The Motivating Question
Like many professors, I have spent the last year pondering all the ways that GenAI technologies dramatically change how we train the next generation of engineers.
I continue to be amazed at how quickly GenAIcan assist back-of-the-envelope engineering analyses of everyday observations. For example, my family likes to make homemade yogurt with leftover milk in our refrigerator. While making yogurt this week, I asked ChatGPT a curiosity-driven question:
At home, when I have milk that’s past the “best used by” date—and maybe starting to smell a little… interesting—I like to make homemade yogurt in my Instant Pot.
Specifically, I use a couple of mason jars. I pour the milk into the jars, add a little water to the bottom of the Instant Pot, raise the jars up, and pressure cook on High for one minute.
Then I let everything cool down until it’s about 100°F, stir in a bit of store-bought yogurt with live active cultures, and incubate it in the Instant Pot overnight.
This is a fairly standard homemade yogurt process, and it works well.
Now here’s my question: Help me perform a life cycle assessment (LCA) to understand the impacts of making yogurt at home versus throwing out the milk.
Specifically, what are the energy inputs are required to make the yogurt at home, including washing the jars before preparation and after I’ve consumed the yogurt, heating the milk, heating a little water in the vessel, and letting it incubate overnight?
Compare that against the energy costs associated with instead throwing out the milk and buying the equivalent amount of yogurt at the grocery store.
For some location-specific context: I live in Granger, Indiana, and I’m usually doing this yogurt-making at night, when I suspect electricity maybe slightly less carbon intensive—but please look this up if you can.
I would normally buy store-brand milk and store-brand yogurt, nothing fancy.
Finally, I’m on a septic tank, so any milk poured down the drain is going into my septic system.
Please also redo the analysis assuming I lived within the city of South Bend, where the wastewater treatment plant has a digester and produces methane that’s used to power some city vehicles.
Classroom Activity?
Within minutes, I got a very detailed answer (reproduced below). This raises the next natural question: how can I use this in the classroom? I am on sabbatical this semester, but here is what I am thinking for when I return to the classroom next fall:
I would tell the students that today's lecture is built around a team-based activity. I would then give the students a paper handout with the prompt on one side, and tell everyone to work with a partner for 7 minutes to sketch out their analysis plan and then upload a picture of their work to Gradescope (graded for completeness to incentivize participation).
As a class, we would then spend 10 to 20 minutes sharing solution approaches, with me capturing key ideas on the chalk board. Armed with these ideas, I would pass out another handout with the detailed ChatGPT analysis and ask students to critique it, with a few suggestions:
- Compare and contrast the overall analysis approach to the ideas we collected as a class.
- Do you agree with the assumptions made by ChatGPT?
- Perform some engineering calculations with paper and a calculator. Did ChatGPT make any math mistakes?
I would give teams 15 minutes to critique the output and upload their work again to Gradescope. Next, I would ask the pairs to form groups of four and share with each other their analysis. This would likely take 10 minutes.
This would then leave about 20 minutes (in a 75 minute class period) to regroup as a class, discuss impressions, answer questions, and conclude on the importance of critiquing technical analysis, whether produced by a colleague or GenAI, in modern engineering practice.
Full ChatGPT Output
Home yogurt vs. toss milk + buy yogurt: a practical LCA (with numbers)
Below is a consequential comparison: “What changes in the world if I make yogurt vs. dump milk and buy yogurt?”
Key takeaway
In almost every realistic case, turning “would-be-wasted” milk into yogurt is dramatically lower impact than dumping it and buying replacement yogurt—because dairy’s footprint is dominated by milk production, not your kitchen electricity. Multiple yogurt LCAs find that the raw milk supply chain is the biggest contributor (often the large majority) of yogurt’s climate footprint.
Functional unit (what we compare)
Let’s compare 1 quart (0.95 L) of milk that you either:
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Scenario A (home yogurt): turn into ~0.9–1.0 kg of yogurt at home (plus a small “starter” spoonful), or
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Scenario B (discard + buy): discard that milk and buy ~1 kg of store-brand yogurt to replace it.
(Yogurt mass depends on straining; I’ll assume “normal” unstrained yogurt.)
1) Electricity emissions for your Instant Pot process (Indiana)
Indiana grid carbon intensity
Indiana’s reported CO₂ emissions rate for electricity in 2024 is 1,393 lb CO₂/MWh.
That’s about 0.63 kg CO₂ per kWh (average).
Nighttime can be different, but without pulling a live hourly feed for your exact node, the honest way is to treat 0.4–0.8 kg/kWh as a reasonable sensitivity range for “overnight in coal+gas territory.” (Indiana is heavily coal/gas in recent mixes. ) MISO publishes near-real-time and hourly state-level emissions data, but it’s not a single static number.
Energy use: reasonable bounds (per ~1 quart / 2 jars batch)
Instant Pots are typically ~700–1200 W peak depending on size/model.
But incubation is low-power cycling.
A realistic breakdown (you can tighten this with a plug-in meter):
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Pressure cook / heat-up: ~0.15 to 0.35 kWh
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Overnight incubation: ~0.05 to 0.20 kWh
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Total Instant Pot electricity: ~0.20 to 0.55 kWh per batch
CO₂ from electricity (Indiana average 0.63 kg/kWh):
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0.20 kWh → ~0.13 kg CO₂
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0.55 kWh → ~0.35 kg CO₂
So, call it roughly 0.1–0.4 kg CO₂ for the Instant Pot electricity per quart-ish batch (depending on your model, fill level, and incubation temp).
2) Jar washing energy (before + after)
This swings wildly based on whether you:
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run a full dishwasher load, or
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hand-wash under hot running water, or
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rinse with mostly cold water.
ENERGY STAR and DOE emphasize that dishwasher energy is a combination of machine electricity + (often dominant) water-heating energy.
A practical way to allocate washing energy:
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If you add jars to an already-full dishwasher load you would run anyway, the incremental energy is tiny.
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If you run a dishwasher because of the jars, that’s big.
Typical dishwasher per-cycle energy is commonly in the ballpark of ~1–2+ kWh depending on cycle and drying, but you should treat that as per full load, not “per jar.”
Reasonable incremental estimate for “a couple mason jars”:
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Best case (piggyback on existing washing): ~0.00–0.05 kWh equivalent
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Typical case: ~0.05–0.20 kWh equivalent
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Worst case (running a cycle mainly for jars / lots of hot water): can be much higher (but that’s a choice, not inherent to yogurt)
Using the “typical” range (0.05–0.20 kWh), that’s ~0.03–0.13 kg CO₂ on Indiana average electricity—or potentially more if your water is heated with electricity.
3) What you avoid by not buying store yogurt
Published LCAs often put yogurt around ~2 kg CO₂e per kg yogurt (varies by system, packaging, allocation, etc.). One example reported 1.94 kg CO₂e per kg.
And LCAs consistently find raw milk dominates yogurt’s footprint.
So in Scenario B, buying ~1 kg yogurt likely adds on the order of ~2 kg CO₂e.
Scenario A still uses some store yogurt as starter, but that’s usually a few tablespoons—say 20–50 g—which is only ~0.04–0.10 kg CO₂e if you scale it by kg.
4) What about “dumping the milk” (septic vs South Bend digester)?
Septic (Granger)
Milk down a septic system becomes organic load. Some fraction will convert anaerobically to methane/CO₂ in the tank/leach field, but it’s not typically captured for energy. Net climate effect is uncertain without system-specific data; practically, this term is usually small compared to the footprint of buying replacement dairy.
South Bend municipal wastewater with digestion + beneficial use
Anaerobic digestion at wastewater plants can produce biogas (methane) that can be used for energy, sometimes as renewable natural gas for vehicles or for on-site power/heat.
South Bend’s wastewater system information is published by the city, and the general pathway (digestion → methane → energy use) is real in Indiana.
Even giving South Bend “credit” for energy recovery, the amount of energy you’d recover from a quart of milk is still unlikely to outweigh the emissions from producing and purchasing a whole extra kilogram of yogurt. It helps a little, but it doesn’t flip the conclusion.
Bottom-line comparison (order-of-magnitude)
Per ~1 quart batch:
Scenario A: Make yogurt at home
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Instant Pot electricity: ~0.1–0.4 kg CO₂
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Incremental jar washing: ~0.0–0.1+ kg CO₂ (depends how you wash)
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Starter yogurt: ~0.04–0.10 kg CO₂e (rough scaling from yogurt LCA)
Total: roughly ~0.2–0.6 kg CO₂e (typical), could be lower if washing is “free” and your pot is efficient.
Scenario B: Dump milk + buy ~1 kg yogurt
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Purchased yogurt: ~2 kg CO₂e per kg (ballpark)
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Plus wasted milk (you already bought it): this is “sunk” financially, but environmentally it means you’re effectively causing additional dairy production to meet your yogurt needs.
Total incremental impact: roughly ~2 kg CO₂e (often more once you account for the fact that the dumped milk’s upstream footprint produced no food benefit).
Conclusion: Home yogurt is very likely ~3–10× lower climate impact than dumping milk and buying replacement yogurt, even on Indiana’s relatively carbon-intensive grid.
How to make this LCA yours (quick measurement plan)
If you want, I can turn this into a one-page calculator you can reuse. The two best measurements:
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Plug-in power meter (Kill A Watt-style) on the Instant Pot for one batch
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Decide how to allocate washing:
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“Incremental” (added to existing dishwasher load) vs
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“Dedicated cycle for jars”
Those two choices will tighten the uncertainty more than anything else.
If you tell me:
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Instant Pot model/size (3/6/8 qt) and
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whether you dishwasher or hand-wash jars (and hot water source: gas or electric),
I’ll plug in more specific numbers without changing your method.
Final Thoughts
While writing this up for the group website, I learned (or remembered) that:
1) I should probably pour milk outside in the garden instead of down the drain to prevent possible plumbing issues;
2) I still need to practice my ChatGPT image generation refinement skills;
3) homemade yogurt tastes really good!