Source: Eirik Newth, via Wikimedia Commons.

Two years ago, I picked out 10 stocks to form the ideal retirement portfolio To date, that portfolio has returned 35%, and is outperforming the S&P 500 by almost 6 percentage points.

Recently, however, I've decided to part ways with one of the 10 stocks: Coca-Cola (NYSE:KO). My reasoning has less to do with financial concerns and more with my personal thoughts on the company's products.

In Coke's place, I'm considering adding shares of SodaStream (NASDAQ:SODA), maker of the at-home soda maker. There are two big reasons I'd be excited to own the stock, and one that makes me question if it would be a good fit for my retirement portfolio.

I can sleep at night with this investment
In deciding to sell shares of Coke, I stated that I simply wasn't comfortable profiting from a company whose drinks are a vehicle for massive amounts of sugar and high-fructose corn syrup that are adding to our country's obesity and diabetes epidemic.

Some might think that an investment in SodaStream would be hypocritical, but there are important differences between these two companies.

The first is that users of the product don't need to make soda at all to enjoy having a SodaStream machine, which simply carbonates the water. My wife and I fill up our SodaStream bottle three times a day and simply mix it with juice.

And even if users purchase the company's syrups to flavor their water, they have far fewer calories, grams of sugar, and carbohydrates than a similar beverage from Coke or PepsiCo would.

Furthermore, the use of SodaStream's machines eliminates hundreds of millions of plastic bottles from needing to be manufactured, recycled, or landfilled. The company has gone out of its way to highlight this benefit through the use of public displays like this -- showing how many bottles could be saved over five years of SodaStream use.

Source: Sodastream. 

Positive momentum
One of the biggest concerns with investing in SodaStream is that it is simply a fad that will soon pass. But the product's history would beg to differ. Though the product is relatively new in the U.S., it has been around -- and is still going strong -- in Western Europe for more than 40 years.

Results show that moves into both the Asia-Pacific region as well as North America have won over millions of converts. During 2012, revenue increased 50% for the company, which is about the same as it's done over the past three years. And as more and more machines are sold, the razor-and-blade model of selling consumables ensures continuing revenue.

Take a look at how machine sales have accelerated over the past five years.

Source: SEC filings, in thousands. 

And though the stock now trades at about 27 times earnings, I think that's a fair price given SodaStream's history and potential.

One reason to worry
I guess you could call this the Green Mountain Coffee (UNKNOWN:GMCR.DL) effect. Green Mountain is the maker of the Keurig coffee machines and the cups that go along with it. For years, these machines made their way into people's kitchens, and the company's stock rose almost 3,000% between 2007 and late 2011.

But once the company's patents expired, and the competition could both make their own machines and offer up their own cups for the machines to use, the stock plunged more than 80% by June 2012. Though it's recovered somewhat since then, it is a cautionary tale.

SodaStream has no protection from other companies making the high-margin syrups that help drive profits. And though it generally has an agreement in place with third parties to be exclusive refillers of CO2 cylinders, a German court has already said these agreements are not lawful.

If SodaStream were to participate in a race to the bottom for its consumables, the stock itself might not be worth what it is today.

I'll be making a decision for my retirement portfolio by the end of the month. In the meantime, it's worth digging deeper into what happened to Green Mountain to help inform my decision.