Effect of drying methods on the physicochemical properties and adhesion performance of water-washed cottonseed meal

Industrial Crops & Products 109 (2017) 281–287

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Effect of drying methods on the physicochemical properties and adhesion performance of water-washed cottonseed meal

MARK

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Ningbo Lia, Sarocha Prodyawonga, Zhongqi Heb, , Xiuzhi S. Sunc, Donghai Wanga a b c

Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA Southern Regional Research Center, USDA Agricultural Research Service, 1100 Robert E. Lee Blvd., New Orleans, LA 70124, USA Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Cottonseed Biobased adhesive Freeze drying Oven drying Spray drying Seed meal

Water-washed cottonseed meal (WCSM) showed the potential for being used as environment-friendly wood adhesives. However, the influence of WCSM preparation process on its adhesion performance is not well known. This work studied the effect of different drying methods on the several key physicochemical features and adhesion performance of WCSM. Defatted cottonseed meal was washed with 8 folds of water for 3 cycles to remove the water-soluble components and dried with oven, freeze dryer, and spray dryer, respectively. Whereas the major chemical composition was unchanged, oven-dried WCSM showed protein degrading denaturation per sodium dodecyl sulfate polyacrylamide gel electrophoresis and differential scanning calorimetry data. With hot press temperature at 100 °C, oven-dried WCSM showed poor adhesion performance when compared with its freeze- and spray-dried counterparts. However, the difference among the products with the three drying methods became smaller, and even none with the press temperature at 150 and 170 °C. The adhesion performance could be further improved by pH 4.5 adjustment and removal of large residual hull particles. This study proved spraydrying and freeze-drying more suitable to make high quality cottonseed meal-based adhesives for a variety of operation conditions. On the other hand, the more economic oven-drying may be applied to make WCSM product for bonding at higher press temperature (e.g. 170 °C) without undermining WCSM’s adhesion performance.

1. Introduction In the past three decades, concerns related to the environment, human health risks, and interests in resource recycling and sustainability have propelled the resurgence of research on bio-based adhesives (He, 2017). These advances would pave the paths for enhanced utilization of natural products and byproducts for global sustainability and a greener environment. Typical vegetable sources of seed proteins used in wood adhesives studies include but are not limited to soy (Luo et al., 2015; Qi et al., 2017), wheat (Khosravi et al., 2015), peanut (Li et al., 2015a), canola (or rapeseed) (Li et al., 2017; Yang et al., 2014) and cottonseed (Cheng et al., 2013, 2016). Protein isolates are generally prepared from seed meals or flours by alkaline extraction and acidic precipitation (He et al., 2013; Wang et al., 2009) so that they are more expensive than seed meals (Cheng and He, 2017). Thus, seed meals have also been widely investigated for wood bonding (Lorenz et al., 2015; Shi et al., 2017). In addition to protein, seed meals contain significant amounts of carbohydrates which are believed to be the major contributor of the poor water resistance of meal-based adhesives

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Corresponding author. E-mail addresses: [email protected], [email protected] (Z. He).

http://dx.doi.org/10.1016/j.indcrop.2017.08.035 Received 2 February 2017; Accepted 18 August 2017 0926-6690/ Published by Elsevier B.V.

(D’Amico et al., 2010; Lorenz et al., 2015). Thus, water washing have been developed to remove soluble carbohydrate components from seed meal for the purpose of improving the water resistance of meal-based adhesives (He et al., 2016a, 2014c). Previous data (He et al., 2016b, 2014a,b) have shown that the adhesion strength and water resistance of water-washed cottonseed meal (WCSM) are comparable to those of cottonseed protein isolate while water washing is more cost-efficient and environment-friendly than protein isolation. Thus, to promote the industrial application of the more promising WCSM as wood adhesives, He et al. (2016c) scaled up the production of WCSM from laboratory 10-g levels to the pilot production level with 10-lb (454 g) starting material. Wet samples and products are frequently dried for stability and easiness in analysis, disposition and storage. Frequently used drying processes include freeze-drying, oven-drying and spray-drying (Chranioti et al., 2016; Dail et al., 2007; Ma et al., 2014). Dail et al. (2007) reported that the oven-drying process caused moderately stable P in poultry manure to either become more labile or more recalcitrant, compared to that of freeze-drying. Ma et al. (2014) compared the

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Electrophoresis was performed at 40 mA and 150 V for 120 min. The gel was stained in 0.25% Coomassie brilliant blue R- 250 and destained in a solution containing 10% of acetic acid and 40% of methanol.

properties of spray-dried and freeze-dried whey protein concentrate hydrolysate. Their data indicated that both dryings could decrease the hygroscopicity of whey protein hydrolysate; however, spray-drying is a more potential approach with better hygroscopicity-improving and bitter-masking effect. Chranioti et al. (2016) compared the effects of spray-, freeze- and oven-drying on reducing bitter aftertaste of encapsulated steviol glycoside products derived from Stevia rebaudiana Bertoni plant. They found differences in microencapsulation efficiency, moisture content, structure and solubility values among products by the different drying methods, concluding that the spray-dried steviol glycoside products presented the best physicochemical characteristics and the most appealing sensorial features. In laboratory, we have used freeze-drying to dry cottonseed meal products (WCSM and protein isolate) to ensure the physicochemical and stability of the products (He et al., 2013, 2014c, 2016c). However, freeze-drying usually takes enormous amounts of time, labor and expenses. Thus, oven- or spray-drying would be an option to lower the processing cost of WCSM if there is no difference in the adhesion performance between these WCSM products.

2.4. Differential scanning calorimetry (DSC) Thermal transition properties of samples were measured with a DSC Q200 V24.4 instrument (TA Instruments, New Castle, DE, USA) that was calibrated with indium and zinc before making sample measurements. Dried powder (7–15 mg) used for FTIR was weighed in a hermetic aluminum pan under a nitrogen atmosphere with a gas flow rate of 50 mL min−1. The powder was heated from 25 °C to 280 °C at a heating rate of 10 °C min−1 in an inert environment. All experiments were performed in duplicate. 2.5. Thermal gravimetric analysis (TGA) Thermal gravimetric analysis (TGA) of samples conducted with a Perkin-Elmer TGA 7 (Norwalk, Conn.) in a nitrogen atmosphere. The sample powder in the 10-mg level was weighed into a platinum cup and scanned from 25 °C to 700 °C at a heating rate of 10 °C min−1. The maximum degradation rate was calculated as mass (%) at peak temperature divided by peak temperature.

2. Materials and methods 2.1. Preparation and drying of WCSM Mill-scale produced defatted cottonseed meal was provided by Cotton, Inc. (Cary, NC, USA). WCSM was prepared from the defatted meal in a pilot scale as described in He et al. (2016c). Briefly, defatted cottonseed meal was washed with 8 folds of water for 3 cycles. After removal of the soluble ingredients by the washing, the wet WCSM product was desiccated by freeze-, oven- or spray-drying. The ovendrying was conducted in an oven with circulated air at 60 °C. Spraydrying conditions were set with inlet temperature at 190 °C, and outlet temperature at 90 °C. After drying, the products were ground by a hammer mill (Model W-6-H, Schutte Buffalo Hammermill, Buffalo, NY, US), passed through a 0.5-mm screen. Part of the freeze-dried sample was further fractionated with a RX-29 To-Tap Sieve Shaker (Tyler, Inc., Mentor, OH, US) with sieves of No. 18 (1 mm), No. 30 (0.60 mm), No. 40 (0.42 mm), and No. 50 (0.30 mm) to remove hull from the meal. The particle’s morphological characteristics were examined by an IPhone 6 (Apple Inc., Cupertino, California, US) equipped with a macro lens (Model CamRah iPhone Camera Macro Lens, CamRah, Texas, US). The fraction with particle size of less than 0.30 mm was denoted by dehulled WCSM. All the products were stored at room temperature (22 °C) until use.

2.6. Wood specimen preparation and bonding Cherry wood boards (127 mm × 50 mm × 5 mm) were supplied by Veneer One Inc. (Oceanside, NY, US). These samples were first preconditioned in a controlled-environment chamber (Model 518, Electrotech systems, Inc., Glenside, PA, USA) for 7 d at 25 °C and 50% relative humidity (RH). The adhesive slurry was made by suspending WCSM in water (12% of solid content) and stirring for 2 h at room temperature (He and Chapital, 2015). The slurry pH was adjusted by 0.1 M HCl or 0.1 M NaOH as needed in the pH effect experiment. The adhesive slurry was brushed separately along the edges of two pieces of cherry wood with an application area of 127 mm × 20 mm until the entire area was completely covered. The adhesive amount applied on each piece of the covered area was approximately 0.06 g (dry basis). The brushing and setting procedure followed the method described by Mo et al. (2011). The brushed areas of the two pieces were assembled together at room temperature for 15 min, then pressed at 2.0 MPa at 100 °C, 150 °C, or 170 °C for 10 min using a hot press (Model 3890 Auto ‘M’, Carver Inc., Wabash, IN, USA).

2.2. Fourier transform infrared spectroscopy

2.7. Adhesion performance measurements

The Fourier transform infrared (FTIR) data were collected in the region of 400–4000 cm−1 with a PerkinElmer Spectrum™ 400 FT-IR/ FT-NIR spectrophotometer (Shelton, CT, USA). Before analysis, the WCSM samples were further ground using a cyclone mill (Udy Corp., Fort Collins, CO, USA) into particle < 0.25 mm, and KBr discs with the ground samples (about 100:1 for KBr:sample) were made. The absorbance spectra of 32 scans of each KBr disc were collected at a resolution of 1 cm−1 in the transmission mode. All samples were tested with duplications. The spectra were normalized and averaged.

After pressing, the glued-wood assemblies were conditioned at 23 °C and 50% RH for 2 days, then cut into five specimens, each measuring 127 mm (length) × 20 mm (width) × 5 mm (thickness). The cut specimens were conditioned for another 5 days at 23 °C and 50% RH before the dry test. Three adhesion strengths (i. e., dry strength, soak strength, and wet strength) were tested using an Instron (Model 4465, Canton, MA, USA) (Li et al., 2015b). The crosshead speed of Instron for adhesion strength testing was 1.6 mm min−1. Adhesion strength was recorded as tensile strength at the maximum load. Results were reported as an average of five sample measurements. Dry strength testing was tested according to the standard method of ASTM D2339-98 (2002). Wet and soak strengths were determined according to the protocols of ASTM D1183-96 (2002) and ASTM D115100 (2002), respectively, and used as the measurements of water resistance. For wet strength, preconditioned specimens were soaked in tap water at 23 °C for 48 h, then tested immediately. For the soak strength test, specimens were soaked in tap water at 23 °C for 48 h, then conditioned at 23 °C and 50% RH for an additional 7 days before testing.

2.3. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDSPAGE) SDS-PAGE was performed on a 4% stacking gel and 12% separating gel with a discontinuous buffer system as in studies of other oil-seed proteins (Qi et al., 2011). The protein sample was mixed with a buffer containing 2% of SDS, 25% of glycerol, and 0.01% of bromphenol blue. SDS-PAGE was carried out under non-reducing conditions. Molecular weight standards (10–250 kDa) were run with the samples. 282

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Fig. 1. Particle size distribution of freeze-dried WCSM. The meal ground by a hammer mill with a 0.5-mm screen was fractionated with a RX-29 To-Tap Sieve Shaker examined by an IPhone 6 equipped with a macro lens.

2.8. Statistical analysis Data from the evaluation of mechanical properties were taken from an average of five samples. Data from experiments carried out in duplicate were analyzed through analysis of variance (ANOVA) and least significant difference (LSD) at the 0.05 level according to procedures in the SAS statistical software package (SAS Institute 2005, Cary, NC, USA). 3. Results and discussion 3.1. Particle size distribution of WCSM Cottonseed contains about 16% of crude oil, 45% of meal, 27% of hulls, 8% of linters and 4% of other wastes (He and Cheng, 2017). Based on our previous study (He et al., 2016c), the protein content in WCSM was increased to 46.3% from 31.4% in the raw meal. Other components were 16% of crude fiber, 27% of acid detergent fiber, 35.4% of neutral detergent fiber, 9.4% of acid detergent lignin, 17.6% of cellulose, 8.4% of hemicellulose, and 1.0% of residue oil. The particle size distribution of freeze-dried WCSM is shown in Fig. 1. The majority (62.3%) of the WCSM components was in the fine particle (< 0.3 mm) fraction which should include protein and carbohydrates. Although the bulk WCSM product was passed through the 0.5-mm screen with a hammer mill, about 2.5% of WCSM components was still in size > 1.0 mm. They seemed to be forced passed through 0.5-mm screen by hammer beating. Those larger particles looked like residual linters (fibrous materials) and hulls (chunk materials) of cottonseed. The hulllike characteristics were more apparent in the fractions with 0.6–1.0 and 0.6–0.42 mm particle size. Thus, we called the fine pass particle (< 0.3 mm) fraction dehulled WCSM and tested its adhesion performance later.

Fig. 2. Comparison of the FTIR spectra of freeze-, spray-, and oven-dried washed cottonseed meal samples.

samples. Cottonseed proteins were identified and characterized on the curve. Peaks at around 1640 cm−1, 1530 cm−1, and 1236 cm−1 are dominated by the protein’s secondary structures, amide I for eC]O stretch, amide II for eNeH bend, and amide III for CeN stretching and NeH bending, respectively (He et al., 2013; Sun et al., 2012; Yu and Irudayaraj, 2005). The peptide bond of protein is unique containing C] O, CeN, and NeH. The amide I absorption contains contributions from primarily C]O stretching vibration (80%) with a minor CeN stretching vibration, while the amide II absorption appears to be arising from NeH bending vibrations (60%) coupled with CeN stretching vibrations (40%) (Jackson and Mantsch, 1995). Cottonseed protein’s secondary structure of α-helices and β-sheets can be identified after the amide peaks deconvolution; the peak at around 1658 cm−1 was α-helices and

3.2. FTIR analysis FTIR absorbance pattern of the WCSMs were presented in Fig. 2. In general, there is no significant difference in FTIR absorption to all the 283

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peak at 1637 cm−1 was assigned to β-sheets (Sun et al., 2012). Overall, the relative distribution of the secondary structure in cottonseed protein were β-sheet (∼40%), β-turn (∼33%), α-helix (∼14%), and random coil (∼12%) (He et al., 2013). A broad and intense band with the peak of 3280 cm−1 was ascribed to the eOH and NH stretching from both proteins and carbohydrates (Sun et al., 2012). Carbohydrates of cellulose, hemicellulose, and lignin were observed on the curve in the range 800–1250 cm−1 (Yan et al., 2009). The small peaks indicating low concentration localized at 1710 and 1745 cm−1 (C]O stretching) and at 2853, 2924, and 3006 cm−1 (CeH stretching) were detected in the spectra of defatted cottonseed meal which were identified as the cottonseed oil in the meal (Guillen and Cabo, 1997). Especially, the band around 1050 cm−1 was highest peak in all three spectra. It could be assigned mainly to carbohydrate as it is a major component in cottonseed products (He et al., 2014a) although phosphorous in nucleotide and phytate with less contents in cottonseed might also have had some contribution to the band intensity at this region (He et al., 2006; Waldrip et al., 2014). This peak in the pilotproduced meal samples (Fig. 2) was much stronger than the peak in the FTIR spectra of lab-produced washed meal samples (He et al., 2014a). This observation was apparently due to the contribution of cotton fiber residues in the meal products (Liu et al., 2016) as more crude fibers were in the pilot meal raw material than that the defatted raw meal used for lab production (He et al., 2016c). On the other hand, the FTIR spectra showed no impact of the drying method on the fiber component.

usually derived by the coupling of two thiol groups (Sammour et al., 1995), because the bands at 52 and 47.5 kDa were disappeared and the new band at 23.5 kDa was observed if the protein were treated with 2Mercaptoethanol which served as the reducing agent to cleave the SeS bonds in protein’s tertiary and quaternary structures (Sammour et al., 1995). The polypeptide fractions with MW of 52 and 47.5 kDa were storage proteins, which serves as reserves of metal ions and amino acids and can be mobilized and utilized for the maintenance and growth of organisms. Storage proteins in cottonseed can be up to 60% as reported by Cunningham et al. (1978). Cottonseed protein extracted from washed meal under oven-drying condition demonstrated different SDS-PAGE patterns in terms of molecular weight and band numbers. Whereas there is no apparent difference in the SDS-gel pattern between freeze- and spray-dry samples, oven-dried protein showed 5 other than 6 bands on SDS-PAGE for freeze- and spray- dried proteins. To oven-dried protein, the band indicating MW of larger than 255 and around 10 kDa faded away, the band at 52 and 47.5 kDa weakened, and two news bands at 13.5 and 20 kDa was observed. The vanishing of bands and formation of new bands on SDS-PAGE of oven-dried protein was the result of protein degradation caused by enzymes during the drying process. As indicted previously, the oven-drying process was accomplished at about 60 °C for 24 h which promoted the growth of yeast in the meal, leading to protein degradation and molecular weight decreasing. Plating and Cherry (1979) reported similar findings during the fermentation of cottonseed flour. In their work, cottonseed protein was denatured and converted to small polypeptide components with improved solubility and increased free amino acids content because of optimum temperature and the presence of fungi during the fermentation.

3.3. SDS-PAGE analysis The patterns of molecular weight distribution of samples were presented in Fig. 3. Six polypeptide fractions were visible on the SDSPAGE for cottonseed protein extracted from cottonseeds meal under freeze and spray drying conditions. The apparent molecular weight of the bands were calculated as > 255, 52, 47.5, 35, 10.2, and < 10 kDa, separately, which are in agreement with most of the previous studies (He et al., 2013; 2016c; Sammour et al., 1995; Sukor et al., 2007). As shown in Fig. 3, the polypeptides with MW of 52 and 47.5 kDa dominated the cottonseeds proteins as the band of those two fractions on SDS-page were the densest compared to others bands. Those two fractions were proved to be disulfide bonded cross-linked proteins, which is

3.4. Differential scanning calorimetry (DSC) thermal analysis The DSC thermograph of treated cottonseed meals were presented in Fig. 4. All the samples showed a main endothermic peak of protein denaturation. However, the oven-dried sample gave the highest peak number (137.35 °C) with ΔHd 7.69 J g−1 than spray-dried (129.01 °C, ΔHd 7.69 J/g) and freeze-dried (116.37 °C, ΔHd 1.33 J g−1) samples. Protein denaturation during thermal treatment involves the breakage of intramolecular bonds (covalent and non-covalent) by absorbing heat, resulted in the unfolded structure (Li et al., 2015). Based on the data, heating treatment increased cottonseed protein’s denaturation temperature and ΔHd, which might be ascribed to crosslinking effect in

Fig. 3. Molecular distribution of washed cottonseed meal samples examined by Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). A: oven-dried meal, B: Spray-dried meal, C: Freeze-dried meal.

Fig. 4. Differential scanning calorimetry (DSC) thermal analysis of freeze-, spray-, and oven-dried WCSM samples.

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mass loss from 8 to 12%. The water evaporation process finished below 200 °C, and during this process chemical changes like bond breakage, formation of free radicals, carbonyl and carboxyl groups may occur (Azargohar et al., 2013). The peak at 270 °C, which was partially overlapped by the main peak on the same curve, was related to the decomposition of hemicellulose component in the sample. Compared with compositions of protein and carbohydrates in cottonseed meal, hemicellulose was easier to decompose for its ransom, amorphous, and rich branched structure, and it started its decomposition in the temperature range of 190–400 °C, and the decomposition reached the maximum at temperature around 268 °C (Azargohar et al., 2013; Yang et al., 2007). Maximum protein degradation occurred at 340 °C which is in agreement with reports elsewhere (Li et al., 2014; Mo et al., 2011). The protein degradation involves breakage of intermolecular and intramolecular hydrogen and electrostatic bonds, decomposition of protein side-chains, and rupture of weak bonds of CeN, C(O)eNH, C(O) eNH2, and NH2 (Mo et al., 2011). The fourth stage at temperature of 250–450 with peak at around 350 °C was ascribed to the breakage of glucosidic bonds from cotton cellulose, with laevoglucose formation (Ciolacu et al., 2011). This behavior is explained by the fact that the thermal degradation reaction started in the amorphous domain of the cellulosic materials by statically degradation of cellulose (Ciolacu et al., 2010). The spray- and oven-dried meals exhibited two thermal degradation peaks instead of four peaks for freeze-dried meal. The first peak around 80 °C was ascribed to water evaporation as did to freeze dried meal. The second peak which attributed to the hemicellulose, protein, and cellulose degradation for the oven- and spray-dried samples was observed at around 350 °C. The merge of the peaks might be the crosslinking effect of hemicellulose, protein, and cellulose fractions in the meal as a result of the drying and heating process. 3.6. Adhesion performance of water washed cottonseed meal With the press temperature at 100 °C, all the three adhesion strength increased in the order of over-drying < spray-drying < freeze-dry (Table 1). The differences in bonding strength among the three drying methods became smaller with increased press temperature. Especially, the water resistance of oven-dried WCSM was very low shown by the data of both wet and soaked strengths. As discussed in previous sections, part of cottonseed protein might have degraded during the ovendrying process, generating small molecule polypeptide components, resulting in increased hydrophobicity and decreased wet resistance of cottonseed meal based-adhesives. Increase in press temperature benefited the adhesion performance of all WCSM samples. The greater improvement was observed with the oven-dried sample. With the hot pressing set at 150 °C, there was no substantial difference in the three strengths between spray- and freeze-dried samples. With 170 °C as press temperature, the wet and soaked strengths of oven-dried WCSM sample were comparable to those of freeze- and spray-dried samples. Protein-based adhesives can bond substrates through mechanical interlocking, physical adsorption, or chemical bonding mechanisms (Singh et al., 2008; Wool and Sun, 2011). Under mechanical interlocking, protein based adhesives first spread and wet the surface, then penetrate into the cavities, pores, and asperities substrate structure of the solid adherend followed by an adhesive curing step amid curing

Fig. 5. Thermal gravimetric analysis (TGA) analysis of freeze-, spray-, and oven-dried washed cottonseed meal samples. A: weight loss, B: derivative TGA (DTG).

protein molecules and between proteins and carbohydrates in cottonseed meal. Denatured (unfolded) protein molecules also favored the aggregation/association among each other through formation of new intermolecular bonds, giving rise to an exothermic process in DSC thermograph followed by the endothermic denaturation peak of cottonseed protein. Similar protein aggregation exothermic peaks were also observed in other proteins, such as soy protein, sorghum, canola protein, whey protein, and pea storage protein (Li et al., 2015). 3.5. Thermal gravimetric analysis (TGA) analysis TGA and DTG (Derivative thermogravimetric analysis) curves of washed cottonseed meals dried with different methods were presented in Fig. 5. Freeze-dried sample displayed four main thermal degradation stages. The first peak around 80 °C corresponded to the water evaporation of both free water and physically absorbed water with the

Table 1 Effect of drying methods on the adhesion strength of washed cottonseed meal with press temperatures at 100, 150 and 170 °C. Data are presented in average ± standard deviations (n = 5). Press temperature

100 °C

150 °C

170 °C

Adhesion strength (MPa)

Wet

Soaked

Dry

Wet

Soaked

Dry

Wet

Soaked

Dry

Freeze-drying Spray-drying Oven-drying

1.23 ± 0.08 1.02 ± 0.10 0.65 ± 0.17

2.71 ± 0.61 2.22 ± 0.30 0.54 ± 0.11

3.34 ± 0.22 3.16 ± 0.16 2.76 ± 0.21

1.49 ± 0.08 1.40 ± 0.06 1.23 ± 0.10

3.11 ± 0.23 3.24 ± 0.15 2.25 ± 0.63

3.75 ± 0.18 3.80 ± 0.23 3.36 ± 0.16

2.27 ± 0.04 2.61 ± 0.08 2.22 ± 0.13

3.81 ± 0.08 4.27 ± 0.25 3.86 ± 0.19

4.13 ± 0.27 4.10 ± 0.24 3.66 ± 0.20

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except at the press temperature of 100 °C, indicating protein contributed more than the cellulosic component to the bonding strength. De-hulled meal has higher protein content, leading to more functional groups available and greater adhesion strength (He et al., 2016b). Cottonseed meal at pH 4.5 (close to the pI of cottonseed protein) showed higher wet shear strength (2.53 MPa) (Fig. 7) than that of neutral (2.21 MPa) or alkaline conditions. Protein at its pI has the least surface charge and is the most hydrophobic, thus results in a dense cross-linked network and higher wet shear strength (Qi et al., 2016; Wang et al., 2009). In contrast, at high pH values (alkaline) electrostatic repulsions between protein molecules, which may reduce the interaction among proteins and increase the interactions between protein and water, could be detrimental to the wet shear strength of protein-based adhesives (Wang et al., 2009). 4. Conclusions The objective of this project was to confirm the influence of different drying methods on the adhesion performance of water-washed cottonseed meal-based adhesives. The properties of cottonseed meal treated with different drying conditions showed varied chemical properties and adhesion performance. The findings in this study suggested that the water-washed meal dried with spray dryer and freeze dryer are more effective to produce high quality cottonseed meal-based adhesives with improved bonding strength and water resistance. Cottonseed meal showed great potential for being used as biodegradable high performance adhesives.

Fig. 6. The adhesion strengths of whole freeze-dried washed cottonseed meal and its dehulled portion with press temperatures at 100, 150 and 170 °C. Data are presented in average ± standard deviations (n = 5).

Acknowledgements Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. References ASTM-D1151-00, 2002. Annual Book of ASTM Standards. American Society for Testing and Materials, West Conshohocken, PA, pp. 67–69. ASTM-D1183-96, 2002. Annual Book of ASTM Standards. American Society for Testing and Materials, West Conshohocken, PA, pp. 70–73. ASTM-D2339-98, 2002. Annual Book of ASTM Standards. American Society for Testing and Materials, West Conshohocken, PA, pp. 158–160. Azargohar, R., Nanda, S., Rao, B.V.S.K., Dalai, A.K., 2013. Slow pyrolysis of deoiled canola meal: product yields and characterization. Energy Fuel 27, 5268–5279. Cheng, H.N., He, Z., 2017. Wood adhesives containing proteins and carbohydrates. In: He, Z. (Ed.), Bio-based Wood Adhesives: Preparation, Characterization, and Testing. CRC Press, Boca Raton, FL, pp. 140–155. Cheng, H.N., Dowd, M.K., He, Z., 2013. Investigation of modified cottonseed protein adhesives for wood composites. Ind. Crops Prod. 46, 399–403. Cheng, H.N., Ford, C.V., Dowd, M.K., 2016. Use of additives to enhance the properties of cottonseed protein as wood adhesives. Int. J. Adhes. Adhes. 68, 156–160. Chranioti, C., Chanioti, S., Tzia, C., 2016. Comparison of spray, freeze and oven drying as a means of reducing bitter aftertaste of steviol glycosides (derived from Stevia rebaudiana Bertoni plant)- evaluation of the final products. Food Chem. 190, 1151–1158. Ciolacu, D., Ciolacu, F., Popa, V.I., 2011. Amorphous cellulose-structure and characterization. Cellulose Chem. Technol. 45, 13–21. Cunningham, S.D., Cater, C.M., Mattil, K.F., 1978. Molecular weight estimate of polypeptide chains from storage protein of glandless cottonseed. J. Food Sci. 43, 656–657. D’Amico, S., Hrabalova, M., Muller, U., Berghofer, E., 2010. Bonding of spruce wood with wheat flour glue-effect of press temperature on the adhesive bond strength. Ind. Crops Prod. 31, 255–260. Dail, H.W., He, Z., Erich, M.S., Honeycutt, C.W., 2007. Effect of drying on phosphorus distribution in poultry manure. Commun. Soil Sci. Plant Anal. 38, 1879–1895. Frihart, C.R., Coolidge, T., Mock, C., Valle, E., 2016. High bonding temperatures greatly improve soy adhesive wet strength. Polymers 8, 394 (10 pages). 310.3390/ polym8110394. Guillen, M.D., Cabo, N., 1997. Characterization of edible oils and lard by Fourier transform infrared spectroscopy. Relationships between composition and frequency of concrete bands in the fingerprint region. J. Am. Oil Chem. Soc. 74, 1281–1286. He, Z., Chapital, D.C., 2015. Preparation and testing of plant seed meal-based wood adhesives. J. Vis. Exp. 97, e52557 (52510.53791/52557). He, Z., Cheng, H.N., 2017. Preparation and utilization of water washed cottonseed meal

Fig. 7. Effects of pH of the adhesive slurry on the adhesion strengths of freeze-dried washed cottonseed meal with press temperatures at 170 °C. Data are presented in average ± standard deviations (n = 5).

pressure, time and temperature. With physical adsorption, adhesives and the adherend surfaces are bonded physically or electrostatically because of interatomic and intermolecular forces established at the interface, therefore, an intimate contact is achieved. Chemical bonding formed across the chemical groups, such as eCOOH, eOH groups on wood and eSH, e+NH3, eOH groups on protein, that greatly participate to the level of adhesion between adhesive and adherend. High press temperature gave improved wet, soaked, and dry adhesion performance of cottonseed meal-based adhesives. One reason is that as bonding temperature increased, both the interaction among protein molecules and chemical reactions between the functional group and wood were improved (Li et al., 2015), leading to increased bonding strength. Other reasons include that the higher temperature induces more Millard reactions between the protein and reducing sugar, which has an aldehyde for reacting with the amine on the protein, in carbohydrates in cottonseed meal, creating higher bonding strength (Frihart et al., 2016). Similar findings were also reported by He et al. (2014b). 3.7. Effect of dehulling and pH on the adhesion performance of WCSM To study the effect of cottonseed hull on the adhesion performance of meal, the bonding behavior of freeze dried meal and de-hulled freeze dried meal were evaluated (Fig. 6). De-hulled sample showed substantially higher bonding strength compared to the meal with the hull, 286

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