If a small container of stinky yeast mixture should arrive at your door with a note, make sure you look after him: his name is Herman, and he's the latest food fad sweeping the nation. And not for the first time, either; many will remember Herman cakes from the Seventies when they first became popular.
The culinary equivalent of a chain letter, the mixture is nurtured for nine days, during which it expands, before being divided into four sections. On the 10th day you pass on three of the parts to friends and make a cake for yourself with the fourth. In theory, one starter mix could be passed around for ever.
Each Herman comes with precise instructions that will tell you when he is hungry, when he is thirsty, and when he should be stirred. Over the 10 days, the mixture (which must only be covered with a tea towel and never put in the fridge) will bubble away, emitting a pungent smell not dissimilar to a brewery. When you come to make your Herman into a cake, you are instructed to add ingredients including apples, cinnamon and raisins, although others will also throw in anything from chocolate chunks to glacé cherries or walnuts. The end result is a bit like panettone.
The idea of Herman cakes is believed to have originated with the Amish, who use a similar yeast mix to make sourdough bread, which is then passed around the community. While its current popularity has spread through websites such as Mumsnet, where it has many a fan, it's found an altogether younger demographic, too. Lizzie Boon, a student nurse, was given her starter mixture by her mother. "She was given some from my dad's colleague's wife," Boon recalls. "I think she's a school teacher, so it came from the school. It's unclear what the exact origin of it is, which is all part of the fun, I guess. She told me about it and I thought it sounded bizarre but said to bring it round. I wasn't sure about it at first, but there were some instructions with it introducing itself as Herman, so straight away I felt guilty about chucking it away because you feel like it's a living thing."
It is not an uncommon reaction; having nurtured the Herman, many become attached to their bowls of mixture and feel surprisingly sentimental about letting go of them.
Other Herman enthusiasts get obsessive over the origins of their mixture and attempt to chart its family tree. Almost everyone agrees that it brings people together. "It quite a nice idea really, passing something around that you can all eat. It's definitely a conversation starter," says Boon.
Not everyone is so enthusiastic about receiving the mixture, and complaining about being landed with a container of the mixture is not uncommon. Others worry about how sanitary the mixture is, having been passed around a number of households and left at room temperature.
However, investigations have concluded that there is little risk of contracting any illness from the mixture but common sense should prevail. Good starters are bubbly and smell strongly of yeast; throw away any that become mouldy or develop an orangey hue.
"As I work in a hospital, I think about how clean things are all the time," laughs Boon. "So I had to just let go a bit. I knew my mum and dad hadn't gotten ill so I thought I'd probably be all right."
Some have taken an interest in Herman cake simply because of the unique way it is made. "I didn't know anything about it when a colleague brought it in for me," says Richard Bowley, who works for Citi Group. "But I Googled it and looked up its history even though I was a bit sceptical about the whole thing. I think the appeal lies in the challenge: you have to look after it for 10 days. And I suppose, for blokes, it's a bit like a big kid's science experiment with all the reacting and growing and bubbling."
Social networking is also helping to propel the Herman cake, with converts regularly offering up starter mixtures to friends on sites such as Facebook, which might go some way to explaining its recent revival. "I went up to Cambridge the other week and a friend was cooking a completely unconnected batch," laughs Bowley. "It must be taking over the country."
How to start your own chain of Herman cakes
2 cups (300g) flour
2 cups (550ml) milk
1 cup (225g) sugar
A third of a cup (90ml) warm water
2 tablespoons or 2 packets active dry yeast
Sprinkle 1 tablespoon of sugar over the warm water
Sprinkle the yeast over this and leave in a warm place for about 10 minutes to double in size.
Mix the milk, sugar, flour and yeast mixture in a large bowl, stir using a wooden spoon.
Place in a warm place and stir every day.
Instructions to pass on to friends:
Hello, my name is Herman. I am a sourdough cake. I'm supposed to sit on your worktop for 10 days without a lid on. You CANNOT put me in the fridge or I will die. If I stop bubbling, I'm dead.
Day 1 You get Herman and put him in a large mixing bowl and cover loosely with a tea towel.
Day 2 Stir Herman.
Day 3 Stir Herman.
Day 4 Herman is hungry. Add 1 cup each of plain flour, sugar and milk. Stir well.
Day 5 Stir Herman.
Day 6 Stir Herman.
Day 7 Stir Herman.
Day 8 Stir Herman.
Day 9 Herman is hungry again. Add the same as day 4 and stir well. Divide into 4 equal portions, keep one, and give the other three away to friends with a copy of these instructions.
Day 10 Herman is very hungry. Stir well and add the following:
1 cup (225g) sugar
Half teaspoon salt
2 cups (300g) plain flour
2/3 cup (180ml) of cooking oil
2 tsp vanilla essence
2 cooking apples cut into chunks
1 cup (200g) raisins
2 heaped tsp cinnamon
2 heaped tsp baking powder
Mix together and put into a large greased baking tin. Sprinkle with a quarter of a cup of brown sugar (44g) and a quarter of a cup (70ml) of melted butter. Bake for 45 minutes at 170-180C.