant professor of chemistry and biochemistry, along with Dr. Jinzi Deng, a postdoctoral scientist in Dr. Drori’s lab, and Elana Apfelbaum ’20S are one step closer to understanding why some AFPs accelerate ice growth. They conducted a systematic comparison between different kinds of AFPs to observe the effect of the proteins on the velocity of crystal growth.
Using a custom-made experimental setup that allows for the growth of microscopic ice crystals in the presence of the AFPs, the team measured the velocity of ice growth. “This method includes an infrared laser that melts a piece of the ice crystal and allows us to obtain very accurate and consistent ice velocity measurements,” said Dr. Drori.
The team observed three different effects of AFPs on the growth velocity of ice: some proteins caused faster growth velocities that increased with higher concentrations, while for other AFPs, higher concentrations did not affect the growth velocity, and some caused a slower growth. They correlated these trends with the protein adsorption rate (the speed with which the protein attaches to the ice surface). “We found that for some AFPs, the ice grew faster in one crystallographic direction (c-axis, see image below) as we added more proteins to the solution. However, growth at other directions (a-axis) was inhibited by the AFPs as the concentration was increased. That means that at any given time, ice that grows in the presence of these AFPs is both inhibited and accelerated.”
The team was perplexed by the different effects on the ice crystal growth. If AFPs are meant to prevent ice growth within organisms, then why would some accelerate the growth velocity?
The answer to this question is found in the shape of the ice crystal exposed to AFPs.
These proteins cannot bind and inhibit all the surfaces of the crystal; instead, they inhibit growth in some crystallographic direction while promoting growth of other directions. In this way, a bipyramidal-shaped crystal is obtained, and the surfaces that the AFPs cannot inhibit are the sharp tips of the bipyramidal crystal. “So, growth acceleration helps to achieve the desired bipyramidal crystal faster and at higher temperatures, which in turn stops further ice growth and prevents freezing injuries to organisms,” Dr. Drori explained.
NOTE: The authors dedicate this paper to Dr. Lea Blau a”h, Professor Emerita of Chemistry and Biochemistry at Stern College for Women, Yeshiva University, who passed away in May 2020. She was the heart and soul of the department that she led for 36 years, retiring in 2015. She was a passionate teacher and researcher and mentored countless undergraduate students as well as junior faculty and was a source of strength, dedication, and kindness at the college.