When making cider... We had pressed apples and put juice in a container with a lid (airtight) (forgot about bubbler). Added the yeast after 1 day. Changed the lid on day 2 to one with a bubbler. On day 4, doesn't appear to be any fermentation. Should I be worried? Thanks, Rachel
In my experience, I usually don't begin to see CO2 bubbles until days 3 or 4. Usually by the end of a week you should have pretty aggressive fermentation. I usually keep my Cider fermenting on a temperature that is the lowest that the yeast supports on the packet as I find that the fermentation is "cleaner" and I believe yeast will produce more "fusel" alcohols at higher temperatures.
Factors at play
Factors that will affect yeast starting activity are temperature, starter yeast amount, and pitching stress (in decreasing order of impact). If you poured the yeast directly into the cider, the sugar in the juice can cause osmotic stress to the yeast (I recommend gently rehydrating it in a bit of water with a sprinkle of sugar first and I even do this to my baker's yeast for bread). Combine this with very low temperature (50° F) and it could possibly take up to 1.5-2 weeks to see any visible activity.
I have coincidently done "unintentional lambics" by keeping a UV pasteurized fresh pressed honey crisp cider in a very cold garage and I didn't notice fermentation for a while. If you are doing your fermentation at a very low temperature it may take a very long time for it to start. I think if you can't see any activity; Hydrometer readings (as Robert suggested) are probably the only surefire way to see if yeast is converting sugar to alcohol.
In summary, if you are using commercial brewing yeast (hopefully you aren't using baking yeast) you should follow the instructions on the packet, and I recommend brewing at the lowest optimal temperature for the best flavor.