Welcome to the Cake Decorators Q&A
how much glycerine do you add to cakes to lengthen shelf life
Glycerine doesn’t necessarily lengthen shelf life, what it does is make cakes moister so they don’t stale so quickly. I used to use quarter teaspoon per one large egg but found it left a slight after taste so abandoned that in favour of using a little golden syrup. I learned the glycerine trick years ago as a student working in a large bakery (summer holiday job!). I’ve recently seen Lindy Smith on her site advocating using it in the same quantities as I did. Other people may have different results, but you can just use simple syrup made up by boiling equal quantities of sugar and water or fruit juice and even alcohol. Bring the ingredients to a rolling boil for a minute, cool the syrup and apply to cake layers with a pastry brush. The syrup is absorbed into the layers keeping the cake moist for longer. For fruit cakes add golden syrup and black treacle according to amounts specified in recipes. Glycerine is fine on heavier and more dense cakes like madeira or some chocolate cakes which have whisked egg whites added, in lighter sponges it can alter the texture by making
it too soft or moist and liable to breaking as I found to my cost. Best thing to do is try it on a small cake for your own consumption and see whether you like it.