Cider Musings

Nitrogen and cider

A  topic of the moment with agricultural emissions run off and degradation of waterways and climate change high on the agenda here in NZ is nitrogen.  The health of an orchard depends on many factors. One important one I want to discuss is nitrogen in the soil. Nitrogen is often applied to arable land to promote crops. Growing crops are often limited by nitrogen availability. Protein is built on amino acids. Amino acids of which there are about 20 different forms are called amino because they contain -NH2 radical.

Up until recently nitrogen available for life came nitrogen-fixing bacteria. Ironically for such a common element ( nitrogen is about 80% of air) supply for all living organisms was limited to a few species of bacteria and those have mainly formed association with plants, to provide the environment needed, or special cells called heterocyst in blue green algae. (Algae are a diverse group of organisms close to bacteria that include seaweed. The nitrogen fixing algae live in the pasture soil and humus and provide up to 20% of pasture nitrogen fixation. ) More than 90 percent of all nitrogen fixation is effected by these organisms, which thus play an important role in the nitrogen cycle. This is done at normal atmospheric pressure and powered by the sun! So different to the Haber-Bosch process which was invented only recently in 1909 which requires very high pressures and temperatures and a catalyst. And for completeness there is another source - lightening which converts atmospheric nitrogen into ammonia and nitrate (NO3) that then enter soil with rainfall.

Prior to that the increased demand from nitrogen in farming post the agricultural revolution was from mining guano esp off the coast of Chile.

Several different bacteria have mastered this including Rhizobium species which are associated with clover. Although commonly thought of as producing nitrogen clover itself is only the host in this symbiotic relationship. Clover roots developing special root nodules that contain walls and compounds ( leghaemoglobin)which provides a lot of energy but excludes oxygen radicals that would stop the enzymic reaction!

Rhizobia invade the root hairs of host plants, where they multiply and stimulate formation of root nodules, enlargements of plant cells and bacteria in close association. Within the nodules the bacteria convert free nitrogen to ammonia, which the host plant utilises for its development adding it into amino acids and proteins, building chlorophyll in the leaves that fuel the process, ATP and DNA. The process begins when the rhizobia are attracted to flavonoids released by the host legume’s roots, the bacteria then begin to attach themselves to extensions of root epidermal cells called root hairs. The host legume then senses chemicals produced by the rhizobia called Nod factors that cause the colonised root hairs to curl and form what is called a shepherd’s crook. Then rhizobia penetrate the root hairs and typically form a tubular structure called an infection thread. Once the bacteria reach the root itself, they stimulate cortical cell divisions that lead to the formation of a nodule. As the nodule begins to form, the bacteria become surrounded by a plant-derived membrane and are released inside plant cells forming the nodule. The bacteria subsequently lose their cell walls and undergo a profound change in cell morphology to form large, irregularly shaped branching cells called bacteroids. They then are entirely dependent on the host plant for their energy needs. In return, the bacteria fix nitrogen for the plant.

The reduction of atmospheric nitrogen is a complex process that requires a large input of energy to proceed . The nitrogen molecule is composed of two nitrogen atoms joined by a triple covalent bond, thus making the molecule highly inert and nonreactive. The triple covalent bond of gaseous nitrogen is one of the strongest bonds in the natural world ( bond energy 946 kJ mol-1, needing 16 units of ATP compared to H-C bond of 415 or H-N bond of 390. The nitrogenase enzyme catalyses the breaking of this bond and the addition of hydrogen atoms to each nitrogen atom. 

The conversion of N2 into ammonia occurs at a site in the Mo-Fe-S metallocluster comprising an inorganic Fe6-S9 shell and a core called FeMoco ( abbreviation for the iron-molybdenum cofactor). This is sent energy by Fe complexes. Amazing when Molybdenum is a scare rare earth metal ( 54th in the earth’s crust) 

The rhizobia bacteria in the soil are aerobic and cannot fix nitrogen as the oxygen sensitive nitrogenase will not function in an oxygen environment. The plant cells in the nodules produce leghemoglobin ( the colour and taste of plant based meats) to buffer the concentration of free oxygen in the cytoplasm of infected plant cells to ensure the enzyme can function. The plant cells of the nodule increase in size to decrease the surface area to volume ratio. That being said, nitrogen fixation is an extremely energetically costly process, so aerobic respiration which necessitates high oxygen concentration, is also necessary in the cells of the root nodule! Quite a balancing act, legheamoglobin maintaining a free oxygen concentration that is low enough to allow nitrogenase to function, but a high enough total oxygen concentration (free and bound to legheamoglobin) for aerobic respiration.

Moving from the molecular level to agriculture, nitrogen is generally seen as the farmers friend as it makes plants grow better, faster and in terms of apples - larger. In the orchard 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. 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. Further fertilisers will then act contrary to a traditional cider makers needs. The fruits are larger but less tasty! 

It has been known for many 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". 

So not enough nitrogen stunts growth esp during the trees early life, too much reduces the quality of the cider flavour profile. So there is an optimum level for nitrogen nutrition in fruit trees which changes with the age of the orchard. 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. Esp when the trees are grown as standards not bush. The best cider in awards often go to cider made from old standard orchards. This is moderated further 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'. And especially if the grass sward is maintained by sheep. When the sheep are taken out, there is a loss of nitrogen in the proteins of their bodies. 

 At TeePee Cider, where time is on our side we aim to craft some of the finest traditional ciders from large mature trees with no extra nitrogen inputs in the form of fertiliser. We do graze sheep in the orchard but the nitrogen is not lost to the orchard as we use the sheep only as sward mowers not meat. (They vacation in a neighbouring field for 8 weeks leading up to harvest.) With low nitrogen the carbohydrate made by photosynthesis in the leaves is all converted into sugar or other flavour precursors such as tannins and aromatics in the apple - 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 our view and cider-making terms, quality takes precedence over quantity when nitrogen supply is limited in a mature orchard, and managing the nitrogen balance is important. 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 modern cousins ( this and above factors might explain the increasing tastelessness of Red Delicious!

At TeePee Cider therefore we are aiming for a reduction in nitrogen in the orchard with our standard trees up to 20 years old. 

One further area of nitrogen use is in the fermentation. Yeasts need some nitrogen containing amino acids, 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. This probably morphed into folklore of cider fermentation reactivating after rats drowned in the vats.

Probably not the most hygienic source for nitrogen! In reality, any rats found in cider would probably have fallen in while drinking the liquid, rather than being added deliberately.

Wine and home brew makers measure nitrogen levels or YAN (yeast available nitrogen) and add DAP or diammonium phosphate to reduce the risk of a stuck fermentation as they want a quick fermentation in a few weeks ( and avoid any sulphide smells) rather than the 4-6 months traditional cider maskers use. So far we have not had any stuck fermentations. We do aim for a slow fermentation over several months and help that by keeping the cider room below 18C. However a leg of mutton is at hand, sorry just joking.  

However nitrogen does not remain 'fixed' forever. There is a nitrogen cycle. When plants and animals die or when animals excrete wastes, the nitrogen compounds in the organic matter re-enter the soil where they are broken down by microorganisms, known as decomposers. This decomposition produces ammonia, which can then go through the nitrification process. Nitrifying bacteria in the soil convert ammonia into nitrite (NO2-) and then into nitrate (NO3-). Compounds such as nitrate, nitrite, ammonia and ammonium can be taken up from soils by plants and then re-used in the formation of other plants and thence animal proteins. Denitrifying bacteria completes the nitrogen cycle by converting soil nitrate (NO3-) back to gaseous nitrogen (N2). These bacteria use nitrate instead of oxygen when obtaining energy, releasing nitrogen gas to the atmosphere.

And back to climate change and agricultural health.  The benefits accrued by increased use of nitrogen are balanced harms.

Agriculture may be responsible for about half the nitrogen fixation on Earth through fertilisers and the cultivation of nitrogen-fixing crops. Increased nitrogen inputs (into the soil) have led to lots more food being produced to feed more people – known as ‘the green revolution’.

However, nitrogen in excess of plant demand especially applied nitrogen rather than biologically bound nitrogen from plants can leach from soils into waterways. The nitrogen enrichment contributes to eutrophication.

Another problem can occur during nitrification and denitrification. When the chemical process is not completed, nitrous oxide (N2O) can be formed. This is of concern, as N2O is a potent greenhouse gas – contributing to global warming.

By keeping our nitrogen inputs low we are also "doing our bit for climate change as well as producing the best cider we can craft.