Kiwifruit physiology: why Brix misleads and dry matter does not

Savvas Pastopoulos, Agronomist MSc — Pastopoulos Agronomics

Every October the same scene repeats itself in the kiwifruit orchards of northern Greece: the refractometer reads 6.2 and the grower decides he can pick. The international literature of the past ten years says something very different — and in fact says that the commonest cause of poor shelf quality is not late but early harvest. This article explains what actually happens inside the fruit from bloom to the cold store, why Brix misleads, and which numbers are worth measuring.

1. Maturation and ripening: two words, two different things

Greek uses one word where the international literature uses two, and half the mistakes start from that confusion.

2. The anatomy of the fruit and why it matters in measurement

The four tissue zones of the fruit. The distinction is not academic: firmness is measured in the outer pericarp after the skin has been removed, the inner pericarp is the first to soften, and the core has the highest dry matter of all the zones.

The kiwifruit is a berry derived from a multicarpellate, superior ovary. It consists of four distinct tissue zones:

Around the axis is a ring of locules — elongated slit-shaped chambers running almost the whole length of the fruit and holding the seeds.

3. The timetable of growth

Fruit growth follows a double sigmoid curve. Hopping described three phases:

The three growth stages. Note how large a part of the season the first stage occupies: within those first eight weeks the cell number is locked in, and with it the maximum possible capacity of the fruit. In the diagram the boundaries are rounded; strictly, the second stage begins on day 59 and the third on day 77.

Note the boundaries: stage III begins on day 77, not day 76. In many Greek texts 76 appears both as the end of stage II and as the start of stage III, which is plainly impossible.

A later reading simplifies this to two phases: cell division for roughly the first six weeks, then cell expansion with parallel starch storage until harvest.

4. The calcium window closes in July

Calcium travels exclusively in the xylem stream, which is driven by the transpiration of the fruit itself. In kiwifruit, fruit transpiration falls sharply after about 60 days from full bloom, as the pedicel xylem degrades, the skin hairs die and surface conductance falls.

The statement that «all the calcium goes in by day 50 from fruit set» is often met. It is nearly right but too absolute on two counts: the milestone is measured from full bloom, not from fruit set, and it is not the whole quantity but the overwhelming majority of the final amount.

5. Starch: the fruit's bank

In most fruits sweetness comes from sugars arriving through the phloem. In kiwifruit something more complex and more interesting applies.

During development, incoming sugars are converted into starch and stored in the plastids of the parenchyma. Starch amounts to about 5–7% of fresh weight, that is, 40–50% of dry matter at harvest. It is, literally, the fruit's bank account.

Close to maturity, net starch synthesis stops. Later, during ripening, net breakdown begins through α- and β-amylases, and the products — glucose, fructose, sucrose — give the sweetness of the eating-ripe fruit.

6. Why Brix rises before harvest — and why cold is not the reason

Here lies the most important conceptual correction of recent years, and it has direct consequences for Greece.

The old explanation

The 6.2 °Brix threshold was established in New Zealand in the early 1980s and adopted by almost every producing country. The rationale was that in New Zealand cool autumn weather triggered early starch breakdown, so Brix rose before harvest and served as an indicator of maturity.

The newer reading

The distinction explains why the 6.2 °Brix threshold was transferred uncritically. In most Mediterranean regions the autumn is distinctly warmer than in New Zealand, so the original assumption simply does not hold. And confirmation came from the findings themselves: the rise in Brix proceeds almost linearly in the field, by about 3.6 units from 136 to 198 days after full bloom, with no need for a cold stimulus [1].

Sources: 1

7. Dry matter: the only index that predicts flavour

If harvest Brix does not tell how sweet the fruit will become, what does?

Dry matter. It includes both the soluble fraction (sugars) and the insoluble one (chiefly starch and structural carbohydrates). Because almost all the starch will eventually be converted into sugars, harvest dry matter predicts the final Brix of the eating-ripe fruit.

Non-soluble dry matter

Even more diagnostic is the NSDM index — the percentage of dry matter that is not soluble:

NSDM (%) = (Dry matter − SSC) × 100 / Dry matter

How the two numbers are measured

The formula is of no use without knowing how dry matter and soluble solids are obtained. The protocol follows, as applied in the published work [1].

Sample: at least 30 uniform fruit per replicate. Measurements are made after the fruit has equilibrated to room temperature, not straight from the fridge.

In the field, NSDM starts to fall at around 160 days from full bloom and drops steadily, from about 66% to below 50% by late November. It is the mirror image of soluble solids, and shows clearly that starch has stopped accumulating [1].

Sources: 1

8. Ethylene: climacteric or not?

Kiwifruit is traditionally classed as a climacteric fruit. The reality is more complex and worth knowing, because it explains the behaviour of the store.

The data that create the problem

• Most of ripening happens without ethylene. The fruit softens, starch breaks down and sugars rise while ethylene production remains undetectable.
• Autocatalytic ethylene production appears at the end, when the fruit is already very soft — typically at around 1 kgf or less. That is, ethylene does not start ripening; it completes it.
• The fruit can soften from exposure to low temperature alone, with no ethylene involved.

The practical conclusion

Whatever the classification, one thing is certain: kiwifruit is extremely sensitive to exogenous ethylene. Minute concentrations are enough to raise respiration and collapse firmness. Therefore:

• No damaged or infected fruit goes into the store.
• Fruit from vines with esca, from plants under water stress or from affected blocks ripens prematurely and drags the healthy fruit with it.
• It is not stored together with ethylene-producing fruit.

In trials on Hayward, fruit treated with propylene at 20 °C immediately after harvest produced ethylene with a lag of three days and ripened in 3–5 days, while the controls did not ripen at all within the nine days of the experiment [2].

Sources: 2

9. The four phases of softening

Softening is not one continuous process but four distinct stages. The three-phase description that circulates is incomplete: it leaves out the over-ripening phase.

Four phases under the current description [1]. The transition from phase 1 to phase 2 is the real starting point of ripening.

Fruit is considered eating-ripe when firmness falls below 25 N [1]. For comparison: at harvest firmness is typically 56–65 N, and after six months in controlled atmosphere it falls to 11–16 N [1].

Sources: 1

10. The trial that overturns «pick early»

This section deserves attention, because its conclusion is the opposite of what most growers do.

In a commercial 15-year-old Hayward orchard, ripening was monitored weekly from 136 days after full bloom. Fruit was harvested at three times — 152, 178 and 192 DAFB — and stored at −0.5 °C for 3–6 months in normal atmosphere or 6–10 months in controlled atmosphere, with or without 1-MCP [1].

The researchers summarise the commercial rule as «Last In, Last Out» (LILO): what goes into the store last comes out last [1].

Sources: 1

11. The contrary evidence — and how the two reconcile

It would be dishonest to present only one side. There is also research showing that very late harvest is damaging too.

In a trial on the Black Sea coast, Hayward fruit was harvested at four soluble solids levels and stored at 0 ± 1 °C at 85–90% relative humidity for six months [3]:

In the same work, the beneficial constituents increased continuously the later the harvest — ascorbic acid, total phenolics, antioxidant capacity [3]. That is, nutritional value and storage life do not peak at the same point.

Sources: 1 3

12. Water: over-irrigation costs firmness

This section is, in my view, the most underrated in Greek practice.

In a trial with four irrigation levels — 100%, 68%, 57% and 40% of evapotranspiration (ETc) — on Actinidia chinensis var. chinensis, with sap flow sensors and fruit diameter gauges, the results were striking [4]:

Different letters indicate a statistically significant difference [4].

The VPD threshold

The same work identified something connected directly to canopy and microclimate management: there is a vapour pressure deficit (VPD) threshold at about 1.8 to 2.0 kPa, above which the stomata begin to close regardless of water availability [4].

Below 2 kPa, well-irrigated vines keep their stomata open. Above it, watering does not help — the plant closes down.

The picture is not universal: recent work in southern Italy found no restriction of stomatal conductance even at VPD up to 4 kPa, possibly because of different vine adaptation [4].

The daily dynamics are also interesting: sap flow continues to rise after the stomata have begun to close, which indicates refilling of the storage tissues — and the fruit refills at night, when competition for water is lower [4].

Sources: 4

13. Calcium as a tool: Greek data

Since calcium no longer enters the fruit after July, the question is whether foliar applications make sense. There is a two-year Greek trial on Hayward that gives an answer [5].

The optimum rate of that particular product was determined at 2–4 kg per tonne/hectare, and the authors link the higher calcium concentration with better postharvest retention [5].

Sources: 5

14. Cooling, controlled atmosphere and 1-MCP

The store does not improve the fruit. It can only slow the loss.

Temperature

Stability matters more than the absolute value. Small fluctuations around 0 °C disturb the softening pattern. An important and counter-intuitive finding: fruit softened faster at 4 and 8 °C than at 16 °C during the first three weeks of storage — probably because at those temperatures the fruit is more sensitive to ethylene.

The statement «from 4 to 12 °C the softening rate stays constant, while at 5 °C the fruit softens faster» is sometimes met. The two sentences cancel each other out: if the rate is constant over 4–12 °C, 5 °C cannot stand out.

Controlled atmosphere

In trials on Hayward at 0 °C for 60, 120 and 180 days, three systems were compared [2]:

Soluble solids increased sharply in the first 60 days and then stayed almost constant in all treatments [2].

1-MCP, an inhibitor of ethylene receptors, reduces the softening rate and extends storage life, particularly combined with CA [1].

Sources: 1 2

15. What the law says

The marketing standard for kiwifruit is set out in Commission Implementing Regulation (EU) No 543/2011. The maturity requirements are [6]:

Two observations for the grower:

• 6.2 °Brix is a legal minimum, not a quality target. The law sets the floor below which the product may not be marketed; it does not guarantee good fruit.
• The Regulation's 15% dry matter is far from the >17% the research proposes for fruit that will spend months in storage [1]. For long storage and export, the legislation is not enough.

Sources: 1 6

16. The new non-destructive measurements

The basic problem with the classical measurements is that they destroy the fruit: a sample of a few dozen fruit is measured and generalised to tens of tonnes.

Hyperspectral imaging allows dry matter to be predicted before harvest, without cutting the fruit. In recent work, using nine characteristic wavelength bands from 538.93 to 856.80 nm and partial least squares regression, R² = 0.92 with an error of 0.41% was achieved on the training set and R² = 0.85 with an error of 0.50% on the test set [7].

Sources: 7

17. Climate is changing the rules

Kiwifruit is among the crops under most pressure from climate change, and the mechanisms bear directly on fruit physiology [8].

Among the adaptation measures recorded are shade netting, to reduce sunburn and create a favourable microclimate, and covering systems, which also reduce VPD late in the season [4][8].

Sources: 4 8

18. A harvest decision protocol

19. What to take away

The quality of Greek kiwifruit is not decided on the day of harvest. It is decided in spring, when cell number is set; in summer, when how much water to give and how much dry matter will accumulate are settled; and by the discipline of autumn, when the temptation to pick early because the refractometer read 6.2 has to be resisted.

20. Sources

The numbers in the text refer to the list below.

• [1] Goldberg, T., Agra, H. and Ben-Arie, R. (2021). Quality of ‘Hayward’ kiwifruit in prolonged cold storage as affected by the stage of maturity at harvest. Horticulturae 7(10):358. Source for the linear increase of ~3.6 SSC units from 136 to 198 DAFB, for the fall in NSDM from ~66% to <50%, for the softening rates of 0.1 and 0.5 N per day with an inflection at 175 DAFB, for the criteria DM>17%, SSC>7%, TA 2.0–2.6% and NSDM>40%, for the finding that the 152 DAFB fruit came out the softest, for the LILO rule and the improvement from delaying to ~200 DAFB, for the four softening phases, for the eating-ripe limit below 25 N and for the values of 56–65 N at harvest and 11–16 N after six months in controlled atmosphere.
• [2] Antunes, M.D.C. and Sfakiotakis, E.M. (2002). Ethylene biosynthesis and ripening behaviour of ‘Hayward’ kiwifruit subjected to some controlled atmospheres. Postharvest Biology and Technology 26(2):167–179. Source for the comparison of normal air, controlled atmosphere at 2% O₂ + 5% CO₂ and ultra-low oxygen at 0 °C for 60, 120 and 180 days, for the three-day lag and ripening in 3–5 days after propylene, for the faster softening in normal air during the first 60 days, and for the inability of fruit from ultra-low oxygen to ripen fully.
• [3] Bakoğlu, N. and Tuna Güneş, N. (2024). Impact of harvest time on cold storage performance in kiwifruit. Journal of Food Composition and Analysis. Source for the four harvest dates at SSC of 5–6%, 6.1–7.0%, 7.1–8.0% and 8.1–9.0%, for the six-month storage at 0±1 °C at 85–90% relative humidity, for the finding that both early and late harvest gave faster loss of firmness and weight, for the optimum window of 6.1–8.0%, and for the continuous increase in ascorbic acid, phenolics and antioxidant capacity with delayed harvest.
• [4] Fernandes, R.D.M. et al. (2025). Kiwifruit ecophysiological adaptations under moderate and severe deficit irrigation. Scientia Horticulturae 347:114193. Source for the four irrigation treatments at 100%, 68%, 57% and 40% of ETc, for the quality table with the greatest diameter, firmness and acidity at 68% ETc, for the lowest dry matter under full irrigation, for the VPD threshold of 1.8–2.0 kPa above which the stomata close, for the absence of restriction up to 4 kPa in other work in southern Italy, and for the night-time refilling of the fruit.
• [5] Sotiropoulos, T. et al. (2023). Effect of preharvest foliar calcium sprays on quality properties and nutritional status of kiwi cv. ‘Hayward’. Notulae Botanicae Horti Agrobotanici Cluj-Napoca 51(2):13109. Source for the increase in flesh firmness, dry matter, soluble solids and acidity with organic calcium, for the increase in leaf N, P, K and B, for the increase in fruit N, B, K and Ca and the decrease in Mg, for the absence of effect on Mn, Zn, Fe and Cu, and for the optimum rate.
• [6] Commission Implementing Regulation (EU) No 543/2011, Annex I, specific marketing standard for kiwifruit. EUR-Lex. Source for the minimum maturity requirement at packing: 6.2 °Brix or an average dry matter of 15%, so as to yield 9.5 °Brix on entering the distribution chain.
• [7] Yang, H. et al. (2024). Determination of dry-matter content of kiwifruit before harvest based on hyperspectral imaging. AgriEngineering 6(1):52–63. Source for the prediction of pre-harvest dry matter using nine characteristic bands from 538.93 to 856.80 nm and partial least squares regression, with R²=0.92 and an error of 0.41% on the training set and R²=0.85 with an error of 0.50% on the test set.
• [8] Rajan, P. et al. (2024). Climate change impacts on and response strategies for kiwifruit production: a comprehensive review. Plants 13(17):2354. Source for the risk of insufficient winter chill, for spring frosts, for the inhibition of respiration above 44.5 °C, for increased pressure from Psa, and for adaptation measures such as shade netting and covering systems.