Tomato tar is a tacky yellow powder that gets onto your skin when you handle tomato plants, and causes yellowish green stains on fabric afterwards. You can solve the problem by washing your hands with a little vinegar before you go near the soap – but where’s the fun in that?
It’s bound to happen at least once every year: a cry goes up ‘who’s got these green stains all over this clean towel?’ Everyone denies it, and you keep a very low profile because you were handling tomatoes. You don’t remember your hands being that dirty when you pinched out the sideshoots – a quick spot of water and they looked fine. Could they really have been that dirty?
The culprit is a substance that commercial growers call ‘tomato tar’. It’s a powdery deposit that is all over the plant – leaves, stem, and fruit. On your skin (which is slightly acid) it is a faint yellow, inoffensive enough to be all but invisible on the skin of most gardeners unless you get really covered in the stuff. If you realise it’s on you and try to wash it off with soap (which is usually strongly alkaline) it turns a really intense green, so you scrub… and scrub… and scrub. You can wash and rinse three or four times and still see it in the rinse water.
The real trouble is when there’s just a little on your skin, and you don’t realise it’s there at all. You get back to the house and give your hands a quick rinse under the tap. So far, so good – water’s pH neutral, so you don’t see the stain. Then you wipe your hands on a towel and head off to another task, leaving the tomato tar you just left on the towel to react slowly with the alkaline detergent residues on the cloth. Half an hour later, your negligence is revealed.
The culprits here are trichomes, tiny appendages on the surface of the plant that look like miniature water towers (you can see a charming microscopy photograph at the Solanum Trichome Project website). These contain a bunch of chemicals including essential oils that give your skin that lovely fresh tomato smell, but also compounds called acylsugars which form the basis of a fast-drying glue. This helps defend the plant from microscopic predators, stopping them wandering too far by giving them the microscopic equivalent of concrete boots. Acylsugars aren’t soluble in water, which is why they’re so hard to wash off.
The solution to tomato tar is to get some acid onto your hands before you go anywhere near the towel, which dissolves the acylsugars. Commercial tomato growers squish an unsaleable green tomato in their hands and rub the gunk well in, but if you’re already inside (or don’t wish to sacrifice a precious tomato) then a couple of tablespoons of vinegar will do the job just as well. Wash your hands with the vinegar first, then rinse it off before using soap. Hey presto, no green colour – and no towel-based guilt later.