The study was conducted in the laboratories of the Department of Food Engineering & Technology, Food Science & Nutrition, Food Processing & Preservation and Agricultural Chemistry and Biochemistry, Hajee Mohammad Danesh Science and Technology University, Dinajpur, Bangladesh. Fresh sweet potato: Fresh Sweet potato of white variety was collected from Local market. The Sweet potatoes were washed with clean water to remove dirt, sand and other undesirable materials before use.
Wheat flour: Commercial wheat flour of 'Pata' brand (12-13% moisture and 8-9%protein) was used for biscuit preparation.
Chemicals, solvents and ingredients: Several chemicals and ingredients were used. Wheat flour (12-13% moisture and 8-9% protein) was commercial white flour, eggs, sugar, baking powder, shortening (dalda), sweet potato flour, salt, and other ingredients were procured from the local market. High density polyethylene bags were used for package and storage of samples. Other minor ingredients were used from laboratory stocks.
Preparation of Sweet potato flour The sweet potato was processed to remove dirt and other field damaged portion. Cleaned sweet potato were chopped into small pieces with knife and blanched in hot water at 1000C for three minutes containing 2% salt and 0.01% citric acid. Then water was drained and sweet potatoes were spread in trays and dried in a sun up to 5-10% moisture level at 32°C-330C for 2 days and also dried in oven at 65°C, 16 hours. After cooling to room temperature, the dried sweet potatoes were ground into flour in a grinder. Then they were sieved and packaged in polythene bags and stored at room temperature for further use in the preparation of biscuit. Finally, it was obtained four categories of sweet potato flour as follows: 1) Sun drying with peel sweet potato flour 2) Sun drying without peel sweet potato flour 3) Oven drying with peel sweet potato flour 4) Oven drying without peel sweet potato flour.
Chemicals analysis of sweet potato flour and biscuits: The fresh sweet potato, sweet potato flour and biscuit prepared by incorporating sweet potato flour were analyzed for moisture, protein, fat, ash, crude fiber, total sugar, titrable acidity, and pH and carbohydrate contents. The moisture, protein, fat, ash contents were determined as per the methods described Rangana (1994), AOAC (2000). The carbohydrate content was determined by subtracting the measured moisture, protein, fat, ash from 100 (Pearson 1976). All the determinations were done in triplicate and the results were expressed as the average values.
Product development Formulation of biscuit incorporating sweet potato flour: The basic formulation of sweet potato biscuits (Atkins, 1971) is in table1. The additions of wheat flour in the formulations were made with 10%, 20% and 30% of sweet potato flour.
Biscuit making procedure: Biscuits were prepared by wheat flour with 10%, 20% and 30% of different sweet potato flour samples in the basic formulation of biscuit (Table 1) as per method of Rajchel et al. (1975). The flour, sweet potato flour and other ingredient for each biscuit were weighed accurately and the sugar and shortening were mixed in a mixing machine for 20 minutes to produce a cream. In the next stages, other ingredient, and flour were mixed using at low speed (145 rpm) for 10 minutes to ensure even distribution of the components. The bowl was scrapped and butter were mixed for an additional two minutes at medium speed (250 rpm). Then the designer was designed biscuits. The biscuit were kept in tray and were baked in National forced convection oven for 40 minutes at 1700C.
Chemical analysis of biscuit containing Sweet potato flour: The biscuit containing 10%, 20% and 30% of sweet potato flours were analyzed for moisture content, protein, ash and crude fat as per the methods of AOAC (1975). Carbohydrate content of the biscuit samples was determined as total carbohydrate by difference method. All determinations were done in triplicate and the results were expressed as the average value.
Evaluation of biscuit by objective analysis: Biscuit volume was initially used as an important parameter of biscuit quality. The biscuit volume was determined by seed displacement method (Ott, 1987). Moisture content was determined according to the methods outlined in AOAC (1975). The weights and specific volume of baked biscuit were also measured.
Subjective (sensory) evaluation of biscuit: The symmetry and the characteristics of crust and crumb of the biscuits supplemented with bran samples were evaluated and recorded. Biscuits were evaluated organoleptically for color, flavor, texture, and overall acceptability. A 1-9 point hedonic rating test was also performed to assess the degree of acceptability of biscuit containing sweet potato flour in different level. Biscuits were presented to panelists randomly as coded samples. The panelists were asked to rate the sample for color, flavor, texture, and overall acceptability on a 1-9 point scale, where 9=Like extremely; 8=Like very much; 7= Like moderately; 6=Like slightly; 5= Neither like nor dislike; 4=Dislike slightly; 3= Dislike moderately; 2= Dislike very much; 1=Dislike extremely. The results were evaluated by analysis of variance and Duncan?s New Multiple Range Test Procedures of the Statistical Analysis System (SAS, 1985).