Thermodynamic description of the chemical leavening in biscuits

Abstract

In this paper we describe the chemical reactions of leavening agents in baking biscuits on a sound thermodynamic basis. The model is part in a sequel targetted at physical understanding of biscuit baking with the purpose of reformulation of biscuits with respect to sucrose and sodium levels. The chemical leavening gases, CO2 and NH3, originate from the dissociation of sodium and ammonium bicarbonate. Next to water vapour, these produced gases create gas bubbles in the biscuit dough. The concentrations of the leavening agents and added salt lead to high ionic strength. Consequently, the activities of ions participating in the leavening reaction deviate strongly from ideality. The non-idealities are described using the Pitzer equations. The values of many parameters of the Pitzer model and equilibrium constants are obtained from the strong developed field of CO2 sequestering in ammonia solutions. The model describing the chemical reactions is coupled to a cell model describing the expansion of gas bubbles. Model simulations show that most of the produced gas originates from the bicarbonate, and the ammonium contributes significantly less. The functionality of ammonium as leavening agent is not quite clear, but it may help in reducing sodium levels.

Keywords: Baking; Leavening agents; Non-ideality; Thermodynamics.

Fig. 1

Henry constant for $C{O}_2$ and $N{H}_3$ as function of temperature, cf. (Kawazuishi and Prausnitz, 1987).

Fig. 2

a) Water activity $a_w$ and b) Mean activity coefficient ${\gamma }_{\pm }$ of NaCl solution at $T={25}^{o}C$ as function of molality m, with predictions made by Pitzer model (solid lines). Symbols represent experimental data by Robinson and Stokes (2002).

Fig. 3

Effective $p{K}_1$ and $p{K}_2$ of aqueous solutions at 25 ​°C with 0.1M NaHCO3 and NaCl at molality $m_{NaCl}$. Symbols represent data from (Thurmond and Millero, 1982), and lines are predictions using the Pitzer model.

Fig. 4

Speciation of ions in solution of ammonium carbonate in comparison with experimental data of (Halstensen et al., 2017).

Fig. 5

Development of the molalities $m_i$ of various ions and dissolved gases during the baking of the AACC 10–53 biscuit as computed by the reduced Pitzer model, coupled to the Amon-Denson cell model for the expanding gas bubble.

Fig. 6

Development of the gas pressures $p_i$ and the porosity ${\phi }_{gas}$ as function of temperature T during the baking of the AACC 10–53 biscuit as computed by our model.

Fig. 7

Development of the gas pressures $p_i$ and the porosity ${\phi }_{gas}$ as function of temperature T during the baking of the biscuit with 0.50 ​M sodium bicarbonate and 0.02 ​M ammonium bicarbonate.