Antimicrobial actions of degraded and native chitosan against spoilage organisms in laboratory media and foods

Abstract

The objective of this study was to determine whether chitosan (poly-beta-1,4-glucosamine) and hydrolysates of chitosan can be used as novel preservatives in foods. Chitosan was hydrolyzed by using oxidative-reductive degradation, crude papaya latex, and lysozyme. Mild hydrolysis of chitosan resulted in improved microbial inactivation in saline and greater inhibition of growth of several spoilage yeasts in laboratory media, but highly degraded products of chitosan exhibited no antimicrobial activity. In pasteurized apple-elderflower juice stored at 7 degrees C, addition of 0.3 g of chitosan per liter eliminated yeasts entirely for the duration of the experiment (13 days), while the total counts and the lactic acid bacterial counts increased at a slower rate than they increased in the control. Addition of 0.3 or 1.0 g of chitosan per kg had no effect on the microbial flora of hummus, a chickpea dip; in the presence of 5.0 g of chitosan per kg, bacterial growth but not yeast growth was substantially reduced compared with growth in control dip stored at 7 degrees C for 6 days. Improved antimicrobial potency of chitosan hydrolysates like that observed in the saline and laboratory medium experiments was not observed in juice and dip experiments. We concluded that native chitosan has potential for use as a preservative in certain types of food but that the increase in antimicrobial activity that occurs following partial hydrolysis is too small to justify the extra processing involved.

FIG. 1

Inactivation of Candida sp. (A) and Rhodotorula sp. (B) in saline solutions at 25°C (▵), in the presence of 0.5 g of native chitosan per liter (□), and in the presence of 0.5 g of a chitosan hydrolysate per liter prepared by using papaya latex (○).

FIG. 2

Growth of S. ludwigii in laboratory media at 25°C in the presence of no chitosan (control) (○), 0.2 g of native chitosan per liter (■), and chitosan hydrolysates with viscosities of 96 s (□), 24 s (▴), 17 s (▵), and 9 s (●). The hydrolysates were prepared by the oxidative-reductive degradation method. The results are means based on five replicate values for absorbance at 620 nm (A₆₂₀). Bars indicate standard error.

FIG. 3

Growth of Z. bailii in laboratory media at 25°C in the presence of native chitosan (□) and hydrolyzed chitosan at concentrations of 0.1 g/liter (A), 0.2 g/liter (B), and 0.3 g/liter (C). The viscosities of the hydrolysates were 154 s (■), 96 s (▴), 54 s (▵), 11 s (●), and 0 s for the control containing N-acetylglucosamine plus hydrogen peroxide (○). The hydrolysates were prepared by the oxidative-reductive degradation method. The results are means based on five replicate values for absorbance at 620 nm (A₆₂₀). Bars indicate standard error.

FIG. 4

Growth of S. cerevisiae in laboratory media at 25°C in the presence of native chitosan (□) and hydrolyzed chitosan at concentrations of 0.1 g/liter (A), 0.2 g/liter (B), and 0.3 g/liter (C). The viscosities of the hydrolysates were 154 s (■), 96 s (▴), 54 s (▵), 11 s (●), and 0 s for the control containing N-acetylglucosamine plus hydrogen peroxide (○). The hydrolysates were prepared by the oxidative-reductive degradation method. The results are means based on five replicate values for absorbance at 620 nm (A₆₂₀). Bars indicate standard error.

FIG. 5

Growth of Z. bailii (A) and S. cerevisiae (B) in laboratory media at 25°C in the presence of 0.1 g of native chitosan per liter (■) and lysozyme-degraded chitosan (□). The controls consisted of unsupplemented laboratory medium (●), autoclaved lysozyme (▵), and chitosan plus autoclaved lysozyme (▴). The results are means based on five replicate values for absorbance at 620 nm (A₆₂₀). Bars indicate standard error.

FIG. 6

Effects of native chitosan and degraded chitosan at a concentration of 0.3 g/liter on the natural flora of apple-elderflower juice stored at 7°C. The total mesophilic organisms (A), lactic acid bacteria (B), and yeasts (C) were enumerated on selective media. Symbols: ▵, juice alone; □, juice treated with native chitosan; ○, juice treated with chitosan degraded with papaya latex. The results are means based on duplicate viable count values.

FIG. 7

Effects of native chitosan and degraded chitosan at a concentration of 5.0 g/liter on the natural flora of a houmous-water slurry stored at 7°C. Total mesophilic organisms (A), lactic acid bacteria (B), and yeasts (C) were enumerated on selective media. Symbols: ▵, houmous alone; □, houmous treated with native chitosan; ○, houmous treated with chitosan degraded with papaya latex. The results are means based on duplicate viable count values.