<1 volt step up transformer

That's 1/4W out of a fruit . I don't think you will get there without using multiple fruits in series.
Additionally, you could *try* to use a joule thief to bump the voltage up, but I still think you are over-reaching for that kind of power.
A *Battery Pack* is your best option I think.

 

You missed my intention... I should have said Fruit-Pack. instead of *battery-pack*
The problem you will run into, is that a capacitor will only charge to the same point as the fruit's output voltage. The time it takes is based on the capacity of the capacitor, and the current capability of the fruit. The only benefit this has is offering a higher 'peak' current output when you finally decide to draw from it.

I would most definitely encourage the use of multiple fruit in series to increase the voltage output. If you had a higher voltage to start with, you have more freedom over the methods you use to store and manipulate it later. (Think that a regular diode would drop 0.7V... which is pretty much all of the potential from a single fruit.)
You can either use the fruit as-is, or juice them and use their juice in a custom enclosure that you have more control over. You can use electrodes with more surface area, and could create multiple 'cells' similar to how a car battery is designed.

 

It's entirely up to you how you tackle this. A custom enclosure is not a requirement unless you only want to use one fruit at a time, then you need to split up the fluid. You could simply setup more than one 'fruit-holder' and impale a fruit on each holder.

**Note : : There is a video circulating showing a lemon with multiple electrodes in at the same time. Please remember that it's a physics troll. By doing this, you are simply increasing the surface area of of the anode and cathode by having more than 2 electrodes in the fruit. If you connect them in 'faux-series' like in the video, you end up shorting most of them out. The *cells* must be electrically isolated in order to be able to connect them in series which is why I recommend multiple fruits, or juicing the fruit and putting the fluid in isolated containers. This allows you to completely control the flow of electrons by using 'jumpers' between the anode of one container and the cathode of another.

Best of luck! If you make something cool out of it, please share ^^
We like seeing juicy pictures.

 

Basically you need to reach the bases threshold voltage of around 0.6v, with a joule thief you may get a very dim glowling red led going with one lemon.

Buy a germanium transistor and you might get a couple of ma out of a lemon