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Homemade Marshmallows

Photo Credit: Heather Katsoulis (hlkljgk/Flickr)

Photo Credit: Heather Katsoulis (hlkljgk/Flickr)

Whether you prefer them toasted over a campfire, bobbing in a cup of hot chocolate, or roasted over a bed of sweet potatoes, marshmallows are an ooey-gooey fluffy treat that just finds a way warm the cockles of your heart.

Marshmallows, like other well-known aerated confections – think mousses, ice cream, meringues –  are essentially made of four basic components: sugar, water, air, and a hydrocolloid.  Hydrocolloids, often called “food gums” are polysaccharides, or typically large-branching proteins, that form thick gels when they interact with water. [1]

Their ability to bind to water molecules makes them hydrophilic (or “water-loving”), and their ability to remain suspended and dispersed evenly in the water (without settling to the bottom) makes the substance a colloid. Thus, food gums are hydrophilic colloids, or hydrocolloids.

Photo Credit:  Daniel Campagna (Chefpedia)

Photo Credit: Daniel Campagna (Chefpedia)

Hydrocolloids are added to many foods we eat – as thickening agents in pie fillings or gravies, gelling agents in puddings and jams, foam stabilizers in beer and meringues, film formers in sausage casings, emulsifiers in salad dressing, and even fat replacers in frostings and muffins.  Common examples of hydrocolloids are starch, xanthan gum, locust bean gum, alginate, pectin, carrageenan, and agar, which all influence the texture and mechanical stability of many foods.  [1][2]

In marshmallows, the hydrocolloid responsible for the chewy, bouncy texture we know and love is gelatin. While gelatin is one of the most popular commercial hydrocolloids, it is definitely not the most glamorous.  Gelatin is made of collagen, which is the structural protein derived from animal skin, connective tissue, and bones. In fact, mainstream gelatin is usually obtained from pigskin, cattle bones, and cattle hide. [3] Gelatin is unique because not only does it function as a foam stabilizer for the marshmallows [4], but when it is mixed with water, gelatin forms a thermally-reversible gel.  These gelatin gels have a melting temperature just below body temperature (< 35°C or 95 °F), so the gel product literally melts in your mouth and releases intense flavor immediately as it dissolves, which is a difficult quality to replicate with other hydrocolloids. [3]  

Gelatin makes marshmallows chewy by forming a tangled 3-D network of polymer chains.  Once gelatin is dissolved in warm water (dubbed the “blooming stage”), it forms a dispersion, which results in a cross-linking of its helix-shaped chains.  The linkages in the gelatin protein network, called “junction zones” trap air in the marshmallow mixture and immobilize the water molecules in the network . The result? The famously spongy structure of marshmallows! [1]  This is why the omission of gelatin from a homemade marshmallow recipe will result in marshmallow crème, since there is no gelatin network to trap the water and air bubbles.

And for the gelatin-averse, worry not! There are indeed many hydrocolloid alternatives to gelatin. However, since gelatin has so many different functions (gelling agent, emulsifier, stabilizer, thickener, etc.), its alternatives are not universal. Rather, substitutes are specific to each specific food application. In our case, some have suggested pectin – a polysaccharide from the cell walls of plants – as the ideal replacement for gelatin in marshmallows [1].

Agar agar is a commonly used vegetarian alternative for jellies.  Photo Credit: I Believe I Can Fry (johnnystiletto/Flickr)

Agar agar is a commonly used vegetarian alternative for jellies.
Photo Credit: I Believe I Can Fry (johnnystiletto/Flickr)

Pectin, carrageenan (a polysaccharide from red seaweeds), or combinations of both can replicate the elastic texture and intense flavor release that gelatin provides for marshmallows. However, since the melting points of both pectin and carrageenan are not the same as the melting point of gelatin – which, as you recall, is slightly below body temperature, marshmallows made with pectin or carrageenan don’t have the quite the same “melt-in-your-mouth” sensation. [1]

* Note: Carrageenan gels are unique in that their melting temperature can be modified, depending on the solution concentration of the carrageenan and the presence of cations, so the melting temperature ranges from 40°C (104°F) and 70°C (158°F).

* Note: Carrageenan gels are unique in that their melting temperature can be modified, depending on the solution concentration of the carrageenan and the presence of cations, so its melting temperature ranges from 40°C (104°F) and 70°C (158°F).

As you can see, none of the gelatin alternatives have the appropriate melting temperatures to replicate gelatin’s melt-in-your-mouth sensation. However, this does prove advantageous in the fact that they can last longer on hot days or in hot, tropical climates and they do not require refrigeration to set.

No matter what you prefer for as a hydrocolloid, pillowy marshmallows can made with the same basic recipe:

Photo Credit: Joy (joyosity/Flickr)

Photo Credit: Joy (joyosity/Flickr)

Ingredients

For the bloom:
3 tablespoons (typically 3 packets) unflavored gelatin powder
1/2 cup cold water

*Vegan Substitution: 2 ½ tablespoons agar agar + ½ cup and 2 tablespoons water
Alternatively, this vegan marshmallow recipe is worth checking out:

For the marshmallows:
3/4 cup water
1 1/2 cups granulated sugar
1 1/4 cup sugar cane syrup or corn syrup
Pinch of salt

For the marshmallow coating:
1 1/2 cups powdered sugar
1/2 cup cornstarch
non-stick cooking spray


Equipment
Bowls and measuring cups
Fork or small whisk
9×13 baking pan or other flat container
4-quart saucepan (slightly larger or smaller is ok)
Pastry brush (optional)
Candy thermometer
Stand mixer with a wire whisk attachment
Stiff spatula or spoon (as opposed to a rubbery, flexible one)
Sharp knife or pizza wheel

Instructions

  1. Prepare pans and equipment: Spray the baking pan with cooking spray. Use a paper towel to wipe the pan and make sure there’s a thin film on every surface, corner, and side. Set it near your stand mixer, along with the kitchen towel and spatula. Fit the stand mixer with the whisk attachment.
  2. Bloom the gelatin/agar: Measure the gelatin or agar into the bowl of the stand mixer. Combine 1/2 cup cold water in a measuring cup and pour this over the gelatin or agar while whisking gently with a fork. Continue stirring until the gelatin or agar has dissolved or reached the consistency of apple sauce and there are no more large lumps. Set the bowl back in your standing mixer. (Alternatively, you can bloom the gelatin or agar in a small cup and transfer it to the stand mixer.)
    * NOTE: You can add about 1 tablespoon of powdered flavorings to your hydrocolloid while it is blooming in the water.

    Photo Credit: Joy (joyosity/Flickr)

    Photo Credit: Joy (joyosity/Flickr)

  3. Combine the ingredients for the syrup: Pour 3/4 cup water into the 4-quart saucepan. Pour the sugar, corn syrup, and salt on top. Do not stir.
  4. Bring the sugar syrup to a boil: Place the pan over medium-high heat and bring it to a full, rapid boil — all of the liquid should be boiling. As it is coming to a bowl, occasionally dip a pastry brush in water and brush down the sides of the pot. This prevents sugar crystals from falling into the liquid, which can cause the syrup to crystallize. If you don’t have a pastry brush, cover the pan for 2 minutes once the mixture is at a boil so the steam can wash the sides.
    Do not stir the sugar once it has come to a boil or it may crystallize!
  5. Boil the syrup to 247°F to 250°F: Clip a candy thermometer to the side of the sauce pan and continue boiling until the sugar mixture reaches 247°F to 250°F. Take the pan off the heat and remove the thermometer.
  6. Whisk the hot syrup into the gelatin / agar: Turn on your mixer to medium speed. Carefully pour the hot sugar syrup down the side of the bowl into the gelatin or agar. The mixture may foam up — just go slowly and carefully.
  7. Increase speed and continue beating: When all the syrup has been added, cover the bowl with a clean kitchen towel and increase the speed to high (the cloth protects from splatters — the cloth can be removed after the marshmallows have started to thicken).

    Photo Credit: Joy (joyosity/Flickr)

    Photo Credit: Joy (joyosity/Flickr)

  8. Beat marshmallows until thick and glossy: Whip for about 10 minutes. At first, the liquid will be very clear and frothy. Around 3 minutes, the liquid will start looking opaque, white, and creamy, and the bowl will be very warm to the touch. Around 5 minutes, the marshmallow will start to increase in volume. You’ll see thin, sticky strands between the whisk and the side of the bowl; these strands will start to thicken into ropes over the next 5 minutes. The marshmallow may not change visually in the last few minutes, but continue beating for the full 10 minutes. When you finish beating and stop the mixer, it will resemble soft-serve vanilla ice cream.
    * NOTE: Add 1- 2 tablespoons of liquid flavorings during the last couple minutes of the beating process. (See Ideas Section below.)
  9. Immediately transfer to the baking pan: With the mixer running on medium, slowly lift (or lower, depending on your model) the whisk out of the bowl so it spins off as much marshmallow as possible. Using your stiff spatula, scrape the as much of the thick and sticky marshmallow mixture into the pan as you can.
    * NOTE: If you want mini marshmallows, after mixing, immediately put the mixture in a piping bag and pipe out your mini marshmallows in the size and shape of your choice.

    Photo Credit: Joy (joyosity/Flickr)

    Photo Credit: Joy (joyosity/Flickr)

  10. Let the marshmallows set for 6 to 24 hours: Spray your hands lightly with cooking oil and smooth the top of the marshmallow to make it as even as possible. Let the mixture sit uncovered and at room temperature for 6 to 24 hours to set.
  11. Prepare the marshmallow coating: Combine the powdered sugar and cornstarch in a bowl.
  12. Remove the marshmallows from the pan: Sprinkle the top of the cured marshmallows with some of the powdered sugar mix and smooth it with your hand. Flip the block of marshmallows out onto your work surface. Use a spatula to pry them out of the pan if necessary. Sprinkle more powdered sugar mixture over the top of the marshmallow block.

    Photo Credit: Joy (joyosity/Flickr)

    Photo Credit: Joy (joyosity/Flickr)

  13. Cut the marshmallows: Using a sharp knife or pizza wheel, cut the marshmallows into squares. It helps to dip your knife in water every few cuts. (You can also cut the marshmallows with cookie cutters.)
  14. Coat each square with powdered sugar mix: Toss each square in the powdered sugar mix so all the sides are evenly coated.

    Photo Credit: Joy (joyosity/Flickr)

    Photo Credit: Joy (joyosity/Flickr)

  15. Store the marshmallows: Marshmallows will keep in an airtight container at room temperature for several weeks. Leftover marshmallow coating can be stored in a sealed container indefinitely.

Ideas:

  • Add Flavorings: You can add about a tablespoon of either powdered or liquid flavorings/food colorings to the marshmallows at Step 2 or Step 8, respectively, in the recipe.
  • Sweet Marshmallows
    – classic: vanilla extract, almond extract, cocoa powder
    – floral: rose water, orange blossom water
    – spiced: cinnamon, pumpkin spice, cardamom, nutmeg, chai tea, peppermint
    – fruity: passion fruit, strawberry, mango, lemon juices
  • Savory Marshmallows
    –  A great base for savory marshmallows: PopSci:Sechuan Peppercorn Marshmallow
    – garlic salt and pepper
    – pesto (I’m imagining a pillowy caramelized pesto-marshmallow roasted on top of a pizza!)
    – hot sauce
  • Add citric acid or cream of tartar to stabilize the inverted sugars in your recipe and prevent them from crystallizing, essentially ensuring that your marshmallows remain fluffy and chewy.
  • Add your sugar syrup into whipped egg whites to incorporate extra air volume and structure for spongier, pillowy marshmallows.
  • DIY Lucky Charms: You can make your own dehydrated marshmallows, similar to the ones found in breakfast cereals (but without all the suspicious additives) by evaporating the water from the sugar solution in your homemade marshmallows.  Various methods are described here.


Recipe adapted from

References Cited

    1. Saha, D., Bhattacharya, S. Hydrocolloids as thickening and gelling agents in food: a critical review. Journal of Food Science and Technology. December 2010; 47(6): 587-597.
    2. Gum“. Food @ OSU.
    3. Karim, A. A., Bhat, R. Gelatin alternatives for the food inudustry: recent developments, challenges and prospects. Trends in Food Science & Technology. December 2008; 19(12): 644-656.
    4. Gelatin. Gelatin Food Science. 14 Dec. 1998.

Eunice LiuAbout the author: Eunice Liu is studying Neuroscience and Linguistics at UCLA. She attributes her love of food science to an obsession with watching bread rise in the oven.

Read more by Eunice Liu


Tri-Color Potato Salad

“There’s so much great food yet to discover that we can grow, so I just love discovering new varieties, crops, things that our customers and myself have never tried before.”
                                                                          – Alex Weiser, 2013 Science & Food course Read more

Fancy Chocolate Treats

Photo credit: Jesús Rodriguez (hezoos/Flickr)

Photo credit: Jesús Rodriguez (hezoos/Flickr)

Chocolate-covered strawberries have an innate beauty in their simplicity, making this snack both sweet and decadent. But this gourmet treat does not have to be expensive nor only savored at special events. Although it’s not quite as simple as dipping strawberries into soupy chocolate sauce, you can easily make chocolate-covered strawberries in your very own kitchen with a basket of strawberries, a bag of chocolate, and a little patience.

To perfect the crafting of chocolate-covered strawberries, it helps to first consider the composition of chocolate. Chocolate contains only a few ingredients: fat, sugars, proteins, and soy lecithin as emulsifier that holds everything together [1,2]. Cocoa butter, a fat that is derived from cocoa beans, makes up the majority of chocolate. Like many vegetable fats, cocoa butter is a mixture of fatty molecules called triacylglycerols. Different types of triacylglycerols—saturated, monounsaturated, polyunsaturated—have their own thermal and structural properties. Roughly 80% of cocoa butter are monounsaturated triacylglycerols [3]. The secret to chocolate perfection lies in the microscopic arrangement of these molecules. The texture (smooth vs. lumpy), appearance (glossy vs. dull), and melting temperature of chocolate (in your mouth at 98°F vs. in your hand at 82°F) all depend on how triacylglycerols pack together in the finished chocolate product.

Triacylglycerols are elongated, spindly molecules that can be packed together in different ways, sort of like long, skinny Legos. The three main ways that triacylglycerols can pack together are named α, β’, and β [3]. A pure mixture of triacylglycerols will form the most stable structure, β [4], and quality chocolate that is hard, smooth, and shiny will predominantly contain this β structure. Unfortunately, cocoa butter isn’t purely one type of triacylglycerol: while the 80% monounsaturated triacylglycerols will tend to pack together nicely into perfect β structures,  the other 20% of cocoa butter fat molecules can interfere and lead to less stable α or β′ structures. As shown in Table 1, chocolate can take on different combinations of α, β′, and β structures, categorized in order of increasing stability as crystals I-VI [2,3]. Crystal V possesses only the β structure, and so it boasts the most desirable chocolate characteristics, such as good sheen, satisfying snap, and melt-in-your-mouth smoothness.

Table 1. Properties of chocolate crystals (adapted from [2]).

Crystal Structure Melting Temp (°F) Chocolate Characteristics
I β′sub(α) 63 Dull, soft, crumbly, melts too easily
II α 70 Dull, soft, crumbly, melts too easily
III β′2 79 Dull, firm, poor snap, melts too easily
IV β′1 82 Dull, firm, poor snap, melts too easily
V β2 93 Glossy, firm, best snap, melts near body temp
VI β1 97 Hard, takes weeks to form

Unfortunately, getting chocolate to form the desired crystal type is easier said than done. When chocolate is melted and then left alone to re-harden on its own terms, uncontrolled crystallization occurs: any and all of the six crystal types will form at random. Chocolate that has been allowed to set this way ends up clumpy and chalky. To control crystallization and select for crystal V, the chocolate must be tempered. Through the tempering process, chocolate is first heated to 110-130°F to melt all the different crystal types. Most importantly, the temperature has to be higher than 82°F to melt the inferior crystals I-IV. Melted chocolate is then cooled down by adding “seeds” of chocolate that already contain only crystal V. These seeds are usually just pieces of chocolate that has already been tempered. Any piece of chocolate—chips, buttons, or chopped— can be used, as the majority of chocolate on the market has already been tempered. These seeds slowly cool the melted chocolate and act as a molecular template from which additional crystal V structures can grow [3]. As the chocolate cools, the stable crystal V will come together into a dense, even network, creating that lustrous, firm chocolate coating.

But beware: a drop of water can ruin all that hard work and perfectly tempered chocolate by causing it to seize. During the manufacturing process, water is removed from the chocolate, leaving behind a blend of fats and sugars. Introducing water to melted chocolate causes the sugar molecules to clump together in a process known as seizing [1]. These wet, sticky sugar clusters result in a grainy, thick batch of chocolate.

Seizing can happen when chocolate is melted in a double boiler, as water from the steam can get into the chocolate. It can also happen when pockets of chocolate are accidentally burnt. Burning is a chemical reaction that oxidizes the fats and sugars to produce carbon dioxide and water. Water that forms in the burnt pockets of chocolate will cause the rest of the batch to seize. But have no fear! Seized chocolate is not completely ruined: it can be saved by adding even more water or other liquids such as cream. Though it may seem counterintuitive, adding more water actually dissolves the sugar clumps, breaking them apart so that the chocolate can become smooth and creamy again [1]. Unfortunately, because there is now moisture in the chocolate, it will not dry and harden into a chocolate shell anymore. Chocolate rescued in this way can be used for hot chocolate, icings, fillings, or ganaches, which means you can still make an impressive chocolate treat even if the chocolate-covered strawberries don’t work out.


Chocolate-Covered Strawberries

1 lb. strawberries
16oz milk chocolate chips
Thermometer (optional, but would be helpful)

1. Melt half to two-thirds of the chocolate chips…

…In a double boiler: Stir constantly. Be sure steam doesn’t escape and sink into the chocolate. Do not cover.

…In the microwave: Heat on high 1 minute. Do not cover. Remove from the microwave and stir. If all the chocolate has not melted, heat again for 5-10 seconds. Repeat until completely melted
Note: If possible, avoid using a heat-retaining container like glass, which may burn the chocolate. Plastic is preferred.

2. Once completely melted, carefully continue heating until the temperature is 90-95°F.

3. Remove from heat, then add chocolate chips. Stir until the chips have melted and the chocolate is 82-88°F.

4. To test if the chocolate is ready, spread a thin layer on the back of a spoon or a piece of paper. It should harden in less than 3 minutes. If it doesn’t, stir in more chocolate chips.

5. When the chocolate is ready, carefully dip in strawberries. Make sure the strawberries are dry, before dipping. Allow dipped strawberries to dry on a sheet of parchment paper.


References Cited

  1. Corriher, S. Chocolate, Chocolate, Chocolate. American Chemical Society: The Elements of Chocolate. October 2007; <http://acselementsofchocolate.typepad.com/elements_of_chocolate/Chocolate.html>
  2. Loisel C, Keller G, Lecq G, Bourgaux C, Ollivon M. Phase Transitions and Polymorphism of Cocoa Butter. Journal of the American Oil Chemists’ Society. 1998;  75(4): 425-439.
  3. Rowat A, Hollar K, Stone H, Rosenberg D. The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation.  Journal of Chemical Education. January 2011; 88(1): 29-33.
  4. Weiss J, Decker E, McClements J, Kristbergsson K, Helgason T, Awad T. Solid Lipid Nanoparticles as Delivery Systems for Bioactive Food Components. Food Biophysics. June 2008; 3(2): 146-154

Alice PhungAbout the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.


Spherification Potluck

There are times when gourmet edges more towards the laboratory than the kitchen; spherification is one of those times. In this culinary technique, liquids are transformed into globular semisolid gels thanks to a hydrocolloid gum extracted from seaweed. When these gel-encased balls are broken, the liquid contents gush out, akin to biting down on mochi or a Gushers candy. In theory, almost any liquid can be spherified, so the possibilities are endless. Ever wanted to eat plum juice caviar, spherical crème brûlée, or mojito spheres? With food-grade sodium alginate, calcium solution, and some creativity, it’s possible.

At the Spherification Potluck last month, graduate students Liz Roth-Johnson and Kendra Nyberg delved into the process on the molecular level. Gelation is made possible through the interaction between alginate and calcium ions. Alginate is a long, negatively charged, noodle-like molecule. When mixed into a liquid, alginate floats about freely, its elongated structure creating a thick, jelly-like consistency. Calcium ions are single calcium atoms with two positive charges, enabling each ion to link together two alginate molecules. Many calcium-linked alginate molecules gives rise to a more solid structure—the gel skin that encases a gooey center.

SpherificationLecture

Liz (left) and Kendra (right) explain the nuts and bolts of spherification.

Spherification - Options

Students brought a variety of beverages, sauces, and condiments to the potluck.

Attendees at the student event opted for items found in kitchen pantries and grocery store shelves, such as pomegranate molasses, rose water, coffee drinks, milk tea, sodas, guava nectar, and hot sauce.

In the first attempt at spherification, coffee was mixed with the sodium alginate to produce a rather thick goop. Plopping globs of this dense solution into the calcium chloride baths gave comical results, as the mixture adamantly refused to form any shape remotely resembling a sphere. Some blobs even broke upon removal from the calcium chloride baths.

CoffeeSpherification

Students prepare an alginate solution (left) and attempt to create spherified coffee (right)

Milk tea and Jarritos orange soda gave the best results in terms of shape and stability. Initially, the center of the milk tea spheres was thicker than expected, yielding a much chewier texture than bargained for. Minimizing incubation time in the calcium chloride solution managed to fix this halfway, somewhat decreasing the thickness of the gel casing. A quick search also revealed that our recipe used twice the sodium alginate other spherification recipes called for. If less alginate was added to the milk tea or orange soda, the spheres would have definitely been gooier.

Spherification - Jarritos sphere

A student shows off a fairly successful attempt at spherified orange soda.

The most difficult to work with was Tapatio, and not just because of the spicy fumes that emanated from the mixing bowl. Hot sauce is acidic, meaning it is full of positively charged hydrogen ions. Mixing it with alginate neutralizes the negative charges, hampering the interaction between alginate and calcium. No alginate-calcium interaction, no cross-link formation, no gel. Dropping the Tapatio-alginate mixture into calcium chloride resulted in nothing more than dissolved Tapatio swirling around in solution.

Spherification encompasses a high degree of flexibility. Besides the gamut of foods that can be used, there are also technical alterations—the ratio of liquid to sodium alginate in the pre-sphere goop; the concentration of the calcium chloride solution; the amount of time the spheres are left sitting in the calcium solution. And this is only the direct method. Other variations on this technique include reverse and frozen reverse spherification. With spherification kits readily available online, why not try spherifying your own recipe? Share your spherification adventures with us in the comments below!

Brownie Hacks & Cookie Engineering

brownie-comparison-relish

Get ready for the holidays! Check out these helpful guides to engineering your perfect brownies and cookies. Read more

Simple Vegan Strawberry Shortcake

BirthdayCake

With increasing numbers of people embracing dietary-restricted, vegetarian, and vegan lifestyles, birthday parties are becoming more complex. Consider the simple birthday cake. Everyone should be able to partake in its delicious sugary goodness; however, this can prove difficult. Specifically, how can you bake a cake if you cannot use one of the most important components, eggs? Read more

Ceviche

Through the process of cooking, molecular transformations alter the macroscopic properties of our food. Consider what happens when you fry an egg: the transparent, liquid egg whites become an opaque white solid. These striking changes in the egg’s color and texture are a result of protein denaturation. Read more

Homemade Ice Cream

Phase transitions—transformations from one state of matter to another—are ubiquitous in food and cooking. Butter’s phase transition from a solid to a liquid results in flaky pie crusts, while water’s phase transition from a liquid to a gas can be used to steam vegetables. There are various ways to manipulate these phase transitions, such as by altering temperature, pressure, or salt content. In this classic home experiment, we will make ice cream by using salt to alter the phase behavior of water.


Objectives

  • Understand how solutes (salt) affect the phase behavior of a solvent (water).
  • Use freezing point depression to make a batch of amazing ice cream.


Materials

  • 1 cup cream
  • 1/2 cup sugar
  • 200 grams ice
  • Kosher salt
  • 1 quart Ziploc bag
  • 1 gallon Ziploc bag
  • Thermometer
  • Scale


Part 1: Use salt to lower the melting point of ice

To successfully freeze ice cream without the help of a freezer, we need a way to efficiently transfer heat out of the ice cream. Liquid water is much better than solid ice at transferring heat, so an ice-water bath will absorb heat from our ice cream better than solid ice. To effectively freeze ice cream, however, we need stable temperatures well below 0˚C.

How is it possible to have a mixture of water and ice at a temperature below 0˚C, water’s freezing point?

When you take ice straight out of the freezer, the ice will be roughly the same temperature as the freezer itself. The temperature in a home freezer is typically between 0˚C and -20˚C. As the ice sits out, it will absorb heat from its surroundings and slowly get warmer until it reaches 0˚C and begins to melt. Adding impurities like salt to ice will lower its melting point.  This means that salted ice will start melting at temperatures below 0˚C. As a result, a salty ice-water bath can stay liquid at temperatures well below 0˚C and efficiently freeze our ice cream. We refer to this phenomenon as “freezing point depression.”

We can use the freezing point depression equation to calculate how much a solvent’s freezing point will drop as a solute is added:

∆Tf = b · Kf  · i

∆T    Freezing point depression, defined as Tf of  pure solvent – Tf of solution.
K f        Cryoscopic constant of the solvent. This is an intrinsic property of the solvent.
b          Molar concentration of the solute: the number of moles of solute per kilogram of solvent.
i           Number of ion particles per molecule of solute, also known as the “Van’t Hoff factor”.
Salt is made up of one sodium ion and one chloride ion, so its Van’t Hoff factor is 2.

  1. Use the freezing point depression equation to calculate how much salt (our solute) is needed to decrease the freezing point of water (our solvent) from (a) 0˚C to -5˚C, (b) 0˚C to -10˚C, (c) 0˚C to -15˚C, and (d) 0˚C to -20˚C.
  2. Plot the magnitude of freezing point depression (ΔTf) versus salt concentration (Results from 1a, b, c, and d). Remember to use units!
  3. Based on your answer from 1d, calculate how many grams of salt are required to create a -20˚C freezing point depression for 200g of ice. This is the amount of salt you will use in Part 2.

Some useful values:
Freezing point (Tf) for pure water: 0˚C.
Cryoscopic constant (Kf) for water: 1.853 ˚C*kg/mol.
Molecular weight of salt (NaCl): 58.44 g/mol.

Click here to check your answers.


Part 2: Use freezing point depression to make ice cream

  1. Combine cream and sugar in the quart-size bag and mix well. Place this bag inside the gallon bag.
  2. Record the initial temperatures of the ice and the cream mixture.
  3. In the gallon bag, pack the ice around the quart-size bag, and then sprinkle the calculated amount of salt over the ice. Be careful that the salt does not fall into the cream mixture.
  4. Gently shake the bag until the cream mixture solidifies into ice cream.
  5. Record the final temperatures of the ice-salt-water mixture and the ice cream.
  6. Enjoy your homemade ice cream!


Questions

  • What was the final temperature of the ice cream? Did it end up below 0˚C? How does its temperature compare to the temperature of the salt-ice-water mixture?
  • What was the final temperature of the ice-salt-water mixture? Is warmer or colder than the ice you started with? How does the temperature compare to the freezing point depression you calculated in Part 1?


Discussion

In this experiment, we used salt to lower the freezing point of water. By adding salt to ice, we were able to achieve a salt-ice-water mixture that was able to freeze our ice cream.

Why does ice cream need temperatures colder than the freezing point of water in order to freeze?

When water freezes, it forms a well-ordered crystalline structure (an ice cube). This unique crystalline structure is what gives solid water a slightly lighter density. Although ice cream is a combination of  cream, sugar, and flavorings, it is still approximately 60% water. The remaining 40% is a mixture of sugar molecules, fat globules, and milk proteins [1]. This liquid mixture is emulsified: the water molecules are dispersed among sugar molecules, milk protein complexes, and large clusters of fat globules.. When this mixture is brought to the freezing temperature of water, the fats, proteins, and sugars hamper the freezing process by interrupting the formation of ordered crystal water structures. The ice cream mixture thus remains a liquid, requiring even colder temperatures below 0˚C to successfully solidify [2].

Structure of ice cream. (A) an electron micrograph of ice cream showing air bubbles, ice crystals, and the sugar solution [3]. Fat globules and milk proteins are not visible at this resolution. (B) Diagram of ice cream structure adapted from University of Guelph.

How did the salt in our experiment create a salt-ice-water mixture below 0˚C?

At 0˚C, ice and water are “at equilibrium” with each other. The total amount of water and ice remains relatively constant, but individual water molecules are constantly switching states: as some water molecules melt and become liquid, other water molecules freeze and become solid. Adding a solute like salt shifts this equilibrium. Solutes essentially “trap” water molecules in the liquid state, preventing them from readily switching back to the solid state. On a macroscopic scale, salt causes solid ice to melt faster and at temperatures below 0˚C, resulting in a salt-ice-water mixture below 0˚C. To get a better feel for how this process works at the molecular level, check out this interactive demonstration of how temperature and solutes affect the water-ice equilibrium.

Contrary to popular belief, the addition of salt to ice does not actually make the ice any colder!

The temperature that you recorded for the salt-ice-water mixture was probably colder than the temperature of the pure ice you started with. How is this possible? When you take the temperature of solid ice, you are not really measuring the temperature of the ice itself—you are measuring the average temperature of the ice, the air around the ice, and any water that has formed from the ice melting. The true temperature of the ice depends on the temperature  freezer it came from (typically between 0˚C and -20˚C) and the length of time the ice has spent out of the freezer.


Online Resources

  1. Interactive explanation of how temperature and solutes affect water-ice equilibrium
  2. “Ice Cream Structure” from University of Guelph

More from On Food and Cooking

  • McGee, Harold. On Food and Cooking. Scribner, 2004. (39–44).

References Cited

  1. Goff HD (1997) Colloidal aspects of ice cream—A review. International Dairy Journal 7: 363–373. doi:10.1016/S0958-6946(97)00040-X.
  2. Hartel RW (1996) Ice crystallization during the manufacture of ice cream. Trends in Food Science & Technology 7: 315–321. doi:10.1016/0924-2244(96)10033-9.
  3. Clarke C (2003) The physics of ice cream. Physics Education 38: 248–253. doi:10.1088/0031-9120/38/3/308.

Liz Roth-JohnsonAbout the author: Liz Roth-Johnson is a Ph.D. candidate in Molecular Biology at UCLA. If she’s not in the lab, you can usually find her experimenting in the kitchen.

Read more by Liz Roth-Johnson


Ricotta Cheese

Protein networks are responsible for the structure and mechanical properties of many foods such as eggs and meat. Even bread gets its chewy texture from the formation of springy gluten protein networks. As we will see in this recipe, protein network formation is vital for the successful production of cheese. Read more