Dry Ice as an Alternative Leavening Agent for Pancakes to Demonstrate Phase Transitions or Chemical Transformations

Abstract

A culinary exploration of the role of CO₂ in leavening is described. This demonstration substitutes dry ice for chemical leaveners in order to achieve the same pancake fluffiness. Under the universal framework of food and cooking, we developed this activity to bring aspects of phase transitions and chemical transformations to a broad audience.

Introduction

The kitchen offers an accessible laboratory for educators hoping to connect their curriculum to the universal human experience of food.¹ Many general audience books tap into the same relatable context by offering readers chemical explanations for culinary reactions.²⁻⁴ Here, we introduce a new gastronomic classroom demonstration based on leavening which can be useful in discussions of chemical transformations, gases, or phase transitions.

Students often recognize the role of yeast as a biological leavening agent and can point to their role in the rising of dough by generating CO₂ through cellular respiration. On the other hand, the leavening properties of baking soda and baking powder (often referred to as “chemical leaveners”) rely on the acid–base reaction which ultimately leads to the production of CO₂. Instead of a biological or chemical process, the crux of this demonstration relies on the simple physical phase transition of solid CO₂ (dry ice) to gaseous CO₂ (Scheme 1).

Scheme 1. Role of CO₂ in Biological, Chemical, or Even Physical Leavening Agents.

To the chemistry educator, the use of dry ice in demonstrations offers more than just the excitement of the classic bubbles and fog over water.^5,6 It can be used to showcase the acidification of water exposed to CO₂.⁷ The modest pressure of a capped centrifuge tube enables visualization of CO₂ as a solid, liquid, and gas.^8,9 This readily accessible liquid state has been used in a teaching context for the extraction of natural products such as limonene from orange peels,¹⁰ anethole from fennel seeds,¹¹ and eugenol from cloves,¹² as well as other processes like small scale dry cleaning¹³ and thin layer chromatography.¹⁴ The differing phase transitions of CO₂ at ambient pressure (sublimation) versus the elevated pressure of a capped centrifuge tube (melting, evaporation) offer an excellent framework for the introduction of states of matter.

Even without dry ice, exploring the role of leavening agents can be an interesting and informative exercise.^15,16 Yeast, baking soda, baking powder, or even seltzer water¹⁷ can release the CO₂ necessary to give a bread its rise or a pancake its fluff. Such leavening can be specifically measured in a classroom setting by monitoring evolved gases.¹⁶ This demonstration builds upon the foundation of activities showcasing the role of biological and chemical leavening agents^15,17 by including the nontraditional use of solid CO₂ as an eventual alternative source of gaseous CO₂.

Audience

Instructors for introductory audiences such as K–12 outreach, first year undergraduate, or nonmajors culinary chemistry courses may find this demonstration useful in discussions of phase transitions, evidence of chemical transformations, or gas properties. The simple ingredients and equipment make this demonstration easy to carry out in nearly any classroom or outreach setting.

Hazards

Dry ice is extremely cold and should not be handled with uncovered hands. Safety glasses and gloves (cryogenic or thick leather) should be worn when handling dry ice. The cooking surface will necessarily be hot. We suggest that you do not consume any of the pancakes/crepes with dry ice used as a leavening agent.

Discussion

In order to parse the effects of the various leaveners, students can make a series of pancakes, with batters that either traditionally provide a fluff or not, and substitute/add dry ice as an alternative leavener (Table 1). Students can start by comparing recipes for crepes and fluffy pancakes (Supporting Information). They may identify the presence of baking soda and baking powder as unique to the fluffy pancakes and be able to recognize these as the source of CO₂. Students will find that removing the baking soda and baking powder from a fluffy pancake provides a similarly flat product as compared to the crepe recipe, while adding dry ice to the unleavened fluffy pancake recipe or crepe recipe will provide a fluffed product (Figure 1).

Table 1. Flat and Fluffy Variants of Several Pancake Recipes with and without Chemical or Physical Leavening Agents.

Flat Product	Fluffy Product
Crepe recipe	Crepe recipe + dry ice
Fluffy pancake recipe without baking soda/powder	Fluffy pancake recipe
	Fluffy pancake recipe without baking soda/powder + dry ice

Figure 1.

Fluffy pancakes, fluffy pancakes recipe without baking soda or baking powder, and fluffy pancakes recipe with 20 g of dry ice substituted for baking soda and baking powder. Each image shows a 4-stack of pancakes.

This demonstration facilitates discussion of phase transitions (sublimation), states of matter (Figure 2), and evidence of chemical reactions (formation of bubbles) all within the context of cooking, a universal human experience. From K–12 outreach audiences to introductory undergraduate courses (1st semester of General Chemistry as well as the nonmajors Chemistry & Biology of Food & Drink), students engaged in meaningful discussions about chemical reactions and phase transitions. The recipe comparison and predictions of fluffiness generated the most discussion and served to engage students in the subsequent cooking demonstration. After the lesson, students walked away with an understanding of leavening agents, and more importantly, several real-world examples of phase transitions and chemical transformations.

Figure 2.

Phase diagram of CO₂.

Demonstration in the Classroom

We tailored this demonstration to several different audiences. While we used the same recipes for each, the fundamental framing of the activity was different depending on the level of background knowledge and the take away points that we hoped to make.

When we animated this demonstration for a combined third and fourth grade classroom, we framed the demonstration around states of matter and chemical reactions. We combined the pancake demonstration with other classic dry ice and liquid nitrogen activities. We started by placing ice in a hot pan and asking what would happen. Students suggested that the ice would melt into water and then boil. We then showcased the boiling of liquid nitrogen in a countertop beaker mentioning that different substances boil at different temperatures. When it came to dry ice, we placed it on the counter and asked what would happen. Several students suggested that it would melt, although of course it instead sublimed. We later put a piece of dry ice in a beaker of water to show that it was bubbling (turning into a gas). We did not get into a full phase diagram discussion with the grade school students, but we did mention that substances differ in when they melt and boil, and that some do not even melt at all. Then, finally, we got to the pancake demonstration, and we asked why pancakes were fluffy. One student suggested bubbles, so we put up the list of ingredients for “fluffy pancakes” and asked if anyone knew which ingredients made bubbles. When nobody answered, we added baking soda to vinegar in order to showcase the formation of bubbles (and that a chemical reaction had occurred). We then asked what would happen if we took the baking soda out of the pancake recipe. One of the students correctly predicted that the pancakes would be flatter. We then asked if the students know of any other way we could make bubbles, and with some coaxing, the students eventually suggested adding dry ice. In this context, the students had very little background knowledge, but we were able to introduce states of matter and phase transitions (although only using the language of melt/boil and solid/liquid/gas), and we were able to demonstrate that pancakes are fluffy because of bubbles in the batter.

When animating the demonstration for a “nonmajors” undergraduate course (Chemistry & Biology of Food & Drink), we assumed a much higher degree of background knowledge (understanding of phase transitions, states of matter, and signs of chemical reactions). In this case, we sought to offer students a comparison between different types of leavening agents. In surveys prior to the course, roughly half the students could identify baking soda/baking powder as a chemical leavening agent for pancakes (Fall of 2020, 4 of 9 students; Spring 2019, 10 of 15), although fewer could describe the acid–base reaction to generate CO₂ (Fall of 2020, 1 of 9 students; Spring 2019, 4 of 15). Meanwhile, nearly all students identified yeast as key toward bread dough leavening (Fall of 2020, 8 of 9 students; Spring 2019, 15 of 15), although again, fewer elaborated to describe that the yeast also are generating CO₂ through fermentation of sugars (Fall of 2020, 1 of 9 students; Spring of 2019, 5 of 15). Therefore, in teaching the different types of leavening (chemical and biological), we sought to highlight the key similarity (generation of CO₂ within the dough/batter) by simply adding finely ground dry ice (CO₂) directly into otherwise unleavened batter. At the end of the course, all of the students could articulate that CO₂ bubbles in dough/batter were responsible for leavening in a chemical context, in a biological context, as well as in the physical context in this demonstration (Fall of 2020, survey, 9 of 9 students; Spring of 2019, final exam, 13 of 15).

When the instructor conducted this demonstration for a general chemistry class (Winter of 2020), the chemical and biological leavening scenarios fit into a different lecture theme (balancing equations) than the physical leavening of the dry ice pancake (phase transitions and phase diagrams). For this reason, the concepts of chemical and biological leavening were introduced earlier through in-class example problems for balancing equations (without a demonstration). Later, when it came time to discuss phase transitions and phase diagrams, we demonstrated the solid–liquid–gas phase transition of water at ambient pressure (melting/boiling of ice on a hot plate), as well as the solid–gas transition of CO₂ at ambient pressure (sublimation of dry ice on the countertop). In order to achieve the same solid–liquid–gas phase transition of CO₂ as had been observed with water, we used the elevated pressure of a sealed centrifuge tube. To connect the idea of CO₂ gas generation (via fermentation or acid/base reaction) with the CO₂ gas generation via sublimation, we conducted the fluffy pancake demonstration described herein. In a subsequent quiz the week after the demonstration, the majority of students (13 of 18) could draw a phase diagram for a generic material with inclusion of the triple point and various phase transitions. Beyond the assessment described above, whether in outreach settings, or in the undergraduate classroom, students anecdotally found the demonstration to be useful and informative.

No matter the setting (outreach, “nonmajors”, or “majors” courses), we used this material as a “demonstration” rather than an exploratory “activity” or “laboratory exercise”, mostly because of the number of batters that need to be prepared for a single cooking surface. However, if your classroom includes multiple cooking surfaces and pans, you could have students explore their own ratios of leavening agents to batter, rather than simply showcasing the optimized recipe yourself. While the optimization process was never included as part of the demonstration, if you wish to have students working on this, you can expect the pancake fluffiness to increase with the amount of dry ice added (20 g > 15 g > 10 g > 5 g), although after 20 g, the dough becomes frozen and unworkable.

Summary

The demonstration outlined here represents a fun and accessible introduction to phase transitions and chemical reactions through the accessible context of cooking pancakes. This demonstration can easily supplement a lesson on leavening agents in cooking, acid–base reactions, or the properties of gases. It has been well received at local K–12 outreach sessions and introductory undergraduate classes.

Supporting Information Available

The Supporting Information is available at https://pubs.acs.org/doi/10.1021/acs.jchemed.1c00896.

• Recipes and additional resources (PDF, DOCX)

• Video of demonstration (MP4)

The authors declare no competing financial interest.