Step 4: Bottle & Ferment
1. Pour your soda mixture into bottles. Give the liquid a stir between each bottle so that the undissolved yeast doesn't settle to the bottom and end up in your last bottle.
2. Leave a small amount of "headspace", or air between the soda and the rim of the bottle. This will allow the yeast to grow a little.
3. Cap the bottles. If you like, give them a shake to oxygenate the liquid and give the yeast a bit of kickstart.
4. Store the bottles for a few days. Make sure they are out of the sunlight. If your storage area is about 65-70 degrees fahrenheit, it will probably take about 2 or 3 days. 70-75 degrees will probably will only take 1 or 2 days.
5. After you have waited, open a test bottle. It should fizz up to the rim before settling back down to the bottom. Taste it and see if it seems fizzy enough. If it is, place the remaining bottles in the refrigerator to stop the fermentation process. Otherwise leave the bottles out for several more hours or days.
Note on bottling and safety:
Many people will tell you that plastic bottles are safer than glass. This is true. In case you let a bottle overcarbonate and explode, plastic will just send soda everywhere and not shards of glass. However, I like the classic feel of a glass bottle in my hand, the sound of a metal cap hitting the floor, and overall, the aesthetic of glass in general. Glass is class. All gourmet sodas come in glass bottles, and your homemade Malta will certainly be gourmet. In addition to this, glass can be reused indefinitely and will not impart any plasticky flavor to your drink. With a few precautions, you can minimize the risk of exploding bottles and enjoy the benefits of glass.
-Use thick glass bottles. Antique bottles are ideal. Any modern returnable bottles will also work great. Most microbrew beer bottles will also come in a slightly thicker glass than say, budweiser. And make sure your bottles are not threaded. The seal created on these bottles will not be as good as that of a pop-top, plus the glass tends to be thinner. Never use jars, growlers, or anything other than pop-top soda or beer bottles. Swing top bottles, if the glass is thick, work great. You can buy them at Ikea or most kitchen supply stores.
-Open a "tester" bottle after just a couple of days. If it gushes over, refrigerate it immediately. Open tester bottles in the sink to prevent making a mess.
-Leave about one inch of headspace when filling your bottles. Leaving too much headspace will make the yeast grow too fast a burst your bottles.
-Once refrigerated, use your soda within about one month. Refrigeration slows fermentation to a crawl, but it does not stop it completely.
-If you don't want to risk glass bottle explosions or you don't have access to a capper, use plastic bottles. Use soda bottles with intact threads and plastic seals. Refrigerate them when they feel tight like a bicycle tire. Do not reuse them more than a couple times as the plastic seal tends to weaken a little each time and will eventually fail.
-Whatever bottles you use, cover them with a towel so that any explosions will be contained.