Marrying malts is a culmination of the production process, when casks of mature malt whiskies are blended, and water can then be added to adjust the alcoholic strength ready for bottling. This may seem an entirely pragmatic stage. However, in addition to the skill and experience of the master blender, certain precautions are also required to ensure a successful (and very happy) marriage.
The first stage is for the master blender to select the casks required to bottle a batch of a particular expression, such as a 12 years old, and every batch must of course maintain the established flavour profile. However, every cask has an individual influence on the maturing malt whisky. Even casks of the same type, which are filled on the same day, and then aged next to each other in the same warehouse for the same length of time can show differences. These may be very subtle differences, but they can also be significant. Moreover, blending malts from various casks also results in interaction between them, which sees some characteristics promoted while others are relegated.
"Selecting the right casks is absolutely vital, and we sample every single cask before deciding whether it will enable us to create a final product that is consistent. I'd be too scared not to sample every cask, in case a batch went wrong," says Gordon Motion, Master Whisky Maker at the Edrington Group.
Once selected, casks can be emptied into a trough, from where the malts are pumped into a marrying vat (large vessel), which is also known as a marrying tun or holding vat.
Marrying vats are typically either stainless steel or oak. If oak, this is 'inactive,' ie. already 'exhausted' and so has no influence on the malts. Any debate about different types of vat generally covers practicalities, such as stainless steel being easier to clean. Malts are typically combined at cask strength (ie. the alcoholic strength they reach in the cask when judged mature). When water is added to malt whisky it's vital to ensure that the two integrate, and that's hardly guaranteed. Water is heavier than alcohol, and this entails a risk of 'layering.'
"Layering refers to layers of water and alcohol that have not fully integrated, which means these layers would have variable alcoholic strengths. As alcoholic strength determines the flavour profile a whisky shows, these layers would also have varying flavour profiles which would be disastrous, and you wouldn't be able to bottle it. But there are various ways of preventing layering, to ensure you achieve consistent strength and flavour profile," says Stuart Harvey, Master Blender, Inver House Distillers.
One option is a 'recirculation' method, in which liquid is drawn from the base of the vat, and pumped back in at the top. Alternatively, marrying vats can be fitted with a 'propeller' device which rotates and gently stirs the liquid.
Another option is to use air rousing (blowing air through the liquid).
"A stainless steel pipe is positioned in the centre of each of the vats, which are tall and cylindrically shaped. Air is blown through the pipe, and emerges from the end which is at the base of the vat. Air bubbles up through the whisky and reaches the surface, it's an even and gentle process which ensures integration of water and alcohol," says Stuart Urquhart, Whisky Supply Manager at Gordon & MacPhail.
When alcohol and water are combined this initially creates a certain turbulence, before an equilibrium is established between them. This sees alcohol molecules forming 'patterns' throughout the liquid. Meanwhile, water molecules, which are smaller, form patterns within the available spaces around the alcohol molecules.
While the integration of whisky and water is absolutely essential, it also has another effect.
"When you add a quantity of water to a quantity of whisky you don't end up with the combined total. One plus one doesn't make two, it makes about 1.9, as there is a small reduction in the total volume due to the water molecules 'fitting in' among the alcohol molecules," says Gordon Motion.
The length of time malt whiskies spend marrying in a vat varies. Some distillers believe more mature malts and more complex recipes require longer marrying, while others believe more mature malts marry more readily, and therefore need less time prior to bottling. Clearly, marrying is a vital process that requires a master blender's skill and judgement, which can only be gained from extensive experience."Marrying is the final stage of a whisky's journey, and you have to know that journey intimately to be able to refine and finesse all the elements. It's the final opportunity to bring together all the flavours and unlock the deepest layers of complexity in the spirit. I have to do the whisky justice, and give the drinker the opportunity to drink the whisky at its best," says Rachel Barrie, Master Blender, Beam Suntory.