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Effect of pH on the morphology and tensile properties of milk protein-based electrospun fibers.

P. Tomasula




Effect of pH on the morphology and tensile properties of milk protein-based electrospun fibers.
S. Akkurt, P. Tomasula*. Dairy and Functional Foods Research Unit, USDA, Agricultural Research Service, Eastern Regional Research Center Wyndmoor, PA.

Electrospinning has been used to produce edible mats from nanosized fibers of calcium (CaCN) or sodium (NaCN) caseinate with pullulan(PUL) added to facilitate fiber formation. The electrospun mats are a promising method to preserve and deliver bioactives to foods; e.g., because of the fiber's large surface area/volume, but fiber imperfections such as bead formation and variations in diameter size would prevent consistent performance of the mats. To improve fiber quality, NaCl has been recommended as an additive to increase shear viscosity for drawing thinner fibers while electrospinning; however, we found NaCl ineffective and hypothesized that NaOH would increase solution viscosity and also strengthen the mats through CN aggregate formation. The objective of this study was to investigate the effect of pH on the morphology and tensile properties of CaCN or NaCN with PUL electrospun fibers. Aqueous solutions of 15 (wt/wt)% CaCN or NaCN, and PUL were prepared at 20�C. Then, 1:1(pH 6.7) and 2:1(pH 6.7) blends of either CN with PUL solution were prepared and adjusted from the initial pH6.7 to pH 8, 9, and 10 with 1N NaOH. A syringe fed 3 mL of each solution to an electrospinning unit at flow rate of 3mL/h and at 20kV, with fibers deposited on a rotating drum. At pH6.7, the mean diameters, D, of the CaCN and NaCN:PUL(1:1) fibers were 226 � 25 and 215 � 18nm, respectively, with beads noted on the fibers. As pH was increased to 8 and then 9, the fibers became thicker with D of 274 � 20 and 234 � 16nm and bead-free, becoming thinner at pH 10, with D of 105 � 16 and 93 � 15nm, respectively. Similar trends were noted for the (2:1) blends. Tensile Strength (TS) and Elongation at Break% (EAB) measurements showed that the initial (TS,EAB%), (1.4 � 0.5MPa, 4.0 � 0.6%) and peak (4.7 � 0.3MPa, 8.4%) values the for CaCN mats occurred at pH 9 with a sudden 60% decline in these values at pH10 suggesting that the negatively-charged CN aggregates partly disassociated. This did not occur for the NaCN aggregates at pH10 most likely due to lower electrostatic repulsion. Studies on CN in fibers will provide insights to their structure and that relative to casein micelles.

Keywords: nanofibers, caseinates, fibrous mats.