Harvest
Starch–iodine testing
Staining a cut apple with iodine solution so that remaining starch turns blue-black, giving a visible map of how far conversion to sugar has progressed.
Also called Iodine staining test, Starch conversion index, Starch pattern index.
- Stage
- Harvest
- Traditional in
- No single tradition — used wherever it suits
- What it most changes
- Not recorded as moving a sensory dimension
- Safety
- None recorded
What it is
A bench test in which a fruit is halved across the equator and the cut face flooded with an iodine solution. Wherever starch remains, the surface stains a dense blue-black; wherever starch has already been hydrolysed to sugar, the flesh stays pale. The result is not a number but a pattern, and the pattern is read against a chart or simply described — fully stained, clearing at the core, a ring of stain under the skin, or clear throughout. It costs seconds, needs no instrument, and is the only routine field method that looks directly at the change ripeness assessment is really tracking.
Described in full
- Shape
- Four labelled curves on a common horizontal axis running from earlier to later on the tree. There is no numeric scale on either axis. Starch begins near the top and falls to almost nothing. Sugar begins low and rises, flattening towards the end. Acid begins high and falls steadily. Firmness begins high and falls, more steeply late.
- Starch
- Falls to zero as the fruit converts stored starch to sugar. This is what the starch–iodine test reads, and it is the most used field indicator of harvest maturity because the change is large, ordered and visible with a cut fruit and a bottle of iodine.
- Sugar
- Rises as starch converts and as the fruit continues to import sugar from the leaves, then flattens. Sugar sets potential alcohol, so a maker pressing early accepts a weaker cider.
- Acid
- Falls throughout, because malic acid is respired by the fruit itself. Waiting for more sugar therefore costs acid, and in low-acid bittersweet fruit that is a microbiological cost as well as a sensory one.
- Firmness
- Falls as the cell walls break down, slowly at first and faster late. It governs how the fruit mills: firm fruit grates cleanly, soft fruit smears and presses badly.
- Why there is no scale
- The directions here are general across cider fruit. The rates are not — they belong to a cultivar, a season and a site, and an axis with numbers on it would invite a reader to read a picking date off a drawing. What the figure shows is the shape of the trade-off, not its timing.
- The point of the figure
- The four curves do not cross a useful threshold at the same moment. Pressing when the starch has gone means pressing before the sugar has finished rising; waiting for maximum sugar means accepting lower acid and softer fruit. There is no moment at which all four are optimal, so harvest maturity is a choice about which curve the maker is prioritising.
Why it is used
- It separates starch conversion from every other signal, which a refractometer cannot do because starch is insoluble and never reaches the reading.
- It gives a spatial answer rather than an average, so a grower can see that the core has converted while the outer cortex has not — information a single sugar figure destroys.
- It is repeatable across a block by cutting several fruit from several trees, which makes it useful for deciding whether a block is uniform enough to pick in one pass.
- For fruit that will be held in store rather than milled at once, it shows how much conversion is still available to happen during the hold.
How it works
- Iodine dissolved with potassium iodide exists largely as the triiodide ion, which threads into the helical coil of amylose and forms a charge-transfer complex absorbing strongly across the visible spectrum — the blue-black seen on the cut face.
- Amylopectin, the branched starch fraction, gives a weaker reddish-brown colour, so a face that has gone from black to dull brown is further advanced than one still inky.
- Starch hydrolysis in a maturing apple begins in the tissue around the core and vascular bundles and proceeds outwards, so the stain characteristically retreats from the centre and survives longest as a band beneath the skin.
- Because the reaction is with starch and not with sugar, the test says nothing about how much sugar is present — only that whatever starch has gone has gone somewhere.
The chemistry and the organisms
What is actually being changed, and by what. Each entry says what that compound or organism does in cider generally; this page is one place it does it.
Compounds involved
Starch
The fruit’s carbohydrate store, unfermentable until ripening converts it to sugar, and the reason picking early costs alcohol and risks a haze that will not clear.
Glucose
The sugar yeast takes first and the smallest of the three in apple juice, which is why the last sugar left in a slow ferment is almost never glucose.
Fructose
The dominant sugar of apple juice and the sweetest of the common sugars, which is why apple juice tastes sweeter than its total sugar figure implies and why a stuck ferment leaves sweetness behind.
Sucrose
The disaccharide of apple juice and the sugar most often added to it, split into glucose and fructose by the yeast’s own invertase before any of it is fermented.
What it is done with
Drawing a sample without spoiling the batch
Every sample is a small hole made in the protection around a batch: something goes in, air goes in with it, and cider comes out — so the technique is about drawing a representative sample while putting nothing back and letting in as little air as possible.
Laboratory measurement versus practical measurement
Gravity, temperature, pH and total acidity are all within reach of a careful person with modest equipment; alcohol by volume, sulphur dioxide for a declaration, patulin, methanol and microbiological counts are not, and pretending otherwise is how numbers on labels become wrong.
What can go wrong
Faults that begin at this step, or that this step makes more likely. Each one is set out with its causes and whether it can be put right.
More on starch–iodine testing
The published index charts that make the test quantitative were developed for dessert and culinary apples destined for controlled-atmosphere storage, where the commercial question is how long the fruit will hold. Cider fruit asks a different question and often sits off the end of the scale: many late bittersweets are gathered well after the point at which the chart would call them fully converted, so the index saturates and stops discriminating exactly where the cider maker needs it to. Tannin-rich flesh compounds the problem, because polyphenol oxidase browns the cut face within moments of exposure and the browning can be mistaken for, or can mask, weak residual staining. Reading the face promptly after cutting, and comparing several fruit rather than one, are the practical answers.
Perry pears are a poorer fit again. Pear flesh carries stone cells and a different balance of stored carbohydrate, sorbitol is a much larger share of the soluble carbohydrate than in apples, and the fruit’s ripening runs from the inside out fast enough that a pear can be starch-clear at the core and already breaking down there while the outer flesh is still firm. Growers in the perry counties tend to treat the test as one input among several rather than the deciding one, and rely more on the fruit parting from the spur and on the characteristic short window between hard and sleepy. The same caution applies wherever a maker transfers a dessert-fruit protocol wholesale onto cider fruit.
The realistic failure is over-reading the result. A face that stains black tells you starch is present; it does not tell you the fruit is unripe in any sense that matters if the crop is going to sit on a barn floor for weeks, because that hold will convert much of what remains. Conversely, a clear face on fruit picked from a tree that has been dropping heavily for a fortnight is not a green light — it is consistent with fruit that is past its point and heading for softening. The test earns its place as a cheap corroborating signal for a decision made on several grounds, and loses it as soon as it becomes the only thing anyone looks at.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
Harvest
Ripeness assessment
Judging when cider fruit has converted enough starch, loosened enough on the spur and hardened enough for the store to be worth gathering.
Harvest
Sweating the fruit
Deliberately heaping or storing gathered fruit so that it softens, loses water and finishes converting starch before it goes to the mill.
Harvest
Fruit storage before milling
Holding a gathered crop between harvest and the mill, and managing what respiration, water loss and spoilage do to it while it waits.
Fruit preparation
Milling
Reducing whole fruit to a pulp so that the press has cell walls it can drain, rather than intact apples it can only bruise.
Juice treatment
Juice clarification
The deliberate use of enzyme, fining agents or mechanical separation to produce a bright juice before fermentation, and what that costs the ferment.
What people ask next
Questions readers ask about the things this page mentions. Each one goes to the section that answers it rather than to a page written to receive the question.
- How are apples milled for cider
- When are cider apples harvested — In the northern hemisphere the cider harvest runs from about September to November, with late bittersweets such as Dabinett and Vilberie coming in last. Cider fruit is generally picked riper than dessert fruit, and often gathered from the ground.
- Which sugar is most of the sugar in apple juice — Fructose, by a long way. Apple juice carries roughly twice as much fructose as glucose, with a smaller amount of sucrose, and fructose is the sweetest of the three on the tongue.
- How is apple juice clarified before fermentation
- How long can apples be kept before pressing
- What does sweating apples mean — Sweating is holding picked fruit in a heap or store for days or weeks before milling, so it softens, loses water and gains sugar concentration. Perry pears in particular are usually held until they yield to the thumb.
Where to go next
- How cider is made — The whole sequence, stage by stage, with the choices open at each one.
- Cider science — The chemistry and microbiology the methods on this page rest on.
- Troubleshooting — What goes wrong, how to recognise it, and whether it can be reversed.
Sources
What this page rests on. Where a source is marked as registered rather than read, CiderHQ is recording that the body is authoritative on the subject without claiming to have worked through the document itself. See our evidence policy for what each state means.
The Science of Cidermaking and associated technical writing
Andrew Lea · reference work · passage verified 2026-08-24
Written by a food chemist who worked at Long Ashton on apple phenolics. Unusual among specialist cider writing in that it is primary-research-adjacent: the author is describing work he did, and cites the literature. This is why it is registered at tier 1 for chemistry while a general cider book is not.
Cornell Cider Research and Extension programme
Cornell University, School of Integrative Plant Science · university · passage verified 2026-08-24
Runs cultivar trials in New York State and publishes juice chemistry for European cider varieties grown in a North American climate — the single most useful counterweight to treating English figures as universal.
The New Cider Maker’s Handbook: A Comprehensive Guide for Craft Producers
Claude Jolicoeur, Chelsea Green Publishing, 2013. ISBN 9781603584739 · reference work · bibliographic record verified, not opened 2026-08-25
Bibliographic record verified on 2026-08-25 against the Open Library union catalogue: Chelsea Green Publishing, 2013, ISBN 9781603584739, one edition recorded. That establishes the citation points at a real book in a stated edition, which is what a citation needs and is all it establishes. No copy was opened and nothing is quoted from it. The book itself is in print and not digitised in any open collection; where CiderHQ needs a figure from this territory it uses an accessible research source instead and says so.