Overview of yam

Yam is a plant of the Dioscoreaceae family and is the fourth most important root crop after cassava, potato and sweet potato. Yam has developed rapidly in many parts of China and is exported to Russia, Japan, Korea, Southeast Asia and the European Union.

Yam is also widely grown in the tropics, in five coastal countries in West Africa, producing over 90% of the world’s output. Yam is rich in nutrients, containing starch, protein, amino acids and other nutrients. In addition, yams contain many bioactive components such as mucins, diosgenin, allantoin, choline, phytosterols, and oligosaccharides.

Fresh yam has a large moisture content and poor storage stability at room temperature, making it susceptible to browning, sprouting or rotting, with a serious loss of yam’s constituents, affecting quality and taste. The price of yam is also affected by the supply and demand, if there is no better preservation method, it will directly reduce the utilization rate of yam, therefore, dried yam powder and yam chips are the main product forms in the market, easy to store for a long time and easy to consume.

Yam (dry weight) consists of about 75%-84% starch, 6%-8% crude protein and 1.2%-1.8% crude fiber. Therefore, starch quality is the most important factor in yam product quality.

In the processing of other yam products, most of them have the peeling process, which produces a large amount of yam skin waste residue, which is easy to cause environmental pollution and resource waste.

Ma Zeyu et al. studied the nutrient composition as well as antioxidant activity of peeled yam powder; Guan Qianqian investigated the physicochemical properties and active components of peeled yam powder; and Liao Xiaoling determined the bioactivity content of Buddha’s hand yam powder obtained from peeling treatment.

In the reported literature, the determination of the relevant components and properties of yam after peeling has not been reported on the relevant properties of yam powder prepared without peeling, whereas yam skin contains a variety of bioactive components such as saponins, polysaccharides, polyphenols and flavonoids a, which affect the properties of yam powder.

Therefore, in this paper, the physicochemical properties and bioactivities of yam powders prepared by peeled and unpeeled pre-treatments were investigated in a more systematic manner, with a view to applying them in different food products according to the characteristics of different yam powders.

Results and analysis

1. The effect of peeling on the total starch content, fast-digestible starch, slow-digestible starch and resistant starch content, and water-binding capacity of yam flour.

The total starch (TS) content of non-peeled yam flour (NPYP) was less than that of peeled yam flour (PYP), which was due to the fact that the starch content in the skin of yam was lower than that in the equivalent mass of yam meat. The rapidly digestible starch (RDS) content of the two yam powders did not differ much.

The slow-digestible starch (SDS) content in NPYP was higher than that in PYP, which may be due to the fact that some substances in the skin increased the spatial resistance within the starch, which delayed the contact between the enzyme and the point of action within the starch, resulting in the slow-digestible property.

For the resistant starch (RS) content, PYP has a higher content, which is resistant to enzymatic degradation.The water binding capacity (194.53 g/100 g) of NPYP was significantly higher than that of PYP, which may be attributed to the higher availability of water binding sites in NPYP, which increases the water binding capacity, or to the high amount of cellulose in the skin, which has a high water-holding capacity, resulting in the higher WBC of NPYP.

2. Effect of peeling on swelling power and solubility of yam flour

Solubility indicates the mass fraction of starch sample dissolved at a certain temperature, and swelling power indicates the mass fraction of water absorbed per gram of dry basis mass of starch at a certain temperature.

The swelling power and solubility of yam flour increased continuously with increase in temperature. This is because as the temperature increases, water enters into the granules faster and the granules absorb water and swell, while causing the uncrystallized portion of the straight-chain starch to gradually dissolve in water due to heat, thus increasing the solubility of the yam flour.

At different temperatures, the swelling force and solubility of NPYP were greater than those of PYP; at 70°C, the swelling force of NPYP increased by 43%, and at 90°C, the solubility of NPYP increased by 18%. This may be due to the fact that the granules of NPYP were looser, resulting in higher solubility; when the starch pasting temperature was reached, the granules absorbed water and swelled more, resulting in higher swelling power.

3.Effect of peeling on the content of bioactive substances in yam powder

The total flavonoids and total soluble polyphenols contents of NPYP were higher, 0.059 mg RE/g and 0.028 mg GA/g respectively, which were significantly higher than that of PYP, this was due to the fact that the skin of yam is rich in flavonoids and other polyphenolic active substances, which would be lost after removing the skin.

7.Effect of peeling on antioxidant properties of yam powder
1) Reducing power of iron ions

The reducing power of NPYP was higher at 0.223mgVc/g, and its antioxidant activity was stronger compared to the yam powder obtained after peeling, which may be due to the fact that the skin is rich in polyphenols increasing the reducing power of the unpeeled yam powder.

2) DPPH – scavenging ability
The DPPH free radical scavenging capacity of NPYP was stronger and could play a better role as an antioxidant.

Conclusion.

In terms of starch related properties, the yam flour obtained after peeling process had a larger total starch and resistant starch content of 45.39% and 31.11% respectively, which is a good source for the preparation of hypoglycemic and hypolipidemic functional foods.

The yam powder without peeling process had higher water binding capacity and swelling power and higher solubility. The peeling process had little effect on the crystalline structure of the yam powder.

In terms of bioactivity, unpeeled yam flour contained higher total flavonoids and total soluble polyphenols, 0.059 mg RE/g and 0.028 mg GA/g, respectively, with higher DPPH-scavenging and reducing abilities, making it a potential source of antioxidant-functional foods, which are becoming increasingly popular today. We can target the application of different yam powders in different foods according to their characteristics.