Many previous twin-gear machines have gone to a great deal of effort to enusure the stainless steel parts of the gears do not touch one another.

Instead they have used nylon-on-stainless-steel - or nylon-on-nylon - as the means of driving the gears.

The Twin Health machine represents a departure from this tradition - and uses a stainless-on-stainless drive mechanism.

Doing so eliminates the nylon components completely - resulting in more durable gears - and simplifying the engineering task of manufacturing them dramatically in the process.

However the manufacturers have had to add a sentence in the instruction manual warning of unspecified dangers if the machine is run too much without putting in any produce to act as an insulator between the gears.

Stainless steel disadvantages

Various cast-iron manual wheatgrass juicers have been produced in the past - and these use cast-iron-on-cast-iron parts - and I'm sure produce innumerable tiny metal fragments - yet I have heard few complaints from consumers.

However, maybe stainless steel fragments are a little less digestible than cast iron ones - so perhaps there is some cause for concern.

Aside from concerns about metal shards there has been some consumer concern that stainless steel is not as inert as it could be - and may release nickel - and other heavy metals.

The study [Nickel and molybdenum contact allergies in patients with coronary in-stent restenosis] is cited is support of this idea.

All twin gear juicers have stainless-steel gears - but rubbing them together seems likely to increase the rate at which wear occurs on them - and indeed some wear which seemed likely to fit this description was present on my sample machine - strongly suggesting that the presence of produce during juicing was not sufficient to prevent wear associated with stainless-steel-on-stainless-steel contact.

What will juicing consumers make of this stainless-on- stainless idea?

I suspect most of them will hardly notice it.

What do I make of it? It is probably better than using nylon. However I can't say it makes me feel terribly comfortable.

Contact avoidance

I hope that there will eventually be scope for a drive mechanism that permits all-stainless steel gears - but with the only points of contact being cogs at either end of the gears.

These cogs could then be located outside the chamber where the juicing process takes place. I suggest one way this could be done in my page of recommendations.

This would eliminate steel touching steel as part of the drive mechanism in regions in contact with the juice.

The Samson Ultra also used nylon parts in contact with the stainless steel screens - so that nowhere was stainless steel in contact with more stainless steel.

Though this may be as site where wear occurs, I'm not so worried about contact and friction there - and would probably rather get rid of the need for nylon parts.

Nylon disadvantages

It should be mentioned that nylon parts probably have some disadvantages of their own.

Eating nylon probably isn't terribly good for you either - and nylon parts are more likely to wear down than the stainless steel is.

Faster wear on the nylon may also result in more pitted surfaces - and greater scope for trapped food and associated bacterial populations.

Lastly, the Twin Health marketing materials point out that part-nylon gears are not likely to be so durable - and can crack. They may well be right about this.