Image lab


Yeast cells immobilised in calcium alginate
Image courtesy: Department of Chemical Engineering, University of Birmingham


Guy says:

I don't know what possessed me to try my hand at making wine, but I'm pretty sure that it had something to do with one of our Chancellors of the Exchequer.

Although my initial attempts produced nothing more than a sweet, murky soup, I can now produce something of which at least my local Berni Inn would be proud.

It was during a visit to the NCBE at The University of Reading that I first heard of immobilising yeast cells. The process basically involves mixing your chosen yeast with a 2% sodium alginate solution and then adding this, a drop at a time via a syringe, to a 1.5% solution of calcium chloride. This traps the microbes inside tiny beads, which allow sugar, alcohol and carbon dioxide to pass through freely, but contain the yeast throughout the fermentation. When the fermentation is complete one simply draws off the wine, leaving the yeast balls in the demi-john.

And so, armed with my bottles of sodium alginate and calcium chloride solutions, I returned home with thoughts of those tiny beads floating up and down to the rhythmic 'plop-plop' of the air lock and a perfectly clear wine every time. Back in the kitchen I set about preparing the living quarters of my yeast whilst my filter kit and finings looked on disapprovingly.

The chosen wine was apple as this had proved successful in the past; and the yeast - it had to be a Sauterne type. The juice was extracted from about 10 lbs of apples (with a few pears thrown in). Enough granulated sugar was added to bring the total sugar content of my gallon of liquid to about 2.5 lb - which should produce a wine of around 12.5% alcohol. The acidity was measured by titration and adjusted until it was about 3.5 ppt. Finally, a yeast nutrient was added followed by my yeast balls - which immediately sank to the bottom of the jar and stayed there. After a couple of days the fermentation was well under way, however, and the yeast balls were floating merrily around on the surface.

I must warn you at this point that the initial fermentation can be quite vigour ous and even a slight shake of the demi-john will cause the 'wine' to come frothing out. It is therefore prudent to allow a good air space in the demi-john for the first week or two.

To my disappointment, once the yeast balls were floating they refused to budge, even towards the end of the fermentation. Nevertheless, when complete, the task of racking off the wine from the yeast was greatly simplified and could almost have been carried out with a sieve but for the fruity sludge at the bottom.

And the wine? Well, it's too early to tell yet, but it certainly looks promising. Oh, and by the way, my yeast balls have been transferred to another demi-john of fruit juice and seem content to sit there all day converting more sugar into alcohol for me.