Structural characteristics of sorghum kernel: Effects of temperature

ABSTRACT

In this study, the effect of cold water, hot water, and steam tempering on sorghum kernel physical and mechanical properties was studied. Single kernel characteristics (SKCS), abrasive hardness, structural changes, and texture of kernels were evaluated as an effect of temperature. At the same moisture level, cold water tempered sorghum had both higher SKCS hardness and abrasive hardness than hot water and steam tempered sorghum kernels. The increase in abrasive hardness, SKCS-hardness index was highly correlated with the moisture content of kernel. The abrasive hardness index, which represents the pericarp properties, did not show any correlation with moisture content but had correlation to the structural changes. The SEM images indicated the structural changes in pericarp after hot water and steam treatment. Steam tempering methods made the pericarp tougher than the cold water and hot water tempering methods and, meanwhile, softened the endosperm by adding moisture to the kernel.

KEYWORDS:

• Hardness
• Sorghum
• Steam pre-treatment
• Tempering
• Texture

Introduction

Sorghum is the fifth most important cereal crop in the world.^[1] In 2015, about 4 million tons of sorghum were produced in the United States (USDA-NASS, 2016). In the US, sorghum is primarily used as an animal feed, while about 12% of sorghum is also used for ethanol production and for human consumption.^[2] However, it is a major cereal crop in Africa and South Asia, due to its severe draught tolerance characteristics. Sorghum is a rich source of phytochemicals, which can reduce the risk of several types of cancer and obesity in humans.^[3] With the expansion of the gluten-free food market, sorghum, a potential source of gluten-free food, has been gaining more importance as human food.^[4]

Sorghum kernels are primarily decorticated and milled into flour, or flaked for further processing. Sorghum have extremely hard endosperm and the pericarp is brittle compared to wheat.^[5] Milling sorghum using a roller mill, by following a similar procedure used for wheat and corn, results in gritty and speckled flour with an astringent taste.^[6] Most methods used in the commercial scale industry are based on the use of a dehuller, for the decortication of grain, and a hammer mill for the production of meal. However, dehullers developed for industrial milling are rather small in capacity and result in huge losses (up to 30%) during milling. Furthermore, the quality of flour obtained through this process is not consistent.^[7] Researchers have attempted to mill sorghum using roller mills without decorticating; however, the separation efficiency of grain constituents has remained very low.^[8] This low efficiency is mainly due to the extremely friable pericarp, the large integral germ, and the highly variable endosperm texture.

Water uptake by cereal grains during tempering is known to occur in two distinct phases: water first adheres to the surface of the kernel and subsequently diffuses to the centre of the kernel through the germ and the pericarp. The addition of moisture toughens the bran, mellows the endosperm,^[9] and results in an increase in kernel volume due to swelling. Differential swelling of parts of the kernel facilitates the structural separation of germ, pericarp, and endosperm.^[10]

Cold water tempered sorghum with high moisture content was found to have a higher rate of weight loss over time upon dehulling, compared with that of untempered sorghum kernels.^[11]Weaker endosperms break into small pieces during grinding, whereas the bran is more resistant and yields larger particles that can be easily separated by sieving. Higher temperature and steam treatment were found to be more appropriate than cold water treatment because of their capability to accelerate the diffusion of moisture into grain kernels and attain a uniform distribution of moisture.^[12] Many studies have reported an increased yield of break flour and bran by steam treatment for wheat. Compared with cold water and hot water tempering, steam tempering is an energy saving and rapid conditioning method, which results in high flour yield.^[13]

Tempering of sorghum, which has a harder pericarp than other common grains such as wheat and corn, for efficient flour extraction remains a challenge. Typically, soft wheats are conditioned to 15–15.5%, hard wheats to 16–16.5%, and durum wheats to 18% (w.b.) moisture.^[13] Cecil^[6] tempered sorghum to 26% moisture content at 60°C for 6 h; however, the moisture content of the flour was excessively high. It is important to develop scalable tempering methods for efficient sorghum flour extraction with minimum bran friability. As a prelude to the development of a scalable milling method, this study evaluated the effects of cold water, hot water, and steam tempering on the sorghum kernel physical and mechanical properties.

Materials and methods

Samples

Sorghum Partner 217 (with mid-level tannin, red colour) sorghum kernels were obtained from Nu Life Market (Scott City, KS, USA). The average initial moisture content of kernels was 12.13% (w.b., wet basis). Moisture content, before and after conditioning, was measured using the ASABE Standard S352.2 of drying 10 g of sample in an air oven at 130°C for 18 h.^[14]

Tempering methods

Cold water tempering: Calculated amounts of distilled water were added to 500 g of sorghum samples to achieve a target moisture content of 18% (w.b.). This moisture content was selected based on preliminary studies (not reported here) on bran yield conducted at 16–18% moisture contents. Amounts of water to be added were calculated using equation 1. After mixing, samples were stored at 21°C for equilibration before conducting physical and mechanical analyses.

(1)

where Q is the amount of water added, g; W is the weight of sorghum kernels (500 g); M_f is the final desired moisture content in dry basis %; and M_i is the initial moisture content of the samples in dry basis %.

Hot water tempering: Calculated amounts of water were added to sorghum kernels for conditioning to a moisture content of 18% (w.b.). Moisture content was the same as in cold water tempering. After adding water, sealed glass containers with the kernels were kept in a water bath at 60°C for 12, 18, and 24 h.^[6] The glass containers with the samples were shaken every 30 min for better moisture equilibration. After conditioning, samples were cooled to room temperature before the subsequent measurements.

Steam tempering: A MIAG laboratory conditioner (MIAG North America, Inc., Minneapolis, MN, USA) was used for steam tempering. Sorghum kernels were loaded into the conditioner drum and steamed for 1, 1.5, 2, and 2.5 min. Steam pressure was maintained at 40 psi. Steam pressure and time values were selected based on preliminary studies (not reported in this manuscript) targeting moisture content in the range of 15–20% (w.b.). After steam treatment, kernels were surface-dried at 60°C with 10 m³/hr airflow for 30 min. Dried sorghum kernels were subsequently cooled to room temperature and stored in sealed Ziploc bags until the experimental measurements.

Geometric characteristics measurement

Sorghum kernel hardness indexes, kernel weights, and diameters were measured using a single kernel characteristics system (SKCS; 4100, Perten Instruments, Hagersten, Sweden). Glumes, broken kernels, and foreign matter were removed by hand before the measurement. The hardness index was measured as the force required to crush sorghum kernels as they passed through a wedge-shaped cavity between a smooth crescent surface and a coarse-toothed rotor in the SKCS.^[15]

Abrasive hardness

A Tangential Abrasive Dehulling Device (TADD, Venebles Machine Works, Saskatoon, Canada) was used to measure the abrasive hardness index (AHI). The TADD uses an 80-grit abrasive disk. For each measurement, 10 g of sample was loaded into the TADD cups (12-cup plates). The time (seconds) taken to abrade 1% of the weight of the grain was considered as the AHI.^[16]

Mechanical structural properties

Texture analysis was carried out as described by Sirisomboon et al.^[17], using a TA-XT plus texture analyser (Texture Technologies Corp., Scarsdale, NY, USA). During each single kernel measurement, kernels were aligned horizontally on the measuring platform. A stainless steel probe (10 mm diameter) was used to compress each sample at a speed of 0.5 mm/s until the breaking point of kernels, up to 60% of the kernel height. Thirty kernels were analyzed with each tempering method. From the force-deformation curve, rupture force, deformation at rupture point, and rupture energy were measured using the TA-XT installed software. The rupture force is the minimum force required to break a sample. Deformation at rupture point is described as the time needed to rupture a sample. Energy used for rupture is the energy needed to break a kernel and is determined from the area under the curve between the initial point and the rupture point.

Internal structure

The effect of tempering on the internal structure of sorghum kernels was evaluated using a cryo-scanning electron microscope (Cryo-SEM).^[18] Sorghum kernels were cut into thick slices (of approximately 2 mm) and frozen in liquid nitrogen. Samples were cut vertically and kept at -120°C for sublimation for approximately 3 min. Thick sorghum slices were subsequently imaged at -90°C with a JEOL JSM-840 SEM (Jeol USA Inc., Peabody, MA, USA) using 5 kV of accelerating voltage, after sputter coating with nitrogen under freezing conditions. Three kernels images for each treatment were acquired under 1000X magnification, and pericarp thickness was measured at three pericarp positions for each kernel using the Adobe Photoshop CS software (Adobe, San Jose, CA, USA). Actual thickness of pericarp was calculated based on the scale shown on the image.

Statistical analysis

All tempering treatments and measurements were performed in triplicate unless otherwise mentioned. Statistical analysis was conducted using PROC GLM with the SAS software (SAS Institute, Cary, North Carolina, USA) to determine whether differences were significant when comparing tempering processes (α = 0.05) based on ANOVA. PROC CORR was used to find the Pearson correlation between moisture content, AHI, HI, diameter, pericarp thickness, rupture force, rupture energy, and deformation.

Results and discussion

The kernel composition and geometric characteristics, including moisture content, SKCS-kernel weight, and SKCS-diameter after tempering pretreatment, are shown in Table 1. With an increase in duration of hot water tempering, moisture content slightly decreased, mainly due to the loss of moisture as vapour during prolonged conditioning at 60°C. In addition, after hot water tempering, samples were removed from the containers and surface dried; for this reason, there was no re-absorption of vapour, and this resulted in a decrease in moisture content. However, with an increase in duration of steam treatment, the moisture content of the samples increased significantly from 15.20% (1 min) to 19.64% (w.b., 2.5 min). Kathuria and Sidhu^[13] reported an increase in moisture content after steaming of wheat kernels for extended steam duration and at different pressures.

Table 1. Kernel physical characteristics.

Treatment	Moisture content (% w.b.)	Weight (g)	Diameter (mm)	Hardness index	Abrasive hardness index
Untreated	12.13 (0.03)^a	20.50 (5.60)^a	23.20 (0.23)^a	75.41 (0.21)^a	16.70 (0.59)^a
Cold water	17.56 (0.13)^bc	20.64 (6.28)^a	24.54 (0.89)^a	64.20 (1.02)^bc	14.27 (0.68)^b
Hot water – 12 h	17.33 (0.74)^bc	20.14 (6.10)^a	25.22 (0.24)^a	59.71 (1.75)^d	10.94 (1.16)^c
Hot water – 18 h	17.13 (0.58)^bc	20.49 (5.80)^a	24.58 (0.24)^a	62.78 (1.44)^cd	11.13 (0.97)^c
Hot water – 24 h	16.90 (0.12)^bc	20.42 (6.42)^a	24.83 (0.24)^a	62.44 (1.74)^cd	11.12 (1.20)^c
Steam – 1.0 min	15.20 (1.40)^ab	21.70 (6.41)^a	24.50 (0.24)^a	65.05 (4.26)^b	14.61 (1.60)^b
Steam – 1.5 min	16.52 (2.50)^abc	20.07 (6.64)^a	24.74 (0.24)^a	62.83 (2.64)^cd	19.43 (1.04)^d
Steam – 2.0 min	18.78 (1.35)^c	18.68 (6.60)^a	25.27 (0.24)^a	62.06 (1.35)^cd	20.64 (0.83)^e
Steam – 2.5 min	19.64 (4.97)^c	20.49 (7.30)^a	26.01 (0.27)^a	53.50 (9.24)^e	22.99 (2.62)^f

Values between parentheses are standard deviations. Within the same column, values with different letters are significantly different (P < 0.05).

Tempering method and moisture content did not have a significant influence on kernel size and weight (Table 1). Only a slight swelling was noticed after tempering, when compared with untreated sorghum, and this finding is in agreement with the results described by White.^[19] Germs always have higher moisture uptake capacity than the other components of grain; the swelling force caused by differential moisture distribution can make germ and pericarp readily separable from the endosperm with minimum damage.^[19]

Kernel hardness

Our results indicated that sorghum endosperm hardness was greatly influenced by moisture content and heat treatment (Table 1). The hardness index of untreated sorghum kernels was significantly higher than that of tempered kernels. The grain resistance to crushing decreased with an increase in moisture content. A similar trend was noticed for steam tempered kernels, where the hardness index decreased with an increase in moisture content caused by prolonged steam duration. The hardness index of steam tempered kernels (1 min) was slightly higher than the one of cold water tempered kernels; this is likely to be related to the low moisture content in steam-treated kernels. Thus, moisture content is the main factor affecting the hardness of the kernel endosperm. However, cold water tempering resulted in a higher hardness index compared with steam (1.5 to 2.5 min) and hot water tempering. Kathuria and Sidhu^[13] reported that heat treatment results in a higher break flour yield, confirming that steam tempering mellows the grain kernels.

Abrasive hardness index

The abrasive hardness index (AHI) of kernels is a parameter that can be used to predict the dehulling performance for grains. In this study, we found that the tempering process has a significant influence on the AHI of sorghum kernels (Table 1). However, the duration of hot water treatment did not have a significant effect on the AHI of sorghum. Steam tempering increased the AHI values that were higher than the ones of cold and hot water tempered kernels. The increase in AHI values indicates that pericarp strength increased after steam treatment, probably due to a toughening of the bran. This trend shows that the pericarp can be made brittle by steam treatment. Untreated sorghum also has a higher value of AHI, but, if untempered, a higher hardness index value indicates that the endosperm remained hard and predicts that a clean separation of the bran from the flour particles is hard to obtain.

Structural changes in sorghum kernels

The sorghum kernels used in this research had a starchy-mesocarp and consisted of several layers of starch-filled cells (Fig. 1). In comparison with untreated kernels, cold water did not alter the pericarp structure (Fig. 1A, 1B). Hot water treatment removed the starch granules present between the mesocarp layers; this is probably due to the partial gelatinization of starch granules and the dissolution of amylose chains at 60°C. Steam-treated sorghum kernels did not show a starchy mesocarp, and cell layers were compact compared with that by other treatments; this is likely due to the high temperature and high pressure during steam treatment. The steam conditioner side wall temperature reached up to 120°C, and this temperature is significantly higher than the starch granule pasting temperature range of 67–71°C.^[20] In addition, the pressure applied during steam treatment was 40 psi; this might have resulted in the compaction of cell layers.

Figure 1. Cross-sectional SEM images of differently tempered sorghum kernels (the yellow arrows indicate the thickness of the pericarp).

Mosier et al.^[21] indicated that hot water and steam treatment increase the pore volume of lignocellulosic materials. Dissolved amylose chains could have penetrated into the endosperm through the porous cell walls. In addition, it has been reported that the outermost layer of kernels gelatinises to a greater extent and that the degree of gelatinisation depends on moisture content.^[22] The extent of gelatinisation of starch is mainly dependent on the severity of the heat treatment and the availability of water.^[23] Earp and Rooney^[24] reported that pericarp thickness and structure depend on grain variety. In addition, they found that thick starchy mesocarps, characterised by a layered structure with abundant starch granules, could easily be peeled off the endosperm by abrasive methods compared with thin non-starchy pericarps. Thus, non-starchy pericarps show longer decorticating times through abrasion, compared with kernels with a starchy-pericarp. AHI results corroborate the structural analysis of grain kernels. In addition, the pericarp thickness of hot water and steam-tempered kernels (Table 2) was significantly reduced compared with cold water tempered and untreated sorghum kernels.

Table 2. Kernel pericarp thickness.

Treatment	Pericarp thickness (μm)
Untreated	57.98 (29.70)^b
Cold water	58.49 (30.77)^b
Hot water – 12 h	31.44 (9.93)^a
Hot water – 18 h	37.60 (9.53)^a
Hot water – 24 h	28.66 (5.60)^a
Steam – 1.0 min	24.49 (4.56)^a
Steam – 1.5 min	26.75 (5.18)^a
Steam – 2.0 min	32.87 (13.80)^a
Steam – 2.5 min	33.40 (11.88)^a

Values between parentheses are standard deviations. Within the same column, values with different letters are significantly different (P < 0.05).

Mechanical structural analysis

The force-deformation curve of sorghum kernels exhibits several points of inflection (Fig. 2), indicating the kernel rupture points during compression. Approximately, at 1 mm displacement, where the major rupture occurred, kernels were crushed into smaller pieces. Before being crushed and broken into smaller pieces, kernels exhibited elastic properties; the force-deformation curve appears first concave, and subsequently convex. From the first derivative of the force-deformation curve, several inflection points were identified at the initial compression of the kernels; the change in slope suggests that these are the rupture points of the bran and aleurone layers.^[25] The first inflection is approximately at 0.05 mm, as our SEM image analysis suggested.

Figure 2. Typical compression pattern of sorghum kernels (steam tempered for 2.0 min).

The rupture force of untreated sorghum is significantly lower than the one of cold water tempered sorghum, and its deformation is higher (Table 3). Cold water tempering shows the highest rupture force and lowest deformation, indicating that the kernel is hard and brittle, even compared to untreated sorghum. The increase in rupture force and energy can be observed with an increase of steam duration. The resistance to deformation decreases with an increase in kernel moisture content. SKCS hardness tests showed a decrease in hardness with an increase in moisture content, and therefore, the main factor increasing rupture force is likely to be kernel surface deformation. With the increase in deformation, force increased rapidly, resulting in an increased energy. Hot water tempering treatment resulted in a relatively low rupture force and the deformation value increased slightly with an extended hot water tempering time. This indicates that hot water treatments result in soft kernels and that an increase in treatment time can reduce the brittleness of kernels. Cold water tempering had fewer effects on the softening of the endosperm and the kernel was still friable. Steam-tempered and hot water–tempered (for 24 h) kernels were less susceptible to breakage, resulting in a better separation of the bran and endosperm of sorghum kernels.

Table 3. Texture of pre-treated sorghum kernels.

Treatment	Displacement at deformation (mm)	Rupture force (N)	Energy (N*mm)
Untreated	0.62 (0.32)^ab	60.81 (14.82)^a	35.40 (15.10)^ab
Cold water	0.49 (0.19)^a	82.43 (21.92)^c	38.87 (14.89)^ab
Hot water – 12 h	0.61 (0.28)^ab	61.52 (13.42)^a	37.41 (18.26)^a
Hot water – 18 h	0.62 (0.30)^abc	75.20 (16.51)^ab	44.70 (20.04)^abc
Hot water – 24 h	0.87 (0.42)^abc	66.46 (19.50)^a	54.08 (24.92)^abcd
Steam – 1.0 min	1.06 (0.66)^abc	66.43 (17.08)^ab	63.41 (31.66)^bcd
Steam – 1.5 min	0.91 (0.37)^c	71.41 (19.33)^a	62.66 (26.15)^cd
Steam – 2.0 min	1.01 (0.46)^bc	73.67 (16.25)^bc	73.45 (37.55)^d
Steam – 2.5 min	1.03 (0.58)^bc	74.13 (18.89)^ab	70.04 (30.80)^cd

Values between parentheses are standard deviations. Within the same column, values with different letters are significantly different (P < 0.05).

Correlation analysis

Moisture content statistically correlated with kernel diameter and hardness index (Table 4). This is probably due to a direct relationship between moisture content and swelling, softening of sorghum kernels. AHI, pericarp thickness, and compressive properties may additionally be influenced by conditioning methods rather than properties. Rupture energy is highly related to displacement (r = 0.94). Such relationships indicate that tough bran and thin pericarp lead to high displacement and energy required to break sorghum kernels.

Table 4. Correlation between sorghum kernel characteristics.

	MC	Diameter	HI	AHI	Pericarp thickness	Displacement	Rupture force	Rupture energy
MC	1	0.93**	–0.92**	0.27	–0.37	0.29	0.60	0.51
Diameter		1	–0.98**	0.33	–0.57	0.47	0.34	0.60
HI			1	–0.23	0.56	–0.39	–0.38	–0.51
AHI				1	–0.04	0.58	0.25	0.68*
Pericarp thickness					1	–0.73*	0.22	–0.65
Displacement						1	–0.07	0.94**
Rupture force							1	0.23
Rupture energy								1

**significant at p = 0.01; * significant at p = 0.05.

Conclusions

This study explains and quantifies the effects of tempering on the mechanical structural properties of sorghum kernels. Due to a low hardness index value and a high AHI (at similar moisture content levels) under hot and steam conditioning, treatment with heat can soften kernel endosperms and increase bran resistance; these kernels are less friable (with high deformation values) compared with those that are cold water tempered. Steam-treated sorghum has a tougher pericarp than hot water tempered sorghum, owing to the structural changes of sorghum kernels during pre-treatment. High temperature results in the gelatinisation of starch granules present inside the mesocarp layers. Increasing steam tempering duration increases the moisture content of sorghum kernels, thus reducing endosperm hardness while toughening the pericarp by changing its structure. These data on pericarp toughness and endosperm hardness of sorghum, as influenced by temperature and pretreatment, will help optimise scalable tempering and milling processes.

Acknowledgements

We acknowledge the help from Dr. P. V. Venkat Reddy, milling consultant; Dr. Hulya Dogan, Kansas State University; Dr. Tom Herald, USDA-ARS, Manhattan, Kansas; Dr. Osvaldo Campanella, Purdue University in conducting the experiments.