ESTABLISHMENT OF HIGH-THROUGHPUT TECHNIQUES FOR STUDYING STARCH FUNCTIONALITIES
Maize is one of the top sources of food starch. Industrial use of starch is mostly in its native form and used due to their functional and structural properties. Native starch properties and functionalities have been altered using chemical. An alternative for the development of native starch substituents with desirable starch properties is the use of mutagenesis techniques to increase genetic variation in maize kernels. With this approach, a highly diverse library of native starches with different properties are produced. Traditional analysis of the functional and structural properties requires generous amounts of material as well as a time-consuming and costly breeding process to obtain enough kernels. To address this difficulty, high-throughput techniques are proposed for studying starch properties and functions which includes a 1) single kernel sampling method for the isolation of milligrams of starch, and techniques for studying starch based on functional properties, 2) retrogradation and 3) shear resistance, using low-volume low-concentration starch pastes.
First, three mechanical approaches were evaluated for the collection of endosperm samples from individual kernels: razor blade, 1.5 mm drill bit, and trephine bur. Furthermore, two methods for the isolation of crude starch from endosperm samples (steeping method and combination of proteases and sonication) were compared. In this study, the mechanical approaches were evaluated using the recovery rate, throughput, and germination rate of sampled kernels. Moreover, yield determination, particle size distribution, and morphological evaluation using a light microscope were performed on crude starch isolated from the endosperm samples. The use of trephine bur to collect endosperm samples and isolation of crude starch using protease digestion and sonication showed the best combination for a high-throughput setting.
Second, a high-throughput technique using milligram sample for the screening of retrogradation-resistant starch was evaluated by comparing two spectrophotometric techniques: turbidity method and molecular rotor (MR). MRs are fluorescent probes with high sensitivity to the viscosity of their environment, polarity of the media, molecular crowding, and free volume. After excitation, MRs relax through rotational movement and reduces the emission of fluorescence. In this study, hydroxypropylated waxy corn starch (WCS) and hydroxypropylated normal corn starch (NCS) were used and their retrogradation kinetics was compared with retrogradation kinetics of native WCS and NCS.
It was found that the molecular rotor 9-(2-carboxy-2-cyaovinyl)-julolidine (CCVJ) was effective to sense changes during slow retrogradation of amylose-containing starch pastes. Development of elastic modulus of retrograded NCS pastes obtained from dynamical rheology showed high correlation with the development of fluorescence intensity of the CCVJ. Furthermore, rate of retrogradation using fluorescence intensity was affected by the introduction of a retrogradation inhibitor, hydroxypropyl groups. Accelerated retrogradation of low-concentration WCS pastes was measured using the turbidity method and fluorescence intensity of CCVJ in a microplate. Accelerated retrogradation was performed by subjecting the low-concentration WCS pastes to six freeze-thaw cycles of -20 ºC for 1 hour and 30 ºC for 1 hour. Overall, development of turbidity resulted in the more sensitive technique to detect rate of retrogradation of amylopectin-containing starch.
The last part of this research studied the use of CCVJ as a technique to identify shear-resistant starch in starch slurries using milligram sample. For this purpose, WCS was cross-linked with sodium trimetaphosphate (STMP) and phosphoryl chloride (POCl3). Low-volume starch slurries having CCVJ were prepared ranging from 0.5% to 1% starch concentration in a 96-well PCR plates and subjected to heat and shear treatments. It was found that fluorescence intensity measured in native WCS pastes were the lowest. Furthermore, fluorescence intensity of the CCVJ in the gelatinized starch increased as the amount of cross-linker increased in the cross-linked WCS. After shear treatments, the same trend in fluorescence intensity increase was recorded in all the crosslinked WCS. Results obtained using fluorescence intensity were compared with rapid viscosity analyzer (RVA) and images from microscope. Results obtained from both techniques corroborated the findings using fluorescence intensity.
In general, the findings of this research provide new insights into the possibilities of developing a high-throughput screening platform of milligram starch sample based on their physical properties.