A Research Note Solute Effects at High Water Activity on Nonenzymatic Browning of Glucose-Lysine Solutions CLAUDIO PETRIELLA, JORGE CHIRIFE, SILVIA L. RESNIK, and ROBERTO D. LOZANO ABSTRACT The effect of different polyols (glycerol, propylene glycol, sorbitol, 1,3-butylene glycol, polyethylene glycol200, and polyethylene glycol 400) and various alkali metal chlorides (KCl, NaCl, LiCl, and CsCl) on the kinetics of nonenzymatic browning in glucose-lysine solutions, of high water activity (a,,,) was studied. No particular inhibitory or promoting effect on browning of glucose-lysine solutions, when ad- justing a, with the various polyols, was found. The alkali metal chlo- rides had a significant influence on the rate of browning and the relative effect of cations (Li > Na > K,Cs) paralleled the hydrated ionic radii in solutions. INTRODUCTION THE QUALITY of intermediate moisture foods is subjected to changes due to nonmicrobial deteriorative reactions such as nonenzymatic browning and loss of nutrients, In the interme- diate and low water activity (a&ange (C 0.85), moisture greatly influences nonenzymatic browning rate. The most interesting feature is a maximum in the intermediate a,,, range attributed to a balance of viscosity-controlled diffusion, dilution and mass action effects (Williams, 1976). Sorbitol could inhibit brown- ing by increasing the medium viscosity and decreasing the reactants mobility. On the other hand, glycerol and other liquid polyols exert a plasticizing effect shifting the a,,, browning maximum downward (Warmbier et al., 1976; Labuza, 1980). The purpose of this work was to study how several polyols and salts affected the nonenzymatic browning kinetics in glu- case-lysine model systems of high a,.,. MATERIALS & METHODS THE MODEL SYSTEMS studied consisted of glucose (0.274M) and lysine (0.00684M) as the reactants to which one or the other of the following polyols was added to adjust the water activity to 0.925 (Chirife et al., 1980; Alzamora and Chirife, 1984): glycerol, propyl- ene glycol, sorbitol, 1,3-butylene glycol, polyethylene glycol200 (PEG- 200), polyethylene glycol 400 (PEG-400). In the other experiments, the concentration of different alkali metal chlorides (LiCI, NaCl, KC1 and C&l) was adjusted to 1M. Phosphate buffer (KH,PO,-NazHPO,, 0.3.5M in phosphate) was used to maintain a constant pH value during heating of the model systems. Screw-cap polyethylene flasks (20 mL capacity) were filled with 18 mL of each model solution and stored at 55°C (f 0.5”C). D-Glucose, sorbitol, propylene glycol, PEG-400 and CsCl were obtained from Mallinckrodt Chemical Works (St. Louis, MO); glyc- erol and the other salts were from Merck (Darmstadt, West Germany); PEG-200 was from Fluka (Buchs, Switzerland) and L-lysine was from Sigma (St. Louis, MO). A spectrophotometric tristimulus color measurement was performed The authors are affiliated with Departamento de lndustrias, Fa- cultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 1428 Buenos Aires, Republica Argentina. Author Petriella is a research fellow, CONICEC Author Resnik is a member of Comision de lnvestigaciones Cientfficas de la Provinica de Bue- nos Aires; Author Lozano is with Division Optica, lnstituto Na- cional de Tecnologla Industrial, Buenos Aires. to measure the color. Spectrophotometric transmitance curves were obtained with an automatic recording Spectrophotometer ZEISS DMC 25 and the CIE tristimulus values, illuminant C, calculated by an automatic integrator (Davidson and Hemmendinger Electronic, Ta- tamy, PA). The CIELUV color function metric saturation (difference of values) (S - S& was used as a suitable measure of browning (Petriella et al., 1985); So is the color value corresponding to the model system without heat treatment. The viscosity of the different polyol solutions at a, = 0.925 and 55°C was measured using a glass capillary viscometer type Ubbelohde (Chirife et al., 1984). Triplicate measurements were done and the mean was reported. RESULTS & DISCUSSION FIGURE 1 shows the development of browning, expressed in terms of the color function (S - S,), (Petriella et al., 1985), as a function of heating time for model systems containing glucose-lysine (pH = 6.0) with a,,,adjusted with different po- 1~01s to 0.925; data for glucose + lysine without any polyol are also included. An initial lag phase or induction period was followed by one of rapid color increase. The comparison in- dicated that all polyols behaved similarly in regards to their influence on the rate of browning. No particular inhibitory or promoting effect on browning of glucose-lysine solutions, by 0 Propylene glycol 1,3-Butylene glycol l without hunectan 0 100 / 200 TIME , Jto/rs Fig. 1 -Effect of heating time on/the increase of the color func- I tion (S - S,J,, for glucose-lyshe solutions of a,., = 0.925 ad- justed with different polyols/ _’ Volume 53, No. 3, 1988;JOURNAL OF FOOD SCIENCE-987 2 / /I /- /