Effect of Mixing During Fermentation in Yogurt Manufacturing

Experimental bioreactors. The bioreactors differed greatly in their heat-transfer characteristics: (a) 3-L fermentor, heated through the walls by an electric mantle and cooled by water circulation through a loop, and (b) 5-L fermentor, heated through a jacketed metal-dished bottom. Arrows indicate the direction of heat transfer. The black dots (●) indicate the location of the corresponding pH electrode.

Temperature (gray symbols) and pH profiles (black symbols) observed within the experimental bioreactors analyzed: (a) the mantle-heated system, and (b) the bottom-jacketed system. Trends corresponding to different sampling locations are marked with specific shapes (rhombus, square, circle, triangle); sampling locations are indicated with the same symbols in the bioreactor schemes presented in the graphs. Solid lines represent trends registered by the pH or temperature sensors built into the bioreactor.

Evolution of pH with time during yogurt elaboration under different experimental conditions: (a) Fermentations that were inoculated by addition of a freeze-dried lactic acid bacterial culture and with no agitation (black circles), or with agitation at 200rpm (gray circles). Curves were shifted in the x-axis to overlap their exponential phases; (b) fermentations that were inoculated by addition of liquid inoculum in the exponential phase. Results from the experiments with no agitation (solid line) and with agitation at 200rpm (dashed line) are presented. Error bars are based on 3 repetitions.

Lactic acid profiles (expressed as mg of lactic/mL) observed during elaboration of liquid yogurt under agitated and nonagitated fermentation experiments: (a) dissociated (dashed line) and nondissociated (solid line) lactic acid profiles in nonagitated (black squares) and agitated (gray squares) fermentation experiments, (b) global lactic acid profiles in nonagitated (black circles), and agitated (gray circles) fermentation experiments, (c) nondissociated (nd)/dissociated (d) lactic acid concentration ratios in nonagitated (black circles) and agitated (gray squares) fermentation experiments.

The pH profiles (▴) and population dynamics of Streptococcus thermophilus (●) and Lactobacillus bulgaricus (■; expressed as ×10³ cfu/mL) during the elaboration of yogurt under (a) nonagitated and (b) agitated conditions. Error bars are based on 3 repetitions.

Growth curves of Streptococcus thermophilus [expressed as ln(cfu/mL)] during the elaboration of liquid yogurt in experiments at 40°C with no agitation (black circles, solid line), and with agitation at 200rpm (gray circles, dashed line). Experiments conducted in the bioreactors shown in Figure 1a and Figure 1b are plotted. If curves were shifted in the x-axis, they would collapse into a master curve, implying practically equivalent kinetic behaviors regardless of agitation conditions.

Growth curves of Lactobacillus bulgaricus [expressed as ln(cfu/mL)] during the elaboration of liquid yogurt in experiments at 40°C with no agitation (black squares, solid line), and with agitation at 200rpm (gray squares, dashed line). Only experiments performed in the mantle-heated bioreactor (see Figure 1a) are plotted. Experimental data points were adjusted to linear trends, and the resulting fitted equations are presented.

Relationship between lactose consumption (circles) and Lactobacillus bulgaricus growth [squares; expressed as ln(cfu/mL)] during the elaboration of liquid yogurt in experiments at 40°C with no agitation (black circles and squares) and with agitation at 200 rpm (gray circles and squares).

Growth curves of Streptococcus thermophilus (circles) and Lactobacillus bulgaricus (squares; expressed as ×10³ cfu/mL) during the elaboration of liquid yogurt in experiments at 40°C with no agitation (solid lines) and with agitation at 200rpm (dashed lines). Only experiments performed in the bottom-jacketed bioreactor (see Figure 1b) are plotted.