Viscosity And Density

Viscosity and Density of Lactulose Solutions'
MICHAEL KOZEMPEL
Engineering Science Research Unit, ARS, USDA, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndrnoor, PA 19038

ABSTRACT

Buma (1) reported the viscosity and density of concentrated lactose solutions. He used the Oswald viscometer that is valid for Newtonian fluids. Those data agreed with earlier published data onlactose. Considering this information, it was reasonable to assume that lactulose solutions would also be NewAbbreviation key: DWB = dry weight basis. tonian. Therefore, calibrated Cannon-Fenske kinematic viscometers (Cannon Instrument Co., State College, PA) were used for viscosity determinaINTRODUCTION tions, and a nominal 10-ml pycnometer was Lactulose is a noncaloric, complex carbohydratecalibrated with deionized water. Solutions were prethat is highly valued as a pharmaceutical with world- pared by weighing the solid and adding sufficient wide markets. The principal worldwide outlets are as deionized water to achieve the desired concentration. a laxative and for treatment of portal systemic en- Each solution was immersed in a constant temperacephalopathy. Lactulose is also used ininfant formu- ture water bath or suspended in ambient air for a t las and as a health food because it induces growth of least 1 h before the viscosity or density was measnonpathogenic bacteria in the colon. ured. Concentration of lactulose solutions ranged Lactulose is normally synthesized by the isomeri- from 20 to 76% solids; a 76% solution is the practical zation of lactose. Recent methods forproducing lactu- limit to solubility of lactulose in water. Solutions of lose involve the use of complexation reagents. Hicks lactulose and boric acid ranged from 9 to 39% solids ( 2 , 3, 4 ) showed that treatment of lactose with boric containing 5, 10, or 15% boric acid, dry weight basis acid in a molar ratio of 1:l (vol/vol) in the presence of ( DWB). The presence of boric acid limits thesolubiltertiary amines or NaOH produces high yields of ity of lactulose in this solution. Lactulose (Sigma Chemical Co., St. Louis, M O ) lactulose. A commercially feasible process has been developed had a purity of approximately 98%. Boric acid was NF to produce lactulose based upon the use of boric acid granular and conformed t o all standards of The Naand NaOH (5, 6, 7 ) . A critical unit operation inthe tional Formulary ( 9 ) . The boric acid was supplied by separation section of the process is evaporation to U. S. Borax and Chemical Corp. (Los Angeles, CA). concentrate the solids (lactulose, lactose, boric acid, Temperature measurements were made with a galactose, and tagatose). It is then possible to crystal- Natural Bureau of Standards calibrated thermometer lize most of the boric acid andto filter it from the that had a range of -1 to 50°C and graduations of solution. During evaporation, the solution becomes 0.1%. quite viscous and dense. To design, operate, and conRESULTS trol the evaporator properly, data on physical properties were needed, specifically, the viscosity and denThe viscosities of lactulose solutions are shown in
Received November 16, 1995. Accepted J u n e 28,1996. 'Reference t o a brand or firm name does not constitute a n endorsement by the USDA over others of a similar nature not mentioned.
1996 J Dairy Sci 79:2152-2154

The density and viscosity of solutions of lactulose and solutions of lactulose and boric acid were determined using calibrated viscometers and a nominal 10-ml pycnometer, respectively. These data were necessary for the development ofthe recovery section of a continuous pilot plant process to produce lactulose, a valuable pharmaceutical, from lactose. ( Key words: viscosity, density, lactulose, boric acid)

sity of lactulose and lactulose plus boric acid solutions in water.
MATERIALS AND METHODS

Figure 1. The data were plotted as the natural log of viscosity versus the reciprocal of the absolute temperature to obtain...