All you need to know about Apple metabolism ( but were afraid to ask)!
Allowing apples to fall from the tree to the ground is not the best way to get the best eating apples! Apples for humans to eat maybe half a world away are carefully picked just before peak ‘ripeness’ - a human term; however apples for cider are often allowed to do this and then are traditionally collected into ‘tumps’ for a few weeks to allow for further maturation before being crushed and pressed in the cider making process in England.
Maturation of apples if allowed will turn into senescence- its peak palatableness for animals that naturally eat them such as bears and deer in the high Tian Shan mountains of Central Asia .
Tian Shan brown bear
Fruits are classified into climacteric or non-climacteric types. Apples are of the former and this means they continue to change post picking or abscission.
For apples destined to be eaten later, the cool store conditions are set to put this stage into hibernation and therefore this maturation is not often discussed except in how to avoid it!
Typical cool store
Although the gene expression and enzyme changes in this later phase of maturation are complex, a lot is predominantly controlled by the plant hormone ethylene. Ethylene production in the apple starts a few weeks before natural abscission and continues post. ( Abscission is also controlled by ethylene production but even more localised in the abscission layer).
In the earliest stage of apple development post pollination there is a rapid rise in the number of cells in the fruitlet, followed by an enlargement of those cells predominantly by starch. The apple tree leaves synthesises sugars, mainly sucrose 10% of total sugars but predominantly sugar alcohols ~ 80%, mainly sorbitol, from the energy produced in photosynthesis. Sorbitol and other sugar alcohols play an interesting and only recently recognised role in plants. They are ubiquitous and are found in unicellular plants and fungi though to flowering plants. A major role seems to be in maintaining osmolality and leaf turgidity. Sorbitol (less commonly known as glucitol is the sugar alcohol analogy of glucose with the aldehyde group (−CHO) replaced the primary alcohol group (−CH2OH).
These are then transported in the phloem of the branches to the apple for storage as fructose and starch. Interestingly sorbitol is converted to fructose in the apple, and the sucrose is converted to glucose then starch. Fructose accumulation in the fruit correlated positively with the activity of sorbitol dehydrogenase (SDH).
The starch level reaches a peak about halfway through fruit development, the sweetness of the mature apple then is related not only to starch breakdown into glucose and fructose sugars but the stored sugars too. The increase in ethylene in the apple is the cause of this then being broken down into sucrose or fructose, the sugars giving the sweet taste on eating; or the substrate needed for the yeasts to make cider from the apple juice. Pears retain more sorbitol which is poorly absorbed by the small intestine and may produce an osmotic diarrhoea. Perry from some varieties such have high amounts . Holmer does hence its nickname Startle Cock! Apple juice contains sorbitol in the range of 2.5-7 g/l whereas pear juice has higher amounts up to 10-25 g/l. Sorbitol is about half as sweet as fructose but it is not metabolised by yeasts so imparts a sweeter taste to the resultant Perry. Whilst there is no explanation I know of why perry pear fuirt has higher sorbitol content than cider apples it is not due to lack of SAH but its activation which is less under normal conditions.
Holmer pear
Approaching maturity ethylene increases for the 2-3 weeks prior to natural abscission and fruit fall but then increases rapidly post detachment from the tree from natural fruit fall and also human picking just prior to maturity. However ethylene may be partly a surrogate maker for a more fundamental process in altered metabolism governed by gene expression especially starch breakdown as some cultivars do not show this association with ethylene.
Ethylene production also affects the organic acids present. Mainly reducing them, Its reduces some esters but the phenolics needs for a good tasting cider are independent.
Ethylene stimulates cell wall hydrolase genes to produce polygalacturonase which causes softening of the fruit and loss of firmness. Not a characteristic humans appreciate but presumably it is an evolutionary adaptation favoured by bears and other large animals the natural eaters of apples in the apple homeland of the mountains of Central Asia, especially Kazakhstan.
With this is an increase in intercellular spaces which accumulate CO2 from the cellular respiration hence apples float. A different process to pears which sink.
Colour development at ripening is not related to ethylene but abscisic acid or ABA which promotes anthocycins and decreases chlorophyll.
So cider needs fruit at its most mature which is beyond when humans would like to bite on a crunchy apple. At TeePee Cider here in New Zealand we delay collecting the fruit until about 25% has fallen. Tumping does not work as our temperatures in autumn are higher than in South West England. The fruit ferments rather than matures!
Perhaps a lot o biochemistry but enjoy the cider
