Influence of processing on digestibility and nano-molecular basis of sorghum grain

Processing have an impact on nano-molecular basis, which increase digestion of starch. Heat processing destroys the crystalline nature of a starch granule making the surface of the starch more available to digestive solvents and enzymes as well as to the rumen microbes (Bdour et al. 2014). The in situ nylon bag technique is widely used to characterize the disappearance of feeds from rumen (woods et al. 2002). In situ incubation is a principle method for the estimation of ruminal degradability of dry matter and crude protein, because it is simple, does not require specific equipment and could be applied in every research laboratory (Alexandrow 1998). In many protein evaluation systems for ruminants, the nylon bag technique is the standard method used for calculating the amount of protein escaping rumen fermentation (Cone et al. 2002). A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the sampleʼs surface topography and composition. Accordingly, scanning electron microscope (SEM) can achieve resolution better than 1 nanometer (Stokes 2008). This experiment was conducted to investigate the digestibility of dry matter and crude protein of sorghum grain in the rumen and therefore, investigate changes in nano-molecular basis.

In this study sorghum grain was processed using methods of steam flaking, roasting and microwaving. Feed samples were dried in an oven at 90°C for 48 h and the dry matter (DM) content was calculated. Then, ground samples were analyzed for ash (AOAC, 2005). Determinations of N were conducted using the Kjeldahl method in an automated Kjelfoss apparatus (Foss Electric, Copenhagen, Denmark). Neutral-detergent fiber (NDF) and acid-detergent fiber (ADF) were determined by the detergent procedures of Van Soest et al. (1991). Acid-detergent insoluble nitrogen (ADIN) was determined as nitrogen in acid-detergent residue. Ether extract (EE) was determined by extracting the sample with ether (AOAC 2005). Degradation characteristics of feeds were calculated after the incubation of 5 g samples of row, steam flaked, roasted, and microwaved sorghum grains (ground at 2 mm) in nylon bags. The bag size was 6×12 cm. The bags were incubated in the rumen of two cannulated sheep for 0, 2, 4, 8, 12, 24, 36, and 48 h. Retrieved bags were washed under running water until the water was clear. Immediately after removal from the rumen, the bags frozen at −18 °C. At the end of the collections, they were unfrozen and washed together with the zero time bags (not incubated in the rumen) for 20 min and then dried at 80°C for 48 h. The residues were weighted and submitted for analysis. For each bag, the residue was analyzed for DM and nitrogen. Feed residues were recovered from each bag and stored for Kjeldahl nitrogen. The model of p=a+b(1-e-ct) was used for the determination of degradation characteristics, in which p is the actual degradation of CP and DM after t, a is the intercept of the degradation curve at time zero, b is the potential degradability of the component of the slowly soluble CP and DM, which will in time be degraded, c represents the constant degradability rate of b at time t, and t is the incubation time. In order to provide SEM images, scanning electron microscopy (Field Emission Tescan mirca 3) was used in the central laboratory of University of Tabriz. The values of a (% solubility) and b (potentially degradable fraction) and deviation of error were determined.  Data obtained from in situ study was subjected to analysis of variance as a completely randomized design by the GLM procedure of SAS software and treatment means were compared by the Duncan test.

In situ results showed that steam flaking of sorghum grain have the most digestibility compared with other treatments, but it was not significant. Steam flaking of sorghum grain reduced nutritional restriction (Teurer et al. 1999) and in many studies has been showed decreases in tannin and phenolic compounds. Microwave processing reduced digestibility; however, it was not significant. Lewandowicz et al. (2002) reported that microwaving reduced starch solubility, because high heat treatment in microwaved starch can create resistance and continual basis in granular structures. Processing significantly decreased crude protein digestibility in all three treatments (P<0.05) that previous studies have confirmed these results (Parnian 2008, Nickhah et al. 2003, Hamaker 1987).
Thermal process reduces protein digestibility in the rumen via denaturing the protein and formation of protein-protein and protein-carbohydrate bonds (Anonymous 2001). Dandy et al. (1995) also concluded wet sorghum baking reduced digestibility. Preston (1998) reported that processing and steaming of corn and sorghum grains reduced solubility of proteins. Also, access to lysine amino acids reduced by flakes density reduces. Scanning electron micrographs in all three treatments showed that processing destructed protein matrix and increased enzymes access to starch granules and increased their digestibility. In steam flaking treatment, granule surfaces were attacked by microorganisms and protein matrix destructed more than other treatments. Also, terminal processing in all three treatments gelatinated the starch and increased average of scale of granules. In addition to the importance of starch digestion, the increase in viscosity during gelatinization may also positively affect physical quality of processed feeds through increasing in binding between feed particles. Svihus et al. (2004) reported that pelleting can improve feed integrity by increasing binding between particles. Gelatinization of starch greatly increases α-amylase accessibility to amylose and amylopectin glucose chains and so, increases the rate of starch digestion. The number of created orifices is explanatory for degradability values. Benmoussa et al. (2006) concluded that number and scale of granules have positive effects on digestive properties. In this study, despite of smaller number and smaller size of orifices in steam flaking treatment than other treatments (against further degradability of steam flaking than others treatments), SEM images of incubated steam flaking showed that granules were processed completely. Therefore, lots of uniformity and meltingcan be justified as its high degradability.

Electron microscopy images showed that the number and size of holes (nano-sized) per unit area of ​​treatments were highly correlated with the degree of degradability. Steam flaked treatment has caused more gelatinization in starch granules as the size of the granules has increased more than the control treatment and other treatments. Steam flake processing shows better degradability properties than other treatments and provides higher amounts of nutrients to microorganisms.