Given a choice between using a good coffee grinder a few days in advance, or one of those whirly-chopper grinders immediately before brewing, which would you choose?
It's really going to be a trade off between the flavor defects, but it also depends on the brewing method, and if it's drip or espresso.
For us, with drip, stale coffee tastes worse than badly ground coffee. We can always tell if coffee has been freshly ground or not, because the characteristics and flavor profile change the longer it's been ground. Having a crappy grinder will affect the flavor as well, with some grounds being over extracted and under extracted. At this point the quality of the coffee wouldn't even matter.
So, it really depends on what you'd prefer to sacrifice. For me? In this situation, I'd probably just drink tea.
Both have downsides:
- Coffee that is ground more than a few hours before brewing loses aroma, which is obviously an important part of flavour.
- A blade grinder doesn't produce evenly-sized particles; big particles will under-extract (losing flavour) and small particles will over-extract (introducing bitterness).
My wife and friends didn't believe me about the over-extraction potential of blade ground coffee, so we did a blind taste test between coffee beans ground with a blade and ground with a burr. I identified the blade-ground coffee every time. That's not the comparison you have, but in the absence of a burr grinder I'd go for pre-ground, for convenience.
Although there are trade-offs for both, I would suggest grinding right before brewing.
Here is why:
As soon as you grind coffee beans, they begin to lose aroma and flavor. This begins to happen as soon as the beans are ground. In my opinion, this is a big problem if you want really great tasting coffee.
Although blade grinders do tend to produce some uneven particles, which can cause uneven extraction (over extraction for smaller particles and under extraction for larger particles), for the everyday cup of coffee, this isn't a significant problem. Good blade grinders ($50 and up) do not have this problem as much.
The everyday coffee drinker will most likely notice less fresh coffee as opposed to an uneven extraction.
A burr grinder is the best way to go, but you need a really good quality one (expensive) to produce a good grind, and there are even downsides to this type of grinder.
In my cafe, we use industrial burr grinders that are very consistent. At home, I use a good quality blade grinder and usually come up with good results, especially if I am grinding more coarsely (drip coffee, french press, etc). I only really get problems with inconsistent grind when grinding finely for espresso. I also find that if you shake the blade grinder (almost like a martini shaker), the grinds mix better during the process and you get more consistency. That is just what I have found though, and you should be very careful when doing this.