Economic Effects of Storing Rubber Cup Lumps in Ponds or Soaking with Water

Introduction

The practice of storing rubber cup lumps (field coagulum) in water — whether in ponds, troughs, or by spraying — is a common intermediate storage method used by smallholder rubber farmers in major producing countries such as Thailand, Indonesia, Malaysia, and Vietnam. This "wet storage" or "maturation" period is often unavoidable due to logistical constraints, as farmers accumulate cup lumps before transporting them to processing factories [1,2]. The economic consequences of this practice are multifaceted, involving trade-offs between quality preservation, processing costs, moisture content, and ultimately the price farmers receive for their raw material.

Quality Effects of Water Storage on Cup Lump Rubber

The quality of cup lump rubber is primarily assessed by two key parameters: Initial Plasticity (P₀) and the Plasticity Retention Index (PRI), which indicate the rubber's resistance to thermo-oxidative degradation during processing [1]. Research has shown that the storage conditions of cup lump significantly affect these quality parameters through microbial activity.

A detailed study by Thongchai et al. [1] examined four alternative storage conditions for cup lump natural rubber: storage in closed bags, outdoor exposure, storage under shade, and storage under shade with water spraying. The study found that the best dry rubber properties — evaluated by P₀ and PRI — were obtained from cup lump stored under shade with spraying water after formic acid coagulation. This condition produced the highest PRI values, indicating superior resistance to oxidative breakdown. Conversely, storage in closed bags resulted in the poorest quality, with the lowest PRI values and the highest microbial populations (1.69 × 10¹⁰ CFU/ml at 3–7 days of maturation) [1].

The mechanism behind this quality difference lies in microbial activity. During maturation, the pH of cup lump initially decreases due to lactic acid production by anaerobic bacteria such as Lactobacillus and Streptococcus species, then gradually increases as these microorganisms decline [1]. The water-spraying condition appears to suppress harmful microbial proliferation while allowing beneficial maturation reactions to occur.

Moisture Content and Its Economic Implications

The price paid to farmers for cup lump rubber depends principally on its moisture content (MC) [2]. In rubber yard markets, traders classify cup lumps into classes based on their MC: "Class A" (dry, with MC ≤ 35%) fetches a higher price, while "Class B" (wet, with MC > 35%) commands a lower price [3]. However, the current practice of estimating MC relies on subjective visual and tactile inspection by human examiners, which is notoriously unreliable and causes disputes between farmers and buyers [2,3].

When rubber cup lumps are stored in ponds or soaked in water, the moisture content increases significantly. This has direct economic consequences because:

Weight-based pricing distortion: Since cup lumps are sold by weight, higher moisture content means farmers are effectively selling water rather than rubber, reducing their effective income per unit of dry rubber content [2].
Price penalties: Buyers apply price discounts for wet cup lumps, as they require additional processing — specifically, more energy-intensive drying — to reach the required final moisture content below 5% [4,6].
Grading downgrades: Cup lumps with excessive MC are classified as lower grades, reducing the selling price [3].

Suchat et al. [2] documented that the initial MC of freshly coagulated cup lump is about 48%, decreasing to 35% after one day of air drying, and to 18.5% after three days. Soaking in water would reverse this drying process, keeping MC high and thus reducing the farmer's net economic return.

Processing Cost Implications

The initial moisture content of raw cup lump has a major impact on downstream processing costs in STR20 (Standard Thai Rubber 20) production. Tirawanichakul and Tirawanichakul [4] demonstrated that the drying of crumb rubber for STR20 production is highly energy-intensive, requiring specific energy consumption that varies significantly with initial moisture content. Higher initial moisture content leads to:

Increased energy consumption: More thermal energy is required to evaporate excess water, raising the cost of drying [4].
Longer drying times: The drying process is extended, reducing factory throughput [4].
Potential quality degradation: High initial moisture content combined with prolonged drying can negatively affect rubber quality, particularly the molecular weight and crosslink density of the final product [6].

Research by Li et al. [6] showed that natural rubber with higher initial moisture content (19.76%) exhibited lower molecular weight (Mₙ and M_w) after microwave drying compared to rubber with lower initial moisture content (3.80%). This degradation in molecular structure can affect the final product quality and potentially reduce its market value.

Furthermore, the dehydration process itself has economic implications. A study on a single-screw dehydrator for wet natural rubber [7] demonstrated that while mechanical dehydration can reduce moisture content to below 20%, the water consumption of conventional processing (three open crepers and a hammer mill) is substantial, and the associated wastewater treatment adds significant cost. The study found that the single-screw dehydrator used only 20% of the water of conventional technology, highlighting the cost burden that excess moisture places on the processing chain [7].

The "Maturation" Trade-off: Economic Benefits of Controlled Water Storage

Paradoxically, the study by Thongchai et al. [1] found that controlled water storage (under shade with spraying water) actually improved certain quality parameters of cup lump rubber. Specifically:

PRI values were highest under water-spraying conditions (up to 70 for acid-coagulated samples at 21 days of maturation), compared to storage in closed bags (PRI ~30–40) [1].
P₀ values were more stable when cup lump was stored with water spraying, maintaining values in the range of 49–51 for acid-coagulated samples throughout 28 days of maturation [1].
The best thermal stability and smallest microorganism population were found in acid-coagulated cup lump stored under water-spraying conditions [1].

This suggests that a controlled, clean water-spraying system under shade — not stagnant pond water — can actually improve the quality of cup lump rubber and potentially yield higher prices. However, this is distinct from the common practice of storing cup lumps in stagnant ponds or natural water bodies, where water quality is poor and microbial contamination is high.

Risks of Pond Storage (Stagnant Water)

Storing cup lumps in ponds or untreated water bodies introduces several economic risks:

Microbial contamination: Stagnant pond water harbors diverse microbial populations, including bacteria and fungi that can degrade natural rubber. The study by Thongchai et al. [1] found that closed-bag storage (which also creates anaerobic, moist conditions) produced the highest microbial counts and poorest quality. Pond storage likely creates similar conditions, promoting the growth of anaerobic microorganisms that produce organic acids and enzymes that damage the rubber polymer structure [1].
Contamination with dirt and debris: Pond water typically contains suspended solids, sediment, and organic matter that become incorporated into the cup lump surface, increasing the impurity content. This can lead to the rubber not meeting the stringent impurity standards required for STR20 grade (maximum 0.05% residual impurity content at 45 µm screen) [7].
Uneven moisture distribution: Soaking in water creates a moisture gradient within the cup lump, with the exterior being much wetter than the interior. This makes accurate moisture assessment difficult and increases the risk of disputes during pricing [2,3].
Environmental concerns: The practice of storing rubber in ponds may lead to water pollution, as rubber latex serum containing formic acid and other coagulants leaches into the water body. This could result in regulatory penalties or community conflicts that impose additional economic costs.

Smallholder Economics and Marketing Implications

The economic impact of storage practices must be understood within the broader context of smallholder rubber marketing. Chambon et al. [8] found that in Thailand, the type of rubber product farmers sell (fresh latex vs. rubber sheets vs. cup lump) is influenced by factors including age, education, experience, and access to farmer organizations. The study by Chambon et al. [8] also revealed that farmers who sell fresh latex (rather than processed rubber sheets) are more dependent on immediate pricing at the point of sale, making them more vulnerable to price penalties for low-quality or high-moisture product.

In the case of cup lump, farmers who store their product in ponds or soak it in water risk:

Lower effective prices due to higher moisture content
Disputes and contention with buyers over subjective quality assessments [2,3]
Reduced bargaining power as the product is more perishable and must be sold quickly before quality deteriorates further [8]

The risk assessment of rubber production in Quang Binh Province, Vietnam, by Tran [5] showed that rubber smallholders face significant economic risks from price fluctuations, with latex price being the most critical variable affecting net present value (NPV) of rubber farming. When quality is compromised by poor storage practices, farmers are even more vulnerable to price declines because their product is less competitive in the market.

Comparative Analysis of Storage Methods

Based on the literature reviewed, the economic outcomes of different storage methods can be summarized as follows:

Storage Method	Quality Impact	Processing Cost Impact	Price Impact
Stagnant pond water	Low PRI, high microbial count, contamination risk	High (excess water, need for cleaning)	Price penalty for wet, low-quality product
Spraying with clean water under shade	High PRI, good thermal stability, controlled maturation	Low (controlled moisture)	Premium for consistent quality
Closed bag storage (no water)	Lowest PRI, highest microbial growth	Moderate	Discount for poor quality
Air drying (shade, no water)	Moderate quality	Low	Standard market price

Recommendations for Economically Optimal Storage

Based on the research findings, the following practices are economically advantageous:

Acid coagulation: Using formic acid for coagulation (rather than natural coagulation) produces cup lump with better thermal stability and smaller microorganism populations, resulting in higher quality and better prices [1].
Controlled water spraying under shade: Rather than soaking in ponds, cup lump should be stored under shade with periodic clean water spraying. This maintains optimal moisture levels for beneficial maturation while preventing excessive wetness [1].
Avoid stagnant water: Storage in ponds or stagnant water bodies should be avoided as it promotes microbial degradation, introduces contaminants, and increases moisture content without the benefits of controlled maturation [1].
Timely processing: The optimal maturation period for cup lump is approximately 21 days under controlled conditions. Longer storage increases the risk of quality degradation [1].
Moisture monitoring: Rapid, non-destructive moisture determination techniques (such as NIR spectroscopy or microwave reflectometry) could help farmers and traders accurately assess the dry rubber content, ensuring fair pricing [2,3].

Conclusion

The economic effect of storing rubber cup lumps in ponds or soaking with water is predominantly negative. While controlled water spraying under shade can improve rubber quality parameters such as PRI and P₀, storage in stagnant pond water promotes microbial degradation, increases moisture content, introduces contaminants, and ultimately leads to lower prices for farmers. The increased moisture content also raises processing costs for factories, reducing the overall value of the raw material. The best economic outcome for smallholder rubber farmers is achieved through acid coagulation followed by storage under shade with controlled clean water spraying, combined with timely processing within 21 days of maturation.

References

[1]Thongchai, N., Pa Mek, K., & Bonfils, F. (2013). Maturation of Cup Lump Natural Rubber: Growth of Microorganisms and Effects on Quality Properties under Alternative Storage Conditions. Advanced Materials Research, 844, 395–398
DOI: 10.4028/www.scientific.net/amr.844.395 [2]Suchat, S., Rittiron, R., & Seehalak, W. (2015). Rapid moisture determination for cup lump natural rubber by near infrared spectroscopy. Industrial Crops and Products, 76, 772–780
DOI: 10.1016/j.indcrop.2015.07.070 [3]Somwong, P., & Chongcheawchamnan, M. (2020). A microwave reflectometer technique for classifying a rubber cup lump. Computers and Electronics in Agriculture, 169, 105152
DOI: 10.1016/j.compag.2019.105152 [4]Tirawanichakul, S., & Tirawanichakul, Y. (2008). Mathematical Model of Fixed-Bed Drying and Strategies for Crumb Rubber Producing STR20. Drying Technology, 26(11), 1388–1400
DOI: 10.1080/07373930802333569 [5]Tran, L. (2020). Risk assessment of the economic efficiency of rubber production: Case of smallholder rubber production in QuangBinh Province, Vietnam. African Journal of Agricultural Research, 16(4), 565–578
DOI: 10.5897/ajar2020.14825 [6]Li, P., et al. (2013). Study on Molecular Weight of Natural Rubber with Different Initial Moisture Contents Dried by Microwave. Applied Mechanics and Materials, 457–458, 988–991
DOI: 10.4028/www.scientific.net/amm.457-458.988 [7]Li, P., et al. (2011). Study on Dehydration Process of Wet Natural Rubber by Single Screw Dehydrator and Properties of Dry Natural Rubber. Advanced Materials Research, 418–420, 544–547
DOI: 10.4028/www.scientific.net/amr.418-420.544 [8]Chambon, B., et al. (2017). Factors influencing rubber marketing by smallholder farmers in Thailand. Development in Practice, 27(6), 865–879
DOI: 10.1080/09614524.2017.1340930

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