Nitrogen accounts for four-fifths of the air that we breathe, and is generally thought of as completely inert. Certainly it is true that humans, animals and plants can do nothing with it directly, and yet in its combinations as nitrate, protein, amino acids or vitamins it is essential to all living processes. No less, then, for cidermaking too!
The role of nitrogen begins before the trees are laid out. As the soil is prepared by digging or ploughing, millions of nitrogen-fixing soil bacteria are activated to pluck the gas from the air and convert it to a form (as nitrate) which the roots of plants can use. Without these specialised bacteria, nitrogen would remain forever an inert gas and unavailable to the plants that need it. So even as the trees are planted, they enter an environment where the nitrogen required for growth is already available. And, if the ground has been further prepared with bonemeal, manure or inorganic fertilisers, all of which contain assimilable forms of nitrogen, then the young fruit tree will have all the nitrogen it needs and maybe more. In its early years, a tree needs relatively large amounts of nitrogen to turn into protein as it grows - later on, its requirements diminish.
It has been known for centuries that the amount of nitrogen in the soil has a significant bearing on the keeping qualities, the colour and the flavour of apples. Thus John Evelyn, writing in his Pomona in 1664, could say that "apples and pears requiring rather a vulgar and ordinary field land than a rich garden-mould, it has been found that kernels sowed in a very high compost have produced large indeed but insipid fruit, hastily rotting on the trees". It is now generally appreciated that there is an optimum level for nitrogen nutrition in fruit trees - too little leads to stunted growth and pale green leaves, while too much leads to excess growth, flavourless fruit and poor keeping qualities. When the trees are young, the amount of nitrogen they take up into the fruit may be up to three times as great as once they settle down into a mature fruiting phase. This is moderated when the trees are grown in a grassed-down sward where the grass can take up any excess nitrogen, thus acting as a form of 'buffer'.
Although modern cider growers tend to favour fruit bulk at the expense of quality, some of the finest traditional ciders came from large mature trees with no extra nitrogen inputs in the form of fertiliser. Moreover, animals such as sheep were often grazed in these orchards, thus removing any surplus nitrogen from the grass and incorporating it into their own body mass. When the sheep left, the nutrients left with them, and the net effect was that there was little excess nitrogen available to the fruit In this event, the carbohydrate made by photosynthesis in the leaves is all converted into sugar or other flavour precursors such as tannins and aromatics - little can be turned into protein and tree growth because there isn't enough nitrogen available. These flavour precursors are a part of what marks out a quality cider.
Hence, in cider-making terms, quality takes precedence over quantity
when nitrogen supply is limiting, and managing the nitrogen balance is
therefore a worthy objective. The majority of the 'soluble' nitrogen in
apples is in the form of asparagine, an amino acid, and can
vary
in total from 44 up to 330 parts per million (ppm) in the juice (when
expressed
as elemental nitrogen). Levels greater than 100 ppm are considered
excessive
and detrimental in traditional cidermaking. There is some evidence that
the 'vintage' cider cultivars, which tend to ferment more slowly,
inherently
take up less nitrogen from the soil than their more rampant cousins.
However,
the actual level of nitrogen in any given apple is hugely influenced by
the state of the soil, the external fertiliser inputs and the weather,
in addition to any inherent abilities of the cultivar.
In practice, cider apple juices contain much less nitrogen than grape juices or beer worts - typically five or ten times less. This has major implications for the conduct of cider making as compared to winemaking or brewing. A traditional British or French cider fermentation takes four or five months for completion, from November or December through to March or April. No modern winemaker or brewer would ever wait that long! As a result, it is now common practice for modern commercial cider makers to add extra nitrogen in the form of ammonium phosphate and thiamin. The purpose of these additions is to give the yeast enough to feed on to ensure a rapid fermentation - there is already sufficient carbohydrate in the form of juice sugar, but the yeast needs assimilable nitrogen (readily supplied by amino acids or ammonium salts) to make protein for its own growth.
Yeast also needs the nitrogen-containing vitamin thiamin (vitamin B1) which plays an essential role as a co-factor during fermentation particularly in the final enzymic conversion of pyruvate to ethanol. Both materials are in short supply in traditional apple juice - so much so that hanging a side of meat in a vat of cider whose fermentation had 'stuck' was once regarded as a normal procedure in English farmhouse cidermaking, because the meat provided both the amino acids and the vitamins which the juice was lacking. There are quaint stories of stuck ciders becoming reactivated after rats drowning in the vats - presumably the extra nitrogen from the corpses was sufficient to get the yeast going again!
Rustic tales apart, the slow fermentation and the resultant stress on the yeast undoubtedly has a major effect on the characteristic flavour profile of a traditional cider. It also offers a potential route to the manufacture of naturally sweet ciders by 'arrested fermentation'. In beer worts this can be done by the use of a low attenuation yeast which leaves untouched the higher 'unfermentable' sugars such as maltotriose and maltotetraose - but in apple juices all the sugars (sucrose, glucose and fructose) are completely fermentable, so this route is not available to the cidermaker. However, by ensuring that the nitrogen levels remain low it is often possible to have a cider fermentation slow enough that it can be encouraged to stick at a gravity of say 1.025. This may be achieved by using fruit from low nutrient orchards followed by 'keeving' (clarification and removal of nitrogenous nutrients prior to fermentation), or by repeated racking which reduces nutrients by taking away the yeast as a 'crop'. Either way, if the fermentation can be slowed sufficiently, it becomes possible to bottle the cider as sweet, sparkling and naturally conditioned.
This technique is still employed commercially in France, but scarcely at all now in England except by a few dedicated amateurs or those working to produce methode champenoise naturally conditioned ciders. It was obviously common practice in the seventeenth century as several of the contributors to Evelyn's Pomona describe it in detail, although of course they had no idea of the role of nitrogen (nor even that of yeast) in the process. But as Sir Paul Neile put it in that volume "It is in your power to give the cider just as much fret as you please, and no more"! By the quaint term 'fret' he meant what we would now refer to as 'sparkle' or as 'natural conditioning'.
Nitrogen, then, inert as it may appear to be in the atmosphere,
plays
a key role in cider making. This is as true for the modern factory
cider
makers who want fast fermented flavourless ciders and therefore add
nitrogenous
nutrients, as it is for the traditionalists who keep their nitrogen
levels
to the minimum. A forgotten element, perhaps, but a critical one
nonetheless!
This page last updated 27th February 2000