1. INTRODUCTION:

Roots and tubers have a wide range of phytochemical components with antioxidant properties which includes phytic acids, flavanoids, saponins and polyphenols. Several epidemiological and clinical studies revealed that the diet rich in vegetables are associated with lowering the incidence of cardiovascular, neurological, degenerative diseases and metabolic disorders^[1].Due to the perishable and seasonable nature, fresh vegetables are subjected to some form of preservation in order to increase shelf life and make them available for later consumption^[2]. Therefore, blanching (pre-treatment) of vegetables is generally regarded as superior food preservation techniques, with respect to retention in sensory attributes and nutritive properties^[3].

It is one of well-established food processing techniques that is typically applied on fruits and vegetables prior to freezing, canning and drying for the purpose of inactivating naturally occurring enzymes which may cause undesirable changes during storage, reducing bacterial content, modifying texture, preserving colour, flavour, and nutritional value^[4].

On the other hand, drying is the traditional and most effective method of food preservation to reduce the water activity of food in order to prevent decay and food spoilage by microorganisms. It also improves the keeping quality of food which may be achieved by sun or using hot-air oven^[5].

Jerusalem artichoke (Helianthus tuberosus L.) belongs to Asteraceae family is commonly known as sunchoke or sunroot^[6]. It is a perennial plant species of sunflower native to temperate regions of North America^[7]. It has tall stem, large leaves, bright yellow sunflower-like flowers and fleshy tubers similar to potatoes. It contains high amount of protein, fibre, ash, vitamins and minerals^[8]. The tuber contains excellent amount of inulin and fructose-oligosaccharides with negligible quantity of starch, which is digested by Bifidobacterium and stimulates the growth of their colonies^[9]. Inulin is a plant polysaccharide that includes all straight-chain fructans consisting of fructosyl units linked by β-D(2-1) glycosidic bond^[10]. It is functional food “ingredient” and can be used as a prebiotic agent, which is associated with expanding the bioavailability of minerals, increase activity of beneficial bacteria and inhibition of pathogenic bacteria in the digestive tract. It also ease the digestion of high protein diets, delay fat absorption, deliver roughage preventing constipation, increase satiety value without having extra calories which results in lowering blood glucose, cholesterol and triglycerides levels^[11]. It also contains high amount of bioactive compounds such as coumarins, sesquiterpenes, flavonoids, phenols, polyacetylenes and their derivatives with various pharmacological activities, such as anti-inflammatory, antimicrobial, anti-diabetic, anti-obesity and anticancer activities^[12,13]. Hence, in the light of the above research facts, the present investigation was undertaken with the objective to evaluate the impact of blanching and drying methods on inulin and sugars content of Jerusalem artichoke tuber.

2. MATERIALS AND METHODS:

2.1. Collection of plant material:

Freshly cultivated Jerusalem artichoketubers were collected from Indian Institute of Vegetable Research (IIVR), Varanasi (U.P.). The tubers were selected considering the absence of any visual damage and infection as well as the uniform size, colour and maturity.

2.2. Preparation of experimental sample of Jerusalem artichoke tuber (JAT):

Sorted and cleaned tubers were stump trimmed off and weighed. The tubers were peeled, washed and cut into thin slices and divided into two lots. First lot was used for the analysis of unprocessed and second lot was subjected to hot water at 95ºC±2 for blanching and cooled at room temperature. One portion of blanched tubers was subjected to drying by using hot-air-oven at 55ºC for 12-14 hours and another left portion of blanched tubers were exposed directly to sun drying at 34ºC±2 for 48 hours. The processed powder was prepared in this following manner;

Figure 1: Flow diagram of development of processed tuber powder using blanching and drying methods

2.3. Determination of inulin:

2.3.1. Free fructose content (F_f):

150µl tuber extract was pipetted into 10ml volumetric flask containing 20mmol L^-1 citrate buffer 6 (5ml) and diluted with water up to 10ml. After 5 min, 150µl of 100 mmol L^-1 potassium iodide was added, and mixture was left for an additional 5 min. The absorption of solution was measured at 350nm using a UV-Vis spectrophotometer.

2.3.2. Total fructose content (F_tot):

0.50ml of tuber extract was acidified with HCL (0.2 mol L^-1) in a final volume of 25ml and kept for acid hydrolysis at 97±2ºC for 45 min. The solution was then adjusted to 7 pH with NaOH before dilution with water to 25ml. The absorption of neutral hydrolysate was measured at 350nm using a UV-Vis spectrophotometer.

The inulin content was calculated using the following equation:

I = k(F_tot-F_f)

Where I is inulin content, F_tot is total fructose content, F_f is free fructose content, k is 0.995, it is a correction factor for the glucose part of water and inulin loss during hydrolysis^[14,15].

2.4. Determination of sugars:

2.4.1. Reducing sugar:

The reducing sugar was estimated by Di-Nitro Salicylic Acid Method (DNS)^[16]. 100mg of tuber powder was weighed and extraction of sugars was done by using 80% hot ethanol twice (5ml each time). Supernatant was collected and evaporated by keeping it on water bath at 80ºC. 10ml of water was added to dissolve the sugars. 0.5-3ml of the extract was pipetted out in test tube and the volume was equalized up to 3ml with water in all the tubes. 3ml of DNS reagent was added and content was heated on a boiling water bath for 5 min. When the contents of the tubes were warm, 1ml of 40% Rochelle salt solution was added. The content was cooled down and the intensity of dark red colour was read at 510nm.

2.4.2. Total Sugar:

The total sugar present in tubers was estimated by phenol-sulphuric acid method^[17]. 100mg of tuber powder was weighed into a boiling tube and was hydrolysed by keeping it on a boiling water bath for three hours with 5ml of 2.5N HCL, and cooled down at room temperature. After, content was neutralized by adding solid sodium carbonate, until the effervescence ceases. The volume was made up to 100ml and the process of centrifuge was done. 0.2, 0.4, 0.6, 0.8 and 1ml of the working standard was pipetted out into a series of test tubes and two separate test tubes in which 0.1 and 0.2ml of sample solution was pipetted out. The volume was made up to 1ml with water in each test tube. A blank was set with 1ml of water. 1ml of 5% phenol solution was added to each tube. Then, 5ml of 96% sulphuric acid was added to each tube and shake well. After 10 min, the content in the tubes was shaken and placed on water bath at 25-30ºC for 20 min. The intensity of colour was read at 490nm.

2.4.3. Non-reducing sugar:

Non-reducing sugar were estimated by subtracting total sugar from reducing sugar.

Non-reducing sugar = Reducing sugar -Total sugar

2.5. Statistical analysis:

The results obtained were expressed as Mean±SD and studentt-test of three determinations and also statistically analysed to ascertain its significance at p˂0.05 levels.

3. RESULTS AND DISCUSSION:

Table 3.1: Inulin content of Jerusalem artichoke tuber flours blanched with different drying methods on dry weight basis

Soluble fiber

(g/100ml)

Unblanched-JATF

Blanched-JATF

Sun-dried

Oven-dried

Inulin

23.29±0.16

21.53±0.07^*

(7.29%↓)

19.38±0.26^*a

(18.16%↓)

JATF: Jerusalem Artichoke Tuber Flour. Values are Mean±SD of triplicate determinations.

^*denotes significant difference when compared with unblanched-JATF at p<0.05 level.

^adenotes significant difference when oven dried blanched-JATF compared with sun dried blanched-JATF.

Table 3.1 shows inulin content (g/100ml) of dried Jerusalem artichoke tuber flours prepared using blanching and drying methods. The data showed that inulin content of unblanched-JATF was 23.19±0.16. This data agrees with the study who reported that raw Jerusalem artichoke tuber contained 21.46g/100g of inulin content^[18]. Blanching resulted significantly decrease in inulin content of both sun-dried (21.53±0.07) JATF and oven-dried JATF (19.38±0.26) by 4.98% and 18.16% respectively at p<0.05 level when compared to unblanched-JATF. Similarly, the study revealed that blanching reported significantly decrease in inulin content (26.14±0.87g/100g) when compared with unblanched Jerusalem artichoke tuber powder (33.81±1.44g/100g)^[19]. The loss of inulin might be due to its solubility in hot water^[20].

Table 3.2: Sugars of Jerusalem artichoke tuber flours blanched with different drying methods on dry weight basis

Sugars

(g/100g)

Unblanched-JATF

Blanched JATF

Sun-dried Flour

Oven-dried Flour

Reducing Sugars

0.63±0.61

0.54±0.20^*

(14.28%↓)

0.39±0.53^*a

(38.09%↓)

Total Sugars

5.94±0.31

4.82±0.21^*

(18.85%↓)

4.57±0.29^*a

(23.06%↓)

Non-reducing Sugars

5.31±0.45

4.28±0.17^*

(19.39%↓)

4.18±0.24^*a

(21.28%↓)

JATF: Jerusalem Artichoke Tuber Flour. Values are Mean±SD of triplicate determinations.

^*denotes significant difference when compared with unblanched-JATF at p<0.05 level.

^adenotes significant difference when oven dried blanched-JATF compared with sun dried blanched-JATF.

Sugars content in terms of reducing sugars, total sugars and non-reducing sugars of unblanched-JATF were 0.63±0.61, 5.94±0.31 and 5.31±0.45g/100g respectively as shown in Table 3.2. Concerning reducing sugars, total sugars and non-reducing sugars, blanched sun dried JATF had 0.54±0.20, 4.82±0.21 and 4.28±0.17g/100g whereas, blanched oven dried JATF had 0.39±0.53, 4.57±0.29 and 4.18±0.24g/100g respectively. By the data,it has been shown that blanching resulted significantly decrease in both reducing sugars, total sugars and non-reducing content in sun-dried JATF by 14.28, 18.85 and 19.39% and for oven dried JATF by 38.09, 23.06 and 21.28% respectively at p<0.05 level when compared with unblanched-JATF. The comparable study stated that reducing sugars (14.03±0.07mg/g)) and total sugars (758.67±3.82mg/g) of blanched sun dried Jerusalem artichoke tuber were significantly reduced when compared with unblanched tuber^[21]. Likewise, the study reported by^[22] that the reducing sugars content of blanched potato cubes was also decreased by 51% and 54% for 3 minutes. Similarly, the boiled root celery had reduced total sugars (500±0.01mg/100g), reducing sugars (250±0.02mg/100g) and non-reducing sugars (240±0.01mg/100g) when compared with unboiled root celery^[23]. The loss of sugar during thermal treatment is associated with leaching of some soluble compounds into the water.

4. CONCLUSION:

The outcomes of the study concluded the fact that unblanched Jerusalem artichoke tuber flour had high inulin and sugars content. However, inulin and sugars content of tuber were significantly decreased after blanching. Though blanched sun dried tuber flour showed minimum loss when compared to blanched oven dried flour. The loss of these nutrients is associated with leaching of soluble compounds in hot water by diffusion as well as high temperature in the oven. Hence, blanching with sun drying method could be the best and superior method of preserve the loss of various nutrients.

5. ACKNOWLEDGEMENT:

Authors are thankful to Prof. Aditya Shastri (Vice Chancellor) of Banasthali Vidyapith for providing all the required lab facilities in the department of Food Science and Nutrition that helped us for the successful completion of the project work.

6. CONFLICT OF INTEREST:

We declared that we have no conflict of interest.

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Received on 15.09.2019 Modified on 21.11.2019

Research J. Pharm. and Tech. 2020; 13(7): 3143-3146.

DOI: 10.5958/0974-360X.2020.00555.7