Removing astringency from Rojo Brillante persimmon

Savvas Pastopoulos, Agronomist MSc — Pastopoulos Agronomic P.C., Neos Mylotopos, Pella, Greece

Rojo Brillante persimmon is harvested firm and intensely astringent, unfit for consumption. Astringency does not disappear with ripening in cold storage, nor with time: it disappears only when enough acetaldehyde accumulates inside the fruit to bind the soluble tannins. What follows explains the mechanism, gives the carbon dioxide and ethanol protocols as they are applied in Spain, and shows what goes wrong when the chamber is not gas-tight, when the batch is not uniform, and when the treatment runs longer than it needs to.

1. What astringency is — and why it is not a taste

Astringency does not belong to the five basic tastes. It is a tactile sensation. The American Society for Testing and Materials defines it as the complex of sensations due to shrinking and drying of the epithelium as a result of exposure to substances such as tannic acids [1].

In persimmon the mechanism is specific. Soluble tannins accumulate in the vacuoles of specialised cells, the tannin cells. In astringent fruit these vacuoles contain 10 to 12% tannins, while the surrounding parenchyma cells contain only traces and are rich in soluble sugars [1].

When the fruit is chewed, the tannin cells rupture and release their contents. The tannins meet salivary proteins and form insoluble protein–tannin complexes. Saliva loses its lubricating property and the surfaces of the mouth dry out. That is astringency.

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2. Why Rojo Brillante stays astringent while Jiro does not

Persimmon cultivars fall into four groups, on two criteria: whether they are astringent at harvest, and whether their behaviour depends on pollination [1] [3].

The difference comes down to the seeds. In PVNA cultivars the seeds release substantial amounts of acetaldehyde, so the fruit de-astringes by itself on the tree. In PVA cultivars the seeds produce a limited amount, which is why astringency disappears only locally, around the seed. In PCA cultivars production is practically nil [1].

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3. The mechanism: acetaldehyde and tannin insolubilisation

Persimmon tannins are high molecular weight group B proanthocyanidins. Their structure was clarified by Matsuo and Itoo in 1978: the monomers forming the polymer are catechin, catechin-3-gallate, gallocatechin and gallocatechin-3-gallate, with C4–C8 bonds predominating [1]. In Rojo Brillante specifically, the prevailing units are epigallocatechin and epigallocatechin gallate [1].

The reaction

In the presence of acetaldehyde, soluble proanthocyanidins polymerise and become insoluble. In vitro trials showed that at pH 6 to 8 tannins react with acetaldehyde within a relatively short time and form a gel [1]. Inside the fruit, acetaldehyde acts as a bridge linking two proanthocyanidin molecules. The resulting complex is no longer soluble, does not bind salivary proteins and is therefore not perceived.

Where the acetaldehyde comes from

Two routes, both exploitable:

• Decarboxylation of pyruvic acid. When the fruit is placed under anaerobic conditions, its respiration turns fermentative. The keys are two enzymes: pyruvate decarboxylase and alcohol dehydrogenase [1].
• Oxidation of ethanol, endogenous or exogenous. This is why the ethanol vapour method also works.

Every de-astringency method — CO₂, nitrogen, ethanol, hot water, vinegar — arrives at the same point: it induces anaerobic respiration and acetaldehyde accumulation [1] [5].

What was actually measured

In an experiment with three cultivars in a 99% CO₂ atmosphere at 20 °C [4]:

Values from Vidrih and co-workers [4]. The cultivars were Kaki Tipo, Lycopersicon and Thiene — not Rojo Brillante — but the mechanism is the same.

Two things in this table deserve attention. First, acetaldehyde increases a hundredfold within the first 20 hours and then levels off: the fuel of the reaction is produced early. Second, the tannins do not fall alongside it — they keep falling afterwards, because polymerisation takes time.

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4. How much tannin the fruit carries and where the threshold lies

Soluble tannin content differs dramatically between cultivars, but also within the same cultivar according to ripening stage.

Values collated from the IVIA literature review [1].

The threshold is not fixed — and that is what matters

For years it was held that concentrations below 1% of fresh weight do not produce a sensation of astringency. Earlier work placed the edibility threshold at 0.1% [4].

In practice this means the laboratory figure does not replace a taste test on a sample from every batch before the consignment leaves.

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5. The de-astringency methods compared

Six families of methods have been tested. All lead to acetaldehyde accumulation; they differ in time, cost, safety and — above all — in what is left of fruit firmness.

Conditions from sources [1] [2] [6] [7] [8]. The values are indicative and are adjusted to the cultivar and the condition of the fruit.

Effectiveness depends substantially on the cultivar. In some cultivars CO₂ outperforms nitrogen, in others the reverse holds [1]. Today exposure to high CO₂ concentrations is the method the industry uses.

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6. The carbon dioxide protocol

The fruit is placed in a sealed chamber. A flow of CO₂ displaces the air. Sensors monitor the concentration and a solenoid valve makes periodic injections so that the level is maintained throughout [1] [7].

The published conditions

The values are indicative and do not constitute a recommendation for application. Calibration is done per cultivar, per batch and per installation, with taste evaluation of the result. Sources [1] [4] [5] [7].

The convergence is striking: twenty-four hours, at 20 °C, with the chamber nearly pure CO₂. The IVIA recommendation emerged from trials at different ripening stages and was found effective at all of them [1].

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7. The factors that determine success

Concentration — and uniformity

Concentrations of 95–100% secure the highest level of effectiveness. What is critical, however, is not only the number on the sensor: it is homogeneous distribution of the gas inside the chamber. The formation of air pockets affects the process adversely, and concentrations below these levels leave fruit with residual astringency [1].

Temperature

Low temperatures slow down astringency removal. A comparison of application at 12 °C against 20 °C in Rojo Brillante showed clearly more effective de-astringency at the higher temperature [1]. The recommended temperature for the cultivar is 20 °C.

Conversely, when harvest takes place on cold days, the fruit must be acclimatised in the packhouse so that flesh temperature is not excessively low [1] [8].

Duration

Duration is the most easily adjusted parameter, and for that reason it tends to be extended in order to offset shortcomings in the control of temperature or concentration. That is reasonable — but it has a limit.

Ripening stage

Early and mid-season fruit, with high firmness, loses astringency more easily than fruit at advanced ripening stages at the end of the season [1]. The riper the fruit, the more treatment hours are required [3].

The practical conclusion is one: the chamber is filled with fruit of similar physiological condition. A non-uniform batch means that some fruit will be over-exposed while other fruit comes out astringent.

What has gone before

Fruit kept for prolonged periods at 15 °C de-astringes with more difficulty than freshly harvested fruit. The cause is structural: the flesh loses cohesion and CO₂ diffusion is impeded [1]. If storage takes place at low temperature and the fruit retains high firmness, the problem does not appear — provided the fruit is acclimatised before entering the chamber.

Even cultural operations count: treatments to advance ripening or uncontrolled water stress in the orchard can undermine the effectiveness of the treatment [1].

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8. The chamber: what it must have

The installation is not complex, but every element of it exists for a specific reason [7] [8]:

• Gas tightness. Without it, gas consumption soars and the concentration is not held.
• A CO₂ sensor connected to a control panel and a solenoid valve, so that injections are made automatically.
• A gas distribution network inside the chamber, for uniform distribution.
• Internal recirculation fans, so that no dead zones remain.
• An extraction fan for ventilating the chamber before anyone enters.
• Heating, so that fruit temperature rises when it enters cold.
• A pressure relief valve.
• An impermeable floor sloping towards a drain that can be sealed.

Supply comes from cylinders or cylinder banks at about 57 bar, or from a cryogenic tank for larger consumption [7].

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9. Safety — CO₂ kills silently

This section is not procedural. A de-astringency chamber is, literally, a space without oxygen.

In practice:

• The chamber is fully ventilated with the extraction fan and with the door open before anyone enters [8].
• During treatment, entry is avoided, even for inspection [8].
• A fixed CO₂ detector with an audible alarm in the chamber area and in adjacent spaces, in case of leakage.
• No work alone. Always a second person outside the space.
• Signage on the door and locking while the programme runs.

The commercial literature itself explicitly acknowledges the greater hazard of CO₂ chambers compared with other methods, both inside the chamber and in the surrounding spaces in the event of a leak [8].

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10. When residual astringency remains

It is the most frequent complaint and it has four usual causes, all of them controllable:

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11. Internal browning from over-exposure

It is the most serious commercial damage the treatment itself can cause, and it appears even without mechanical injury.

The picture is characteristic: browning around the core of the fruit, mainly in the zone near the calyx [3]. De-astringency itself constitutes oxidative stress for the fruit. When it is excessively prolonged, that stress manifests as browning — and is aggravated if the fruit is then stored at low temperature.

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12. Browning or pink spot — what mechanical damage reveals

This is perhaps the most useful diagnostic tool in the whole text, because the appearance of the damage shows when the fruit was bruised.

Distinction and mechanism from the IVIA work on Rojo Brillante [3]. The same behaviour was confirmed in most astringent cultivars, with different intensity per cultivar; non-astringent PCNA cultivars show low susceptibility.

The reasoning behind the recommendation is biochemical. CO₂ treatment leads to an imbalance in the redox state of the fruit. If the oxidative stress of an impact on the line is added, the insoluble tannins oxidise and take on colour [3].

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13. After de-astringency: storage and chilling injury

Here Rojo Brillante has a peculiarity that costs money: it does not tolerate chilling.

Values from the synthesis of IVIA work [2]. 1-methylcyclopropene is an inhibitor of ethylene action; its availability and approval status must always be checked against the national legislation in force.

The symptoms of chilling injury

• Drastic loss of firmness — and the insidious part is that it is not visible while the fruit is in the cold room. It manifests after transfer to marketing temperature, usually within two or three days [1] [2].
• Hard, rubbery texture in some cases, instead of softening.
• Internal browning in the central region, with the formation of compact nodules in the upper part of the fruit.
• Calyx drop.

It is further noted that the increase in acetaldehyde and ethanol after treatment is greater when the fruit is stored at 1 °C than at 15 °C, which is interpreted as one more manifestation of chilling injury [2].

Postharvest fruit management is covered more generally in the article Postharvest nutrition in stone fruit.

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14. The ethanol method and the combined system

The ability of alcohol to remove astringency from persimmon has been known in Japan for more than a hundred years [8].

The Japanese carton method

The fruit is placed in cartons of 5 to 15 kg, sprayed with an aqueous solution of 30–40% ethanol, sealed and stored for 10 to 14 days at 10–15 °C. The result is excellent — firm fruit with a gelatinous texture, sweet flavour and good orange colour — but commercially unviable, precisely because of the time involved [6].

The Spanish industrial version

The method developed for Rojo Brillante is based on holding the fruit in an atmosphere with a suitable ethanol concentration, high humidity and a temperature above 20 °C, for 36 to 42 hours depending on the condition of the fruit [8].

• Product consumption: 5 to 10 litres per tonne of fruit, depending on the tightness of the chamber and the stacking.
• Food-grade ethanol is used, in a ratio calculated so that any risk of ignition is avoided.
• The equipment comprises a network of dispersion nozzles, a dosing pump and a programmer.

The combined CO₂ and ethanol system

It was developed precisely to combine the advantages of the two methods: shorter treatment time and greater effectiveness, which together substantially reduce the likelihood of internal browning. Its advantages are particularly significant at the end of the season, when de-astringency becomes difficult [8].

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15. A low-cost solution for small quantities

For a grower without a chamber, there is a documented method tested at research level that does not require a large investment [5].

The fruit is placed in polyethylene bags 0.2 mm thick and heat-sealed. The bag is pierced at one corner, CO₂ is introduced from the opposite corner through a tube from a cylinder, the air is displaced by successive flushes until the desired concentration is reached, and the bag is resealed.

What the trials showed

• A 24-hour exposure to extreme CO₂ concentrations is enough to start the process, which completes within the following three days [5].
• Lower concentrations, around 70%, need more time but can give better flavour.
• Exposure to CO₂ maintained or even temporarily increased firmness. In Triumph at 95% CO₂, firmness after one, two and three days was 6.4, 6.7 and 9.6 kg/cm², against 2.6 kg/cm² in untreated fruit.
• At three storage temperatures (23, 16 and 4 °C) astringency had gone in all cultivars by day 7. Lower temperatures slowed the process down, but also slowed softening.

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16. How to check whether astringency has gone

Methods from sources [4] [5] and from current practice. The ferric chloride test is indicative and does not replace taste evaluation.

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17. The Greek context

In Greece two persimmon cultivars are grown mainly, with completely different postharvest management: Rojo Brillante, which requires treatment before consumption, and Jiro, which goes straight to consumption after harvest [10].

This difference is not a varietal detail. It is a difference of investment model. Jiro needs nothing. Rojo Brillante needs a chamber, gas, sensors, safety measures and know-how — and in exchange gives a product with longer storage life and export potential.

For the picture of the crop and the cultivar, the blog already carries the articles The Rojo Brilliante persimmon cultivar, Persimmon, the crop and Pruning persimmon in the Rojo Brilliante cultivar.

18. Sources

The numbers in the text refer to the list below.

• [1] Besada, C. and Salvador, A. Tecnología poscosecha del caqui. Chapter 12, Instituto Valenciano de Investigaciones Agrarias (IVIA), 2015. Main source for the PCNA/PCA/PVA/PVNA classification, the structure of the tannins as group B proanthocyanidins, the role of pyruvate decarboxylase and alcohol dehydrogenase, the 95–100% CO₂ / 24 hours / 20 °C protocol, the comparison of 12 against 20 °C, the 0.06% tannin threshold and the two phases of insolubilisation.
• [2] Salvador, A., Arnal, L., Navarro, P. and Martínez-Jávega, J.M. (2006). Estado actual en la conservación de caqui 'Rojo Brillante'. Revista Iberoamericana de Tecnología Postcosecha 7(2):86–92. Source for chilling injury below 11 °C, the storage limits (20 days at 15 °C, less than 15 days at 1 °C, up to 40 days with 1-MCP), the 85% share of marketed persimmon sold as "kaki persimon" and the greater sensitivity of early-season fruit.
• [3] Besada, C., Novillo, P., Gil, R., Navarro, P. and Salvador, A. (2017). Causas de pardeamiento en caqui. Phytoma España 286, VI International Symposium on Persimmon. Source for the distinction between browning and pink spot according to the moment of impact, for browning around the core due to CO₂ over-exposure, for the recommendation to apply the treatment after the packing line and for the need for uniform batches.
• [4] Vidrih, R., Simcic, M., Hribar, J. and Plestenjak, A. (1994). Astringency removal by high CO₂ treatment in persimmon fruit (Diospyros kaki). Acta Horticulturae 368 (Postharvest 93):652–655. Biotechnical Faculty, University of Ljubljana. Source for the course of acetaldehyde (0.02 → 2 mg/100 g) and ethanol (3 → 20 → 60 mg/100 g), for the fall of soluble tannins to 0.02% at 72 hours, for the case of Kaki Tipo requiring 72 instead of 20 hours, and for the tannin determination method at 550 nm.
• [5] Hladnik, J. (2020). Removing astringency in persimmon with CO₂ and the effect of prolonged softening process. IX International Symposium on Agricultural Sciences AgroReS 2020, Proceedings, pp. 78–85. Agricultural Institute of Slovenia. Source for the polyethylene bag method, for the sufficiency of a 24-hour exposure with completion within three days, for the firmness values in Triumph (6.4 / 6.7 / 9.6 against 2.6 kg/cm²), for the drop in soluble solids and for the "fermented flavour" after 14–21 days.
• [6] Kato, K. (1990). Astringency Removal and Ripening in Persimmons Treated with Ethanol and Ethylene. HortScience 25(2):205–207. Source for the Japanese carton method (spraying with 30–40% ethanol, sealing, 10–14 days at 10–15 °C) and for the finding that CO₂ delays fruit ripening.
• [7] Abelló Linde S.A. BIOGON® C — CO₂ para la desastringencia del caqui. Technical application leaflet. Source for the commercial protocol of 90–95% CO₂ / 20–24 hours / 20–24 °C / about 90% RH and for the layout of the installation (relief valve, CO₂ sensor, control panel, solenoid valve, distribution network, cylinders at 57 bar or cryogenic tank).
• [8] CIA. IBERICA BROGDEX S.A. (FOMESA group). Equipo para Eliminación de la Astringencia del KAKI. Commercial technical leaflet. Source for the ethanol vapour method (36–42 hours, above 20 °C, 5–10 litres per tonne), for the natural alcohol content of the fruit (150 mg/100 ml), for the combined CO₂–ethanol system, for the disadvantages of CO₂ (browning, cost, hazard), for the disappearance of residual astringency within two days and for the chamber specifications.
• [9] ELINYAE — Occupational Exposure Limit Values, under Presidential Decree 90/1999 (Greece). Source for the 5,000 ppm exposure limit value for carbon dioxide.
• [10] AgroTypos (28/11/2023). Persimmon: how the new plantings are made, first Producer Group founded in Pella (in Greek). Source for the two dominant cultivars in Greece (Rojo Brillante and Jiro) and for the first persimmon Producer Group in Pella targeting 25 hectares.
• [11] Database of approved plant protection products and biocides, plantprotect.gr, with data from the official database of the Greek Ministry of Rural Development and Food. Source for checking the approved uses on persimmon.