2.2.1. Amylase extraction
The sweet potatoes used were obtained at the local market. The fresh tuberous
roots were washed, peeled and diced. Fresh sweet potato tuber just after collection
were used, to avoid high alpha amylase contamination, considering that alpha amylase
tended to increase during storage [8,25]. For the beta-amylase extraction, 50 g
of the potato pieces were added to 100 mL of cold distilled water and pulverized.
After centrifugation (7000
×
g/20 min/4 ◦C) the supernatant (amylase extract) was
used for all assays. The enzyme extract presented about 4.7 mg protein mL−1 and a
specific activity of 17.05 U mg−1 protein. No further purification methods were used.
2.2.2. Protein determination
Protein concentration was determined by Bradford method [26], using bovine
serum albumin (BSA) as standard.
2.2.3. Enzymatic activity
Enzyme activity was determined using starch as substrate (1% w/v in 100 mM
citrate-phosphate buffer at pH 6.0). Starch hydrolysis (50 ◦C) was monitored by
determination of reducing sugar using dinitrosalicylic acid method at 540 nm [27].
A standard curve was prepared with maltose. One beta-amylase activity unit (U)
was defined as the amount of enzyme capable of producing 1 Mol of maltose per
minute under assay conditions.