Food Science Friday: 5-minute Ice cream
Editor's Note: In case you haven't heard, I've spend the past few months crafting an identity as "Ms. Higgins", high school Chemistry teacher. No, I do not have a background in chemistry. No, the subject wasn't even a part of my undergraduate degree in Sustainable Living. And yes, I must be partially insane for taking this position...but let's talk about that later. I'll share more on the story behind this little career switch and Master's degree that I'm soon-to-be chasing in another post. In the meantime, let's make ice cream! I'd love to share with you some of the fun that we've been getting into in Ms. Higgins' class :)
The following is a revised version of yesterday's 45-minute lab lesson with my 9th & 10th grade boys class. If you're a teacher and would like a copy of the lesson plan, shoot me an email at firstname.lastname@example.org
food Science Friday
freezing-point depression & an edible chemistry challenge
Can you tell the difference between local, organic dairy and its ultra-pasteurized conventional counterpart? How does sodium chloride lower the freezing point of water? What does this have to do with dessert?! This scrumptious science activity involves 3 ingredients, 5-10 minutes, 2 phase changes, 1 batch of tasty handmade ice cream + a brief investigation into our food system and the use of local vs. industrial dairy.
To make ice cream, the ingredients—typically milk, heavy cream (or half & half), sugar and vanilla extract—need to be combined together to form a new liquid substance (an “emulsion”, if you wish) and then cooled down to a solid, frozen state of matter.
The Science of Salt
Let’s take a moment to talk about salt, our familiar friend born of the love interest between alkali metals and halogens. When salt is added to water, it breaks up into two ions: one sodium (Na+), and one chloride (Cl-). These ions are pushy--they move around and take up space between water molecules, which gets in the way of water’s ability to form neat, crystalline links (ice)...effectively lowering the temperature at which water freezes. Weird! This disruption is technically called freezing-point depression, and it’s responsible for crushing your dreams of a Snow Day: in cold climates, salt and sand are spread on the streets in the wintertime to prevent roads from getting slick after a storm. Great. Thanks to the science of sodium chloride, we can melt ice even when the temperature is below the normal freezing point of water and make our way safely to school. Lame, but cool.
Food Safety regulations: this is where things can get a bit “heated” in a discussion on nutrition, but we’ll save that for a later date. Basically, current food safety regulations support and enforce the belief that it’s not okay to consume raw dairy products (a super controversial topic). They require dairy to be heated, or “pasteurized” before it can legally be sold for human consumption. How does it work? In the regular pasteurization process, milk is heated to a minimum of 162 degrees F for 15 minutes. This is said to kill bad things (namely, pathogenic bacteria) and prevent the milk from spoiling too quickly.
In the ultra-pasteurization process, milk or cream is flash-heated through metal pipes almost instantaneously to 280 degrees F, and cooled down nearly as quickly. Ooookay. This paves the way for industrial distribution of “sterile” dairy (it makes the final product shelf-stable for several months), and does some strange things to the actual chemistry of milk. Ultra-pasteurization changes the taste and texture of dairy; high temperatures irritate and deactivate some flavor compounds that are happy in raw milk, and mess with some of the proteins that help to give cream its, well, “creamy” quality. Ugh.
Because of this structural strangeness, some ultra-pasteurized products are loaded with additives like synthetic stabilizers to make up for their broken proteins. Many recipes discourage the use of these products in ice (or whipped!) cream. Let’s put it to the test! What clues might you look and taste for in your ice cream to tell if your dairy was ultra-pasteurized or not?
Materials, Equipment, Ingredients
timer or clock
large ziplock bag
small ziplock bag
gloves, towel, or sweatshirt
salt (the larger the crystals, the better!)
“mystery” half & half (either local, organic OR ultra-pasteurized conventional)
vanilla or almond extract
Pour sugar and flavor extract (¼ - ½ dropper) into your small bag of mystery half and half* (see below for recipe proportions)
Seal the bag (with minimal excess air) and shake gently to mix. Set aside
Pour salt into your large bag of ice. Shake gently to mix.
Place the small ingredient bag into the large salted ice bag. Make sure both are sealed completely.
Put on gloves or wrap the cold bag in a sweatshirt.
Shake the bag for 5-10 minutes, alternating with your buddy every minute.
Check the texture of the ingredient bag every few minutes.
Once the ingredient bag has reached a solid, creamy texture, taste and enjoy!
Compare flavors and textures with other groups
which was your favorite?
- which “mystery” half and half, A or B, do you think was local + organic? why?
* Ingredient proportions used:
- 1 cup half & half
- 2 tbsp sugar (or sugar alternative! Coconut palm sugar is AMAZING for this. Just saying.)
- ¼ - ½ tsp vanilla or almond extract
Key Concepts: states of matter, phase changes (melting, freezing), freezing-point depression, solutions, food science, ultra-pasteurization
Related Concepts: homogenization, emulsions, raw dairy, Organic certification, local food nutrition & economy, food safety & regulations
American Chemical Society “How Does Salt Melt Ice? - Reachtions” https://www.youtube.com/watch?v=JkhWV2uaHaA
Food Safety Site “Safety Education: How Pasteurization Works” http://www.foodsafetysite.com/educators/competencies/general/foodprocessing/processing2.html
Christensen, Emma “Food Science: What’s the Deal with Ultra-Pasteurization?” http://www.thekitchn.com/food-science-whats-the-deal-wi-82428
Scientific American “Scrumptious Science: Making Ice Cream in a Bag” http://www.scientificamerican.com/article/scrumptious-science-making-ice-cream-in-a-bag/
The Sci Guys “Melting Points - Ice Cream in a Bag” https://www.youtube.com/watch?v=s1CpSrXa1EI
Chemistry.About.com “Melting Snow & Ice with Salt” http://chemistry.about.com/cs/howthingswork/a/aa120703a.htm