Engineered Jute Bags for Storing Food Grains within Harrington's Safe Zone of Seed Moisture Content
Harrington's thumb rule states that for every 1% increase in seed moisture content (SMC), seed lifetime decreases by ≤50%. Thus, to avoid post-harvest grain losses, stored seeds must be insulated from water. Although the jute bags typically used to store grains afford an ecofriendly, durable, and low-cost storage solution, their hydrophilic nature makes them vulnerable to wetting from airborne humidity and precipitation events. To address this issue, we treated jute fabrics with alkali and wax to render them water-repellent. A 2-month seed storage experiment revealed that wax-coated jute bags (WCJBs) outperformed control jute bags (CJBs) at safeguarding wheat (Triticum aestivum) grains exposed to 55%-98% relative humidity. Specifically, grains stored in WCJBs exhibited 15%-50% less SMC than grains stored in CJBs, which led to a 21%-66% enhancement in the germination efficacy of WCJB-stored seeds. This engineering solution could enhance the potential of jute bags to reduce post-harvest losses.
INTRODUCTION<!>Materials and apparatus.<!>Wettability tests. Deionized water droplets with and without food coloring and the DSA100E<!>Seed storage experiments.<!>RESULTS AND DISCUSSION
<p>In 2020, the Nobel Peace Prize was awarded to the World Food Programme; this is a testimony to the complete dependence of humans on our food supply 1 . According to the United Nation's Food and Agricultural Organization, developing sustainable agricultural practices that can feed ~10 10 humans in 2050 is the mega-challenge of our time [2][3][4] . To meet the increasing demand for food, annual global food production must increase by around 70% 5 . Grain crops, such as cereals, oil seeds, and pulses, form the basis of global food security [6][7][8] , with two billion tons of grains currently produced annually 7,9 . By 2050, global grain production is projected to increase by >33% to meet food and animal feed needs 3,10 . Increasing crop yields beyond current levels is likely to be difficult given the effects of climate change, deforestation, rising groundwater depletion, and overstressed energy resources [11][12][13][14] . This emerging food security scenario will demand a highly efficient food supply chain from farms to consumption 8 . At present, ~25%-33% of the total grain produced annually is lost between harvest and consumption through "post-harvest losses" (PHLs) 6,15 , which entail quantitative and qualitative food losses within the interconnected post-harvest chain from harvest through to storage, processing, marketing, and finally consumption [16][17][18] . Alarmingly, in developing countries, up to 50%-60% of cereals may be lost during storage due to poor storage infrastructure [16][17][18] , slower adoption of modern technologies 19 , and fragmented information sharing 20 . Conversely, in developed countries, grain losses can be as low as 1%-2% because of better post-harvest handling and improved hermetic storage technologies 9,20 . Nevertheless, total food waste at the consumer level in developed countries is >40% (~222 million tons); thus, the magnitude of food losses in these countries is comparable to that in developing countries and similar to the net food production of sub-Saharan Africa (~230 million tons) 6 . PHLs can be in form of spoilage, seed viability loss, grain depletion, and nutritional loss 21 resulting from biotic (e.g., insects, rodents, and fungi) or abiotic (e.g., temperature and air-moisture/humidity) factors 9 . Fungal infection and mycotoxin contamination are major causes of PHL during seed/grain/cereal storage, accounting for 25%-40% of losses 22 .</p><p>In the context of seed deterioration, Harrington's thumb rule states that for every 1% increase in seed moisture content (SMC), the storage life of the seed is reduced by half [23][24] . This empirical rule underscores the importance of insulating stored grains from water in liquid and vapor forms. Typically, seeds are dried to achieve a SMC of ~13% before storage [15][16] ; when SMC reaches ~16%, Harrington's rule becomes relevant and seeds begin to deteriorate in an accelerated manner 23 . Therefore, to avoid increased SMC, it is important to carefully consider storage structures, packaging materials, storage duration, and relative humidity (RH). 15 Issues arise when moisture-absorbing storage bags facilitate the diffusion of moisture to dry grains and thereby increase SMC. In developing countries, including those in the Indian subcontinent and sub-Saharan Africa, farmers exploit jute bags for grain storage because of their low-cost, mechanical durability, and traditional usage over millenia 9,19,25 . However, jute is hydrophilic and therefore absorbs water due to its chemical components, which include cellulose (~60%), hemicellulose (~20), lignin (~10%), pectin, and water-soluble substances [26][27][28] . Consequently, jute bags are vulnerable to environmental water, e.g., precipitation, RH, and soil moisture, which interacts with conventional storage systems. Indeed, water accumulation and/or vapor transport through jute bags increases SMC and causes fungal infections; the resultant seed deterioration affects seed germination capacity [29][30][31] . In India, about 60%-70% of grains are stored in traditional structures in which 36.0% and 33.5% of total PHLs are encountered for rice and wheat, respectively 32 . In sub-Saharan Africa, PHLs of 59% are estimated for maize grains after a 90-day storage period in traditional storage structures (granary/polypropylene bags) 33 . Thus, lowcost surface modifications that create water-repellant jute bags are required to reduce PHLs during storage.</p><p>To address this problem, we took inspiration from the numerous plants and animals that have developed water-repellency by combining hydrophobic waxy coatings with micro/nanoscale surface roughness. Examples include lotus leaves (Nelumbo nucifera) 34 , the bodies of sea-skaters (Halobates germanus) [35][36] , the cuticles of springtails (Collembola) 37 , as well as certain desert plants 38 and insects 39 among others 40 . Researchers have already developed bioinspired technologies for drag reduction 41 , separation and purification [42][43] , cavitation mitigation 44 , coating-free entrapment of air underwater 37,42,45 , and producing super-waterrepellent cotton 46 . Thus, there is a growing expectation for new agrotechnologies to be inspired by nature [47][48] .</p><p>Herein, we present a bioinspired surface treatment for jute bags that enhances their robustness for improved grain storage. Specifically, jute fabrics were treated with alkalis and then coated with paraffin wax to produce water repellence. The seed storage efficacy of these wax-coated jute bags (WCJBs) was compared with that of control jute bags (CJBs, i.e., untreated jute bags) under controlled humidity conditions during a 2-month storage experiment. This was followed by a germination study in which the viability of seeds stored in WCJBs and CJBs was comparably assessed. The low cost, eco-friendly, and simple treatment process reported here has the potential to substantially reduce PHLs, particularly in developing countries currently reliant on regular jute bags for grain storage.</p><!><p>Paraffin wax w a s o b t a i n e d f rom common white candles manufactured by IKEA (Sweden); hexane and ethanol w e r e purchased from VWR International S.A.S ( Fontenay-Sous-Bois, France); ammonium acetate, glacial acetic acid, and calcium nitrate tetrahydrate (Ca(NO 3 ) 2 .4H 2 O) were purchased from Sigma Aldrich (St. Louis, MO, USA); sodium chloride, potassium sulfate, and sodium hydroxide were purchased from Fisher Scientific (New Jersey, USA). All chemicals were used as received from suppliers. Jute packaging sacks (Indian Sela Basmati Rice, 50 kg) were obtained from the Al Jawahir Company (Saudi Arabia). A Quanta 600 scanning electron microscope system was used for scanning electron microscopy. A Quorum Q150 TS sputter coater was used to coat jute bags (CJBs and WCJBs) with a 3 nm-thick platinum layer to avoid electrical charging during electron microscopy. A DSA100E contact angle goniometer (Kruss, Germany) was used to characterize the wettability of surfaces. Wheat grains (T. aestivum L.) were harvested from a field at King Abdulaziz University's Agricultural Research Station in Hada Al-Sham (which extends between 39°30′ and 40°15′ East and 21°45′ and 22°10′ North, in Makkah, Saudi Arabia).</p><p>Surface treatment of jute. Jute sacks were cut into 14 × 7 cm pieces and hand-sewn to produce jute storage bags. First, the bags were cleaned by immersion in water (10 min) and then 70% ethanol (20 min), before being dried in a thermal convection oven at 80°C (30 min). Cleaned bags were then immersed in 0.5-M ammonium acetate solution (adjusted to pH 5 using 1.0% glacial acetic acid) for 30 min. After further oven-drying at 80°C (30 min), the bags were immersed in 5% (m/v) NaOH solution (1.25 M and pH ~14) for 2 h. These alkali-treated bags were washed with deionized water and again oven-dried at 80°C (30 min). Subsequently, the bags were immersed in solutions of paraffin wax dissolved in hexane (6 g/L), during which they were turned upside-down several times to facilitate uniform coating. After 30 min of this treatment, the bags were left to dry overnight in a solvent hood at 21.6°C ± 0.5°C.</p><!><p>contact angle goniometer were used to characterize the wettability of bag surfaces. WCJBs and CJBs were also exposed to simulated rainfall for ~1 min: they were placed at an angle of ~30° and the simulated raindrops were applied by releasing water droplets (~2 mL) from a syringe placed ~5 cm above the bags. The moisture absorption of WCJBs and CJBs was estimated by measuring their mass.</p><!><p>Clean and healthy T. aestivum grains, devoid of any physical damage, were selected and placed inside paper bags for initial weight measurement. These grains were then dried in an oven at 80°C for 72 h, after which their mass did not change further; thus, SMC in the initial state was calculated as follows: 𝑆𝑀𝐶 = (𝑊 ! − 𝑊 ! )/𝑊 ! , where 𝑊 ! is the initial weight of grains and 𝑊 ! is their final weight after drying. Subsequently, wheat grains were packaged in WCJBs and CJBs, and the bags were placed in airtight plastic containers with the following RHs: 55%, 75%, and 98%. The air humidity level was controlled in confined spaces by equilibrating it with supersaturated salt solutions of Ca(NO 3 ) 2 .4H 2 O, NaCl, and K 2 SO 4 [49][50] .</p><p>Specifically, to obtain 55% RH, 50 g of Ca(NO 3 ) 2 .4H 2 O was dissolved in 100 mL of deionized water by stirring; more salt was added until a saturated solution was formed with a salt:water ratio of 3:1 (w/w). This procedure was repeated to obtain 75% RH using NaCl (but with a 2:1 salt:water ratio) and 98% RH using K 2 SO 4 (at a ~3:1 salt:water ratio). The saturated salt solutions were then poured into the lower compartment of the airtight plastic containers, which were integrated with humidity and temperature sensors (Fig. 1). The bags containing wheat grains were then placed on a plastic mesh tray (barrier) above the saturated salt solutions; therefore, they were exposed to air with well-controlled humidity. Experiments were carried out in triplicate; therefore, nine containers with WCJBs or CJBs were randomly placed and monitored for 2 months. Moisture absorption was determined by weighing each jute bag every 3 days. Total moisture absorbed was calculated in grams per gram of jute bag plus stored seed combined.</p><!><p>Water repellency. Alkali treatment of cellulosic fibers, also known as mercerization, has been exploited to tune various characteristics, such as elastic moduli, brittleness, surface roughness, and readiness for surface coating, of jute and cotton 27, 46, 52-54 . In the current study, the alkali treatment (Fig. 2) of jute enhanced the surface roughness of fibers, presumably due to the dissolution of the hemicellulose content (Fig. 3). Previously, alkali treatment has been reported to dissolve hemicellulose and other fiber components, thereby rearranging them, which results in the swelling and constriction of pores in jute fabrics 27, 55-57 .  We chose an alkali concentration of 5% and treatment duration of 2 h because they enhance the surface roughness of fibers without significantly depleting their mechanical strength 27,55 . For instance, Wang et al. 58 demonstrated that mechanical properties, such as tensile strength, flexural strength, tensile modulus, and flexural modulus, were superior for 6% NaOHtreated jute fabric reinforced composites relative to the mechanical properties of jute fabrics treated at higher alkali concentrations (>6%). High alkali concentrations not only remove lignin and hemicellulose but also change the native cellulose I to cellulose II 27,59 . Such concentrations can also cause excessive peeling of the fiber surface, which reduces the strength and toughness of fibers 58 . In our further analyses, as the alkali-treated fabrics were coated with paraffin wax, the combination of surface roughness and the hydrophobicity of the wax produced robust water repellence 43,60 . The intrinsic hydrophobicity of the paraffin wax is characterized by the apparent contact angles of water droplets on smooth wax-coated surfaces: 𝜃 ! ≈ 105°. When a rough surface is coated with a hydrophobic coating, the resulting water-repellence is enhanced by the entrapment of air inside the surface asperities, which also prevent the penetration of water into the microtexture. After wax coating, the gaps between the WCJB fibers reduced in comparison to the gaps between the CJB fibers (Fig. 3A-C).</p><p>When water droplets of ~8 µL were placed onto CJBs, they gradually penetrated into the fibers in ≤ 2 min (Figure 4A, C, and E). In contrast, this volume of water droplets placed onto WCJBs did not penetrate into the fibers due to the entrapment of air in the microtexture; the drops exhibited apparent contact angles of 𝜃 ! ≈ 130 ° and remained in their sessile position (pinned and 𝜃 ! decreasing) until they evaporated (Figure 4B, D, and F). These air-trapped wetting states are described by the Cassie-Baxter model 61 , which connects the macroscopic apparent contact angles on the rough surface, 𝜃 ! , with 𝜃 ! and the area fractions of liquid-solid (𝜙 !" ) and liquid-vapor (𝜙 !" ) fractions underneath the drop as cos 𝜃 ! = 𝜙 !" × cos 𝜃 ! − 𝜙 !" . The WCJBs and CJBs were also exposed to simulated rainfall. The CJBs were wetted by the impact of water droplets, which they accumulated (Movie 1); however, the simulated raindrops simply rolled-off the WCJBs, thereby preventing water accumulation (Movie 2). In comparison to the CJBs, therefore, the WCJBs showed the potential for superior protection of stored grains against humid air.</p><p>Seed storage experiments. We investigated the potential of WCJBs to enhance the shelf-life and viability of wheat grains (T. aestivum) stored at 50%-98% RH for 2 months by comparing their 13 performance with that of CJBs (Fig. 1). The SMC of the wheat prior to storage was 7.1%, which was well within Harrington's safe zone (6%-16%) 24 . Wheat grains were stored in WCJBs and CJBs (as controls) at 21.6°C ± 0.5°C under three different air humidity conditions: 55%, 75%, and 98% RH. During the 2-month-long storage experiment, we examined the moisture absorption and mold growth on jute bags and stored grains every 3 days (Fig. 5). For the 55% RH treatment, there were no significant differences in moisture absorption between the WCJBs and CJBs and the grains stored within these bags (Fig. 5A and D). Consequently, at the end of the storage period, the grains had not suffered from fungal growth. Similarly, for the 75% RH treatment, the WCJBs and their stored grains did not exhibit moisture absorption or mold growth; however, the CJBs and their stored grains showed dampness and fungal growth (Fig. 5B   and E). For the 98% RH treatment, both the CJBs and WCJBs and their stored grains exhibited dampness and fungal growth (Fig. 5C and F). Nevertheless, although the water droplets condensed and grew on the WCJBs, they rolled off along the surface because of bags' water repellence (Fig. 5F, inset); consequently, fungal proliferation on wheat grains stored in WCJBs at 98% RH was less prominent than that on wheat grains stored in CJBs. The amount of moisture absorbed by the bags containing stored grains and the grains' SMCs were also quantified at the end of the storage period (Figure 6). At 55% RH, the total moisture absorbed through the WCJBs was 46% lower than that absorbed through the CJBs (Fig. 6A); the SMC of grains stored in the WCJBs increased from 7.1% to 7.5%, which remained well within Harrington's safe zone, whereas the SMC of grains in the CJBs increased to 14.9% (Figure 6B). CJBs absorbed twelve times the moisture absorbed by WCJBs; thus, they acted as reservoirs for moisture influx into stored grains. At 75% RH, the total moisture absorbed through the WCJBs was 32% lower than that absorbed through the CJBs; the SMC of grains stored in WCJBs increased from 7.1% to 11%, whereas the SMC of grains increased to 17% in the CJBs, i.e., the latter SMC violated Harrington's rule leading to fungal infestation. At an RH of 98%, the total moisture absorbed through the WCJBs was 45% lower than that absorbed through the CJBs;</p><p>the SMC of grains stored in WCJBs increased to 26%, whereas the SMC of grains in CJBs increased to 32%. Thus, at 95% RH, both storage media failed to safeguard grains.</p><p>Notwithstanding, significant differences were found between WCJBs and CJBs for both total moisture absorbed and SMC (p < 0.05) at all RHs. Seed viability after storage. The germination capacity of seeds, or seed viability, is affected by increasing SMC and fungal molds 29 . For example, the germination capacity of soybean (Glycine max), sorghum (Sorghum bicolor), red clover (Trifolium pratense), and Timothy grass (Phleum pratense) seeds reduces substantially during 6-12 months of storage at 30°C and 75% RH 29 . To investigate the full potential of WCJBs as a reliable seed storage medium, we studied the germination of the stored grains using the following formula:</p><p>100%. Overall, we found that germination was significantly higher for wheat grains stored in WCJBs than for wheat grains stored in CJBs (p < 0.05). Specifically, at 55%, 75%, and 98% RH, storage in WCJBs resulted in 32%, 35%, and 12% improved germination capacity, respectively (Fig. 7).  momentum and mass roll off the enhanced bags; with fewer sessile drops on a porous surface, absorption and mass transfer are also reduced. Secondly, we noticed that the fibers of WCJBs were somewhat swollen in contrast to those of CJBs (Fig. 4). This likely led to partial pore clogging of the treated-jute layer and reduced the flux of water vapor through the pores. In addition, enhanced surface roughness due to the removal of surface impurities and other fiber components by alkali could have increased the contact area between the jute fibers and wax matrix 60 . Consequently, paraffin wax seeping through pores from both surfaces could eventually clog some pores. This hypothesis is consistent with the findings of Ghosh et al. 62 , who studied the durability of jute geotextiles in soil; they noticed that impregnation of nonwoven jute samples with bitumen reduced jute pore sizes because of the formation of a superimposing bitumen binder layer on the surface of both sides of the jute fabric. Additional studies on the efficacy of this approach, in which different seed types, storage durations, and temperatures are applied, as well as on the exposure of WCJBs to running water (time-dependent changes in wetting) 60 , are required to explore strengths and weaknesses of this approach. The mechanical strength of WCJBs also needs to be compared against CJBs to ensure that they will not start physically deteriorating after loading. In summary, WCJBs have the potential to alleviate PHLs due to seed deterioration in regions with a humid climate, e.g., the Indian subcontinent and sub-Saharan Africa. Farmers in these regions extensively exploit conventional jute bags to store grains; these bags are stacked in traditional storage structures or outdoor stockpiles covered with tarpaulin 9,19 . Thus, the stored grains are often exposed to high humidity, especially in the event of precipitation or increased RH, which elevates SMC, accelerates the deterioration of seeds, and reduces germination capacity. In the present study, wheat grains stored in WCJBs for over 2 months experienced small deviations in SMC, i.e., 7.1%-7.5% at 55% RH and 7.1%-11% at 75% RH that were well within Harrington's limits. In contrast, wheat grains stored in CJBs over the same duration experienced much higher SMC, i.e., 7.1%-14.9% at 55% RH and 7.1%-17% at 75% RH.</p><p>Consequently, WCJBs improved the germination capacity of stored grains by 32%-35% in comparison with the germination capacity of grains stored in CJBs under identical conditions.</p><p>Therefore, WCJBs have the potential to substantially extend the shelf-life of seeds under low RH conditions and they facilitate increased resilience under higher RH conditions. Importantly, grains stored in WCJBs remain within Harrington's safe zone for SMC; thus, PHLs due to seed deterioration are prevented. The low cost, eco-friendliness, and availability of jute fabrics, alkalis, and paraffin wax, along with the simplicity of the treatment process, gives wax-coated jute fabrics great potential for use in agro-technologies and other applications.</p><p>Abbreviations used: PHL: post-harvest losses; CJBs: control jute bags; WCJBs: wax-coated jute bags; SMC: seed moisture content; RH: relative humidity.</p>