Tiger nut (Cyperus esculentus L.) is an edible perennial grass-like C₄ plant of the sedge family (Turesson et al., 2010). Tiger nut is tuber usable grass and also called chufa, nut grass, yellow nut sedge, earth almond, edible galingale and ground almond (Defelice, 2002; Sanchez-Zapata et al., 2012). It is widely used for human and animal consumption as a nutritious food and feed in Africa, Europe and America (Sanchez-Zapata et al., 2012).

Tiger nut is rich in starch, oil, minerals, and  vitamins  E and C. The starch and oil are major macronutrients in the tiger nut tuber. High starch content of this plant provide unique functional properties (Manek et al., 2012), cold storage stabilities, and preserves organoleptic properties of foods (Jing et al., 2012). The tiger nut oil also has high monounsaturated fatty acids, similar to olive, avocado and hazelnut oil (Ezeh et al., 2014). These monounstaturated oil has high unsaponifiable matter, phospholipids and other bioactive compounds such as tocopherols,   phytosterols   and   polyphenols  (Sanchez-Zapata et al., 2012; Ezeh et al., 2014). Although tiger nut oil fatty acid profile is similar to olive oil, nut oil has unique gold-yellow color, neutral taste properties, high in phytosterols (Sanchez-Zapata et al., 2012), and better deep frying stability (Sanchez-Zapata et al., 2012; Lasekan and Abdulkarim, 2012) .The nutritional profiles and unique functional properties have made tiger nut as unique food (Ekeanyanwu and Ononogbu, 2010) like beverage, flour (Oladele and Aina, 2007; Chinma et al., 2010), edible oil (Muhammad et al., 2011; Lasekan and Abdulkarim, 2012), and a feed source (Sanchez-Zapata et al., 2012).Although there are numerous example of plants that accumulate high amount of starch or sugars in roots and tubers, tiger nut accumulates a substantial amount of oil in such tissues. As an high oil yield and more adaptable crop, tiger nut have more potential usage as food and industrial materials. Current research focused on functional properties, organoleptic properties, biochemical actives, oil extraction and nutritional value of tiger nut but the information on changing nutrient patterns during its growing cycle is meager. Therefore, the current study was carried out to determine macronutrient profile changes of the field grown tiger nut plant parts during its full growth cycle.

Tiger nut tubers were provided by the Karamay Huili CO., LTD (Karamay, Xinjiang, China). Before planting, the tubers were rinsed with tap water for 2 h, and then soaked at 45°C for 24 h changing the water in the middle. Then it was drained out for backup. After that the tubers were planted with spacing 20×40cm under the condition of sandy soil. The germination appeared after planting of 10days. It was watered at 10 days interval according to their real growth status. After the growth of 60 days, the tiger nut was harvested at 3 days interval. After harvest, the tubers, root and leaves were separated and wrapped in aluminium foil and stored at -80°C.

Instrument and equipment

Soil shovel, magnetic stirrer, resistivity meter, PH Meter (HANAHI98183), paraffin pan, electric oven blast, analytical balance, dryer, weighing bottle, Soxhlet extractor, constant temperature water bath, formwork units, Kjeldahl flask, adjustable electric, water bath, grinder, oven, electronic balance (0.0001 g), conical flask (250ml), volumetric flask, reflux device, acid burette, filtration device and microwave were used in this study.

Preparation of sample

Parts of tiger nuts was slowly thawed and then oven dried, and splintered with grinder.

Analyses of tiger nut

Measurement of soil physical and chemical properties

The soil physical and chemical properties were measured according to the method of Bao (2000). The quantitative analysis of water, fat, total protein, starch and sugar were determined according to the method of Jing (2012) and Nielsen (2010) without modification.

Statistical analysis

The data was replicated three times, and the mean data analyzed by statistical package for the social sciences (SPSS) 17.0. 

Protein content (%)

The protein content was significantly different between the leaves and tubers of the tiger nut (Figure 3). Initially, the protein content of the leaves was lower until 111 days (0.11±0.00) but after that, it increased until 120 days (2.19±0.02). In case of tuber, there was an increasing trend until 152 days (1.11±0.00) and after that, it decreased gradually with the advancement of growing periods.

Starch content (%)

There was an increasing trend of starch content at different parts of the tiger nut among the leaves, roots and tubers, but the increasing values were irregular among the leaves, roots and tubers (Figure 4). The highest content of starch was recorded before harvest, and the values were lower among the leaves, roots and tubers after harvest.

Sugar content (%)

The increasing trend of reducing sugar content of tiger nut was not very clear between leaves and tubers fluctuation  was  observed.  This  phenomenon  might  be due to their irregular changes during the growing of middle time. The highest reducing sugar content (6.99±0.57) was recorded in tubers at 155 days whereas the lower one was recorded (5.73±0.27) for leaves (Figure 5).

However, the starch content of the root was higher as compared to oil content.

Tiger nut tuber development consists of at least two phases. In the first phase, root elongation ceased, and its apex started to swell and eventually developed into a tuber (Turesson et al., 2010). This development is similar to potato tuber development (Viola et al., 2001). For tuber nutrition content changes, starch and oil content accumulation pattern were same with aeroponic system (Turesson et al., 2010). The starch content increased before the 142 day with the highest level of 49% and after that day it decreased with the increasing of growing period (Figure 8). However, tuber oil content gradually increased in entire growth cycle and attained the highest level of 30% during harvesting (Figure 8).

The sugar content pattern was different from aeroponic system due to different sampling way. In the field growing system, although at the beginning sampling was same with aeroponic system, different development phase tuber occurred at the same time. So, the sugar content of the tuber was increased until the 152th days and after that, the decreasing trend was observed (with high sugar content at the early phase and low sugar content at the later phase). The protein content of tuber was very low, and remained at the same level in whole life cycle than other nutrient content (Figure 8) (Turesson et al., 2010).

For moisture content, tiger nut’s leaf and tuber moisture content reduced from the beginning. However, the moisture content of roots increased before the 85th day and then began to reduce (Figure 1). Although the highest moisture content recorded were 82.25, 77.49 and 79.31% respectively for leaves, roots and tubers, the moisture content in tuber was higher than in leaves and root (Figure 1). 

The moisture content of the tiger nut decreased throughout its growth cycle with time with different pattern in different organs. For leaves, moisture content was decreased until harvest time. And it was gradually decreasing before 90th day, and sharply decreasing after that day. The starch content was increased with reducing of fat content. There was no changing trend in sugar content, but protein content was increased around 115^th day, and remaining in that level until harvest. The maximum starch, fat, total sugar, and protein percentages were 26.4, 9, 6.4, and 2%, respectively.

For root, moisture content increased till 90th day, and decreased until harvest time. The oil content showed upheaval features with the rapid increase before 100th day with the maximum value of 8% and gradually was decreased till harvest time. The starch content changed with irregularities, but always kept higher than oil content. Tuber, starch and oil content increased with the increasing of growing time but in the middle of their growth cycle, the changes were irregular. Reducing sugars and protein content of the tuber also showed no significant change. The   maximum   starch,   fat,   total   sugar,   and  protein percentages were 49, 30, 7 and 1.1%, respectively. For tuber optimum yields, it is recommended to harvest the plant at 142nd day for starch. The delayed harvesting may lead to increase in oil content while reducing its total starch contents.

Tiger nut has the potential to provide novel information that can significantly widen understanding about the synthesis of storage reserves, regulating and directing into specific tissues and organs. This kind of information is valuable for work aimed at either increasing the oil content in presently used oil crops or searching for best ways for oil accumulation in organs or tissues that normally do not store oil. Because of this unique nutritional composition of different plant parts, it has the potential to become a model plant to study oil accumulation in non-seed tissues.

The authors are grateful to the financial support of the National Science Foundation of China- A Mutual Fund Project in Xinjiang (No. U1303103), Program from the Chinese Ministry of Education to Promote Research Collaboration with the United States and Canada and High-Level Personnel Training Project. We thanks to the Karamay Huili CO., LTD for supplying tiger nut tubers. Their sincere thanks go to Molla Mohammad Mainuddin, Ph.D. student, College of Food Science and Nutritional Engineering, China Agricultural University (CAU), Beijing, China and other Ph.D. students for their valuable suggestions and technical support.  

The author(s) have not declared any conflict of interests.