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Dear Pablo: I have been wondering for some time what a life cycle analysis would show is the "greenest" way to ship milk. Plastic containers are light, but are not reusable and don't biodegrade; cardboard containers are less light, are not reusable, and don't biodegrade either. Glass bottles are reusable, but are really, really heavy--and so, of course, use much more fuel to ship. My local coop carries all three, and I am conflicted every time I shop. What should I do?
You are right that glass bottles are heavy and you are right to question their use. In a paper that I wrote about the greenhouse gas emissions from wine production and distribution my co-author, Tyler Coleman of DrVino.com, and I determined that transportation emissions can be a very significant part of the product's overall life cycle emissions. But, unlike milk, wine is typically transported over very far distances. So the question is do the heavier glass bottles make a significant difference to greenhouse gas emissions over the much shorter distances that milk is usually shipped?
Milk Container Weights and Materials
I went to the store and picked up some organic milk in a glass bottle, a plastic jug, and a TetraPak
carton. The glass bottle holds 1 liter and weighs 410 grams, the plastic jug holds a quart (or 0.94 liters, so we will round up to 1 liter) and weighs 51 grams, and the TetraPak also holds 1 liter and weighs 57 grams (including the closure and secondary and tertiary packaging).According to the EcoInvent life cycle analysis database, the emissions from glass production are 0.559 grams of greenhouse gases per gram of glass. For the plastic, HDPE, I turned to a report from the Plastics Devision of the American Chemistry Council
and found that the emissions for producing the plastic are 1.478 grams per gram of plastic. Finally I looked up the emission factor for the TetraPak in a life cycle inventory report from TetraPak Inc. Those emissions are 0.136 grams of greenhouse gases per gram of TetraPak.
Manufacturing Milk Containers
By multiplying the container weight by the emissions factor for each material, we can obtain the greenhouse gas emissions from producing the container. For glass, it's 229 grams, for the plastic jug it's 75 grams, and for the TetraPak it's 8 grams of greenhouse gas emissions. It's not surprising that glass creates more emissions because it weighs more and there are higher raw material transportation emissions. Glass also has a higher melting point, requiring more energy to melt it. Most surprising is that the emissions from producing the TetraPak are so low, but this benefit is counteracted by the fact that the packaging material is much less easily recyclable.
Transporting Milk containers
Each type of containers is transported by truck, and we can fairly make the assumption that the distance each travels to get to your local store is roughly the same as well, probably about 60 miles (100 km). Milk homogenized at 50 degrees Fahrenheit has a density of 1,032 grams per liter, so the weight of each filled container is simply the package weight plus 1,032 grams. So the weight as-transported for the glass bottle is 1,442 grams, the plastic jug is 1,083 grams, and the TetraPak is 1,067 grams. Transportation emissions are measured in grams of greenhouse gases per ton per kilometer (t-km) and for a semi tractor-trailer truck, the emissions are 242 g/t-km.So, by doing the math, I found that the greenhouse gas emissions for transporting the milk is 35 grams for the glass, 26 grams for the plastic jug, and 26 grams for the TetraPak. The total greenhouse gas emissions for the manufacture of the packaging and the transportation, all other things being assumed equal, are 265 grams for the glass, 101 grams for the plastic jug, and 32 for the TetraPak. So, while the TetraPak is questionable due to limited recycling capabilities in the US
, it does have the lowest greenhouse gas emissions. What is clear is that the glass creates the most greenhouse gas emissions, even if it is reusable (and indeed gets reused instead of tossed or recycled), mainly due to the higher transportation weight.
Is Home Milk-Delivery Making a Comeback?
Back in the day, milk was delivered to our doorsteps early in the morning by a milkman. With an increase in farmer's markets and "buy local" campaigns, it is only natural to expect a resurgence in home milk delivery
. Besides being a nostalgic throwback and great way to support local dairies, is home milk delivery also more green? The beauty of home delivery, whether it's mail-order shopping, a cloth-diaper service, or grocery delivery, is that it can combine multiple deliveries in a single trip and help you avoid using your personal car.If you compare home delivery of milk with a personal vehicle trip to the store, then the home delivery is much more efficient (particularly when you consider that home-delivered milk would probably come from a local dairy closer than the 60-mile distant creamery used in the math above). But, if you have to go to the store for other groceries anyway, getting milk there is a negligible addition to your greenhouse gas emissions. Of course, there are other factors involved in addition to the greenhouse gas emissions. Locally-produced milk might be more natural, taste better, and certainly supports your local economy more than buying your milk from a supermarket chain that sources its milk from a factory farm.Additional Resources on Milk ContainersGallon Milk Jug Gets Redesigned For a Low Carbon WorldNice Jugs and How to Milk Them For All They're Worth Milk Delivery Returns to Manhattan How to Make Milk Jugs Lighter? Take the Handle OffFurther Resources on Milk ContainersMilk in a bag?TetraPak Environment