Understanding secondary fermentation

This week was extremely exciting (and very nerve racking) - we bottled 13000 bottles of Silverthorn Cap Classique under crown cap for secondary fermentation. What is secondary fermentation? Simply stated this is what sets Champagne style wines apart from all other wine types - it is the method by which the magical bubble is imparted into a still wine. Without getting too technical I will attempt, in point form and with visual support, to reveal some of the 'secrets' of secondary fermentation.


1) After harvesting, whole bunch pressing, primary fermentation, blending, stabilising and filtering the resulting product is referred to as a base wine. It has a very low alcohol content (10.5%) and very low sulphur (10mg/l FSO2) in order to ensure that the yeast can easily complete secondary fermentation in the bottle. The only form of preservative is the base wines naturally high acid and very low pH.

2) We then sugar up the base wine by adding 24g of sugar per liter. This formula is simple - 4g of sugar gives you 1 bar of pressure - we ideally want 6 bars of pressure.

3) We then make up a batch of strongly fermenting yeast over an extended period of 3 days (see image of bubbling yeast) The reason we take so long is to acclimatise the yeast to the alcohol content of 10.5% and to build up a massive population of active microscopic yeast cells.

4) On the day of bottling we thoroughly mix the brew of yeast into the sugared up base wine (see image of cloudy wine) and add yeast nutrition and a riddling agent (bentonite clay which helps settle the lees during the riddling process pre-degorging)

5) The bottling line needs to be specially prepared for the critical action of crown cap application. Due to the fact that we cant filter the base wine (it would remove the yeast) we have to sterilise and sanitise the machinery by steaming it for one hour (see image of steaming bottling jaws) The crown cap applicator needs to be accurately installed and set up.

6) A sparkling wine bottle has two important design features: firstly it has a very deep punt and thick walls to withstand the build up of 6 bars of pressure and secondly the lip design incorporates a rounded ridge which facilitates the application of a crown cap (see image of the gold crown cap securely crimped into place)

7) Once the wines have been bottled and sealed the next important step to consider is storage. The bottles need to be packed into bins lying horizontally (see image of wooden bins in a cool storage room)The bins need to be stored in a constantly cool (14 degrees Celsius) cellar. This is essential in order to ensure slow controlled secondary fermentation resulting in a fine bubble.

8) The minimum time Cap Classique has to legally spend on the lees is 9 months. This process of lees contact is the secret to developing complex rich biscuity flavours from the autolysis of yeast cells (they break up releasing flavour components like amino acids) The other positive benefit of time on the lees is to get gentle fine bubbles. Silverthorn The Green Man and Genie both spend an average of 28 months on the lees.

So quite simply secondary fermentation is the process by which yeast convert sugar into two products - alcohol (from 10.5% to 11.5%) and carbon dioxide (the bubble) Due to the crimped on crown cap the carbon dioxide cant escape and is naturally captured inside the bottle. It is through this incredible process that you as the consumer can enjoy the thrill of Bubbly.