Comparison of Effects of Oligosaccharides on Physicochemical Properties of Corn Starch

Purpose: To investigate the effect of different oligosaccharides on the physicochemical properties of corn starch. Methods: The blue value and retrogradation of corn starch were evaluated following the addition of different oligosaccharides and compared with control. Pasting properties, melting enthalpy and melting temperature were determined by rapid viscosity analysis (RVA) and differential scanning calorimetry (DSC). Results: Maltose, lactose and fructo-oligosaccharides significantly affected iodine binding ability of corn starch. Retrogradation of corn starch was significantly inhibited by maltose, xylooligosaccharide and fructooligosaccharides. RVA analysis showed that the addition of oligosaccharides increased the peak viscosity of corn starch (from 3180 cP to 3687 cP) but decreased that of corn starch (from 896cP to 625 cP). DSC analysis showed that there was a large reduction in recrystallisation of corn starch by oligosaccharide. The samples containing xylooligosaccharide and fructooligosaccharides showed lower melting enthalpy (142.7 J/g and 143.5 J/g, respectively) than that of control (209.4 J/g), and their onset temperature (-10.9 °C and -8.89 °C, respectively) were also lower than that of control (-5.92 °C). In addition, the melting enthalpy and melting temperature of the samples containing oligosaccharides also showed lower values than those of control during storage. Conclusion: The finding that oligosaccharides modify the physicochemical properties of corn starch would be of practical importance in broadening their application prospects of the starch in the food and pharmaceutical industries.


INTRODUCTION
Starch constitutes the major carbohydrate in the corn endosperm. Corn starch serves as a multifunctional ingredient to the food or nonfood industries [1][2][3]. Corn starch is widely used as a thickener, gelling agent, bulking agent or water retention agent [4][5][6]. When heated, starch usually gelatinizes fully or partially and can be transformed from semi-crystalline granular material to an amorphous paste, or to a system containing granular remnants [7,8]. In the presence of sufficient water, starch granules swell and lose their crystallinity during gelatinization until they are completely dissolved [9]. The influence of plasticisers such as water or sugars on the recrystallisation of gelatinized starch is important for the processing of starchbased foods and its application in pharmaceutical industries [10].
Today, food innovation is built on the concept of functional foods, which is based on the use of nutrition knowledge in the food industry with the aim of improving human health [11]. Oligosaccharides are carbohydrate chains containing 2-10 sugar units [12]. Some authors have also described oligosaccharides as carbohydrates with up to 20 residues. It was reported that some oligosaccharides might have anticarcinogenic effects [13]. The Indigestible oligosaccharides, such as fructooligosaccharides and xylooligosaccharide, named as prebiotics, can promote the growth of bifidobacteria and lactobacilli. Besides, the health benefits of indigestible oligosaccharides are similar to those of fermentable dietary fiber and resistant starch [14][15][16].
In recent years, oligosaccharides have been widely used in the food and pharmaceutical industries. The objective of this paper was to investigate the effects of several oligosaccharides on the physicochemical properties of corn starch, including iodine blue value, retrogradation, pasting properties, water activity and melting properties, in order to provide some theoretical references for the application of oligosaccharides in the food industry and medicine.

Materials
Corn starch (amylose content, 25.43 %) was purchased from local market. Sucrose, maltose and lactose were purchased from Tianjin Kemiou Chemical Reagent Co., Tianjin, China. Fructooligosaccharides (FOS) and xylooligosaccharide (XOS) were purchased from Tianrun Biological Technology Co., Henan, China. Other reagents and chemicals used were of analytical grade.

Iodine binding test
The blue value (BV) was measured in triplicate according to the method of Zeng et al [17]. The addition of oligosaccharide was based on the weight of starch, viz, 0.1, 0.2, 0.3, 0.4, 0.5 g per 1 g starch, respectively. The data were averages of three replicates of each starch sample. The sample without oligosaccharide was used as the control (ck).

Retrogradation test
A corn starch suspension (1 %) was heated at 95 °C and maintained for 30 min in a thermostat water bath (Changcheng Instruments Co, Ltd, Zhengzhou,China), followed by a natural cooling to room temperature (about 25 °C). The starch paste was transferred into a 100 mL graduated cylinder. Starch retrogradation property was calculated by supernatant volume (SV) after standing for 24 h. The addition of oligosaccharide was based on the weight of starch, viz, 0.1, 0.2, 0.3, 0.4 and 0.5 g oligosaccharide per 1 g starch. The data are mean of three replicates of each starch sample. The sample without oligosaccharide was used as control (ck).

Pasting test
Pasting properties of the corn starch were determined by using a Rapid Visco Analyzer (RVA Ⅳ, Newport Scientific Pty. Ltd., Warriewood, Australia). A rotation speed of the paddle of 160 rpm was used except that the paddle speed was set at 960 rpm in the first 10 s. Each starch suspension (3 g corn starch, 1 g oligosaccharide and 25 g distilled water) was equilibrated at 50 °C for 1 min, heated to 95 °C in 3.7 min and maintained at 95 °C for 2.5 min. The sample was then cooled to 50 °C in 3.8 min and maintained at 50 °C for 2 min.

Differential scanning calorimetry (DSC)
Gelatinized starch samples prepared according to the above pasting properties method. Starch pastes (containing 3 g corn starch, 1 g oligosaccharide and 25 g distilled water) were stored in a refrigerator at 4 °C for several days (0, 2, 4, 6, 8 d); 5 mg paste was taken and scanned in a differential scanning calorimeter (DSC, Q200, Thermal Analysis Instruments, USA) instrument. Cooling rate was 5 °C per min over the temperature range of 20 to -45 °C and then heated to 30 °C. An empty pan was used as the reference standard. Melting enthalpy (△H), melting onset temperature (To), melting peak temperature (Tp) and melting conclusion temperature (Tc) were measured. The data were recorded as the mean of triplicate measurements for each starch sample. Generally speaking, the greater the SV is, the stronger the retrogradation is. On the other hand, the smaller the SV is, the weaker the retrogradation is, that is, the more slowly the starch ages. From Fig 2, a higher rate of retrogradation was observed in control sample. The SV decreased significantly with the increase of maltose and XOS addition. The samples with FOS also showed lower SV than that of the control. The SV of samples with sucrose and lactose slightly decreased at first and then increased a little.  Table 1 summarized their characteristics. All the oligosaccharides increased the peak viscosity of corn starch with the highest value observed in the sample containing maltose. In addition, the setback of all the samples decreased and the lowest was observed in the sample with sucrose.   However, breakdown of all the samples were higher than that of control except for sucrose sample. The peak time was postponed after adding oligosaccharide while the pasting temperature increased.

Melting transitions
DSC has been widely used to investigate thermal transitions in materials. From Fig 4, the melting enthalpies of corn starch gels with oligosaccharides were lower than that of the control after storing 3 days or 6 days at 4 °C. In other words, the oligosaccharides prevented starch molecules to form hydrogen bonds with one another and hence the hydroxyl groups are free to form hydrogen bonds with water molecules. Moreover, the enthalpies of the samples with sucrose, XOS and FOS increased with the increase of storing time. The enthalpy of the samples with maltose slightly went down during storage. Among the five kinds of oligosaccharides, XOS and FOS showed better anti-aging effects on corn starch because their melting enthalpies were lower than that of other samples. These results indicated that there were larger reductions in recrystallisation of starch molecules when XOS and FOS were added in corn starch. and Tc values of all samples with oligosaccharides were lower than that of the control. The To value of samples with sucrose, maltose, lactose and FOS decreased after storage 3 days, but slightly increased after storage 6 days. After storage for 3 days, the Tc values of corn starch gels with oligosaccharides were higher than that of the fresh gels (Fig 5, 0 d) except samples with sucrose. However, the Tc of all samples with oligosaccharides storage 6 days decreased more than that of storage 3 days. Besides, the To and Tc of samples with XOS and FOS were relative lower than that of These results indicated that the interaction between oligosaccharides and water built a more complex system, thus reduced the freezing point and the melting temperature of the system. Among the five oligosaccharides, XOS and FOS exhibited obvious effects in particular.

DISCUSSION
In the present study, oligosaccharides were used to improve some properties of corn starch. The results obtained indicate that oligosaccharide interfered with iodine binding behavior of corn starch. Retrogradation test also indicates that retrogradation of corn starch can be inhibited by maltose, XOS and FOS. It is well known that the structural arrangement of starch chains within the amorphous and crystalline regions of the ungelatinized granule indirectly influences the retrogradation properties of starch gels, and influenced the extent of granule breakdown during gelatinization and the interaction that occurred between starch chains during gel storage [18]. In this light, the addition of oligosaccharide might affect the structural arrangement of starch gels.
When corn starch paste was stored for a period of time, part of the combined water with starch during pasting would be free from the starch molecules because of starch's recrystallization. As a result, the free water in the starch gel increased. The more the amount of free water is, the greater the melting enthalpy becomes, and the faster the recrystallization rate of starch is. DSC analysis showed that the oligosaccharides decreased the melting enthalpies and melting temperature of corn starch. It was reported that the melting temperature and enthalpy depended on crystal size. In particular, crystal size decrease reflects in melting temperature/enthalpy decrease [19]. From this point of view, the samples with XOS and FOS were easier to form small crystal. Moreover, one of the most important consequences of the crystal melting temperature and enthalpy decrease is the increase in crystal solubility [20]. Therefore, it is important for the solubility of food or medicine containing oligosaccharides. Hence, the interaction between oligosaccharides and water contributed to the reduction in the melting enthalpy and melting temperature of retrograded starch gels which indicated that the samples with XOS and FOS more easily formed small crystals and improved solubility.

CONCLUSION
A certain amount of oligosaccharides can affect the physicochemical properties of corn starch. Among of the five kinds of oligosaccharides, XOS and FOS exert significant effects on iodine binding behavior, melting enthalpy and melting temperature. These results indicate that XOS and FOS have better anti-aging effects, and to some extent, these properties can help to meet various needs in the food and pharmaceutical industries.