5. PHYSICOCHEMICAL PROPERTIES OF AMADUMBE STARCH SUBJECTED
5.2 Materials and Methods
This section contains all materials and methods used to determine the impact of different storage conditions on the quality properties of amadumbe starch.
5.2.1 Post-harvest sample preparations
The tubers were harvested after ten months. The amadumbe tubers were then washed using tap water and allowed to dry at ambient conditions. The corms with defects and harvest injuries were discarded immediately after drying.
5.2.2 Data Collection
Evaluation of all physicochemical properties of amadumbe starch was done at the Food Science and Agricultural Engineering Laboratory, University of KwaZulu-Natal, South Africa.
5.2.3 Experimental design
Three experimental designs were used in this study. The difference between experiments 1 and 2 was due to financial constraints. Experiment 3 differed because temperature becomes an additional treatment when calculating the swelling power and solubility of amadumbe starch.
5.2.3.1 Experimental design 1
The experimental design to determine starch yield, water holding capacity, and oil holding capacity was conducted using a Completely Randomised Block Design. The experimental design had two factors (storage conditions and storage period). Sampling was done every 14th day for 70 days using three replications (The graphical illustration of the experimental design can be found in Figure 3.2 of Section 3.2.3).
129 5.2.3.2 Experimental design 2
The experimental design to determine morphology was similar to Experimental design 1.
However, sampling was only done on days 0 and 70 (The graphical illustration of the experimental design can be found in Figure 4.1 of Section 4.2.3.2)
5.2.3.3 Experimental design 3
The experimental design to determine swelling power and solubility was conducted using a Completely Randomised Block Design. The experimental design had three factors (storage conditions, storage period, and temperature). Sampling was done every 14th day for 70 days using three replications (The graphical illustration of the experimental design can be found in Figure 4.2 of Section 4.2.3.3).
5.2.4 Starch extraction and yield
The method used by Naidoo et al. (2015) was used with minor modification to extract the starch. The amadumbe were brought to the laboratory for analysis immediately after harvest.
The 400g tubers were washed, peeled, chopped into small pieces, and then milled using a blender. The resultant pulp was dispersed in water (1:5), stirred at room temperature for five hours. A double cheesecloth was used to filter the non-starchy components. The resulting filtrate was allowed to settle at room temperature for 48 h. The supernatant was decarded, and the remaining residue was oven-dried at 35Β°C for 48 h. The dried residue was sieved (screen size: 180 ΞΌm) to obtain starch. The starch yield was calculated as the ratio of the starch obtained to the weight of the tuber used (Equation 5.1).
SY = Wππππ
Wπ‘π‘ x100 (5.1)
where
SY = Starch yield (%),
Wfa = weight of starch obtained after drying (g), and Wt = weight of the tuber used (g).
130 5.2.5 Amadumbe starch granule morphology
The granule morphology was determined using the method used by Ngobese et al. (2017).
Granule morphology was determined using a scanning electron microscope (ZEISS EVO LS15; Carl Zeiss Microscopy, United States of America), set at a magnification of 4.50 KX with signal A at SEI, I Probe = 59 pA, and EHT = 20.00kV. The starch was splashed on a double-sided silver tape attached to a 10 mm diameter specimen stub. The samples were coated with gold, using an ion sputter coater Q150 ΠS; Quorumtech, United Kingdom). The obtained granules from scanning electron microscope were then subjected to image analysis (Soft Imaging System GmbH; Olympus America Inc, United States of America) at the Microscopic analysis laboratory (University of KwaZulu Natal, Pietermaritzburg, South Africa).
5.2.6 Amadumbe starch swelling power and solubility
The swelling power and solubility were determined using the method used by Chisenga et al.
(2019c) with minor modifications. The swelling power and solubility were determined at 50, 60, 70, 80, and 90Β°C. A 0.5 g dry starch sample was suspended in 20 mL deionised water in a 50 mL centrifuge tube with a known weight. The centrifugal tube was heated using a water bath (Faithful FWS-30; Huanghua Faithful Instruments Co., Ltd, China) at 50, 60, 70, 80, and 90Β°C for 30 min and spiralling was done every 5 min. The centrifugal tube was allowed to cool to ambient temperature and centrifuged (Avanti J-26S XP; Beckman Coulter, United States of America) at 1867 Γ g for 20 min. The supernatant obtained was used for the solubility test. The sediment mass was measured, and the swelling power (g.g-1) was calculated as the ratio of the sediment mass to the original sample weight (Equation 5.2.) The collected supernatant was placed on a pre-weighed evaporating crucible dish and oven-dried at 105Β°C for 12 h, and the dried mass was measured. The solubility (%) was calculated as the ratio of dried mass to the original sample weight (Equation 5.3).
SP = Wππ
Wππ (5.2)
131 S = Wππππ
W0 Γ100 (5.3)
where
SP = Swelling power (g.g-1), S = Solubility (%),
WO = weight of original amadumbe starch (g),
Ws = weight of wet amadumbe starch sediment (g), and Wds = weight of dry amadumbe starch supernatant (g).
5.2.7 Amadumbe starch water and oil holding capacities
The water and oil holding capacities were determined using the method used by Ngobese et al.
(2017). A 0.5 g of starch was placed in a 15 mL conical pre-weighed centrifuge tube. Five millilitres of deionised water or canola with a density of 1 and 0.89 g cmβ3, respectively, at ambient temperature, was added to the starch. The mixture was stirred with a stainless-steel spatula and allowed to stand at ambient temperature for two hours before being centrifuged at 350 Γ g (Avanti J-26S XP; Beckman Coulter, United States of America) for 30 min. The supernatant was discarded, and the sample was reweighed. The absorption capacity was calculated by taking the ratio of the difference between the initial and final weight to the weight of the initial starch portion used (Equation 5.4).
HC = Wππ βWππ
Wππ Γ100 (5.4)
where
HC = Water or Oil holding capacity (%),
Wi = initial weight of amadumbe starch (g), and Wf = final weight of amadumbe starch (g).
5.2.8 Amadumbe starch colour
The colour of starch was determined using the method used by Oyeyinka and Amonsou (2020).
Standardisation was done using the Hunter Lab colour standards (ColorFlexEZ; Hunter
132
Associate Laboratories Inc, United States of America). The colour of the starch was measured by reading L*, a*, and b*. L*, a*, and b* refer to (degree of lightness), (redness to greenness), and (yellowness to blueness), respectively. To read the colour, starch was poured into a glass cup (04-7209-00 glass sample cup; Hunter Associate Laboratories Inc, United States of America) until it completely covered the bottom part of the cup.
5.2.9 Statistical analysis
Data analysis was performed using the GenStat 18.2.1 software, analysis of variance (ANOVA) was used to test for differences. The separation of means was determined using Duncanβs multiple range test at the 5 % significant level. The relationship between the quality attributes was determined using linear regression analysis (Chisenga et al., 2019).