Kinetics of Crust Color Development
Color is a very important property, which defines the acceptability
of baked products by the consumer; the extent of
browning also determines the flavor of the finished product.
Moreover, as color development occurs largely during the
later stages of baking, it can be used to judge completion of
the baking process (Mundt and Wedzicha 2007).
Particularly in the muffin baking process, it has been observed
that at a first instance, the emulsion begins to increase
its volume until becoming a solid with its own distribution of
pores in the inside (as previously mentioned, this instance
corresponds to baking minimal time). In turn, as a consequence
of heat and mass transfers between the product and
the environment, the product surface dehydrates and color
development is observed as a result of Maillard reaction and
sugar caramelization. As it is known, the extent of these
chemical reactions is largely influenced by the physical mechanisms of heat and water transfer during baking, which
are strongly dependent on the processing conditions during
baking, i.e., time, temperature, air velocity, air relative humidity,
and rate of heat transfer.
The Maillard reaction is important for the formation of
color and aroma in the crust, but may also be associated with
the formation of toxic compounds such as acrylamide (Ahrné
et al. 2007).
Hence, surface color followup during baking is important
to identify the moment at which the product reaches the color
defined as optimum by consumers. In this paper, the followup was done using the BI parameter.
Under all tested conditions, the same tendency has been
observed: an initial period during which the BI presents no
variation, followed by an increase, whose speed depends on
the baking condition, and finally a plateau, where color is
already too dark (burnt) and BI value stabilizes ca. 160 (Ureta
et al. 2014). The experimental results and the adjustment
proposed in Eq. (2) are presented in Fig. 3, demonstrating that
the kinetic model proposed represents surface color evolution
(R2.0.96) with sufficient accuracy. Hence, it is necessary to
analyze the dependence of the two parameters of this model (k and t1/2) with the operative conditions, which, in our case,
comes down to two variables, a continuous one, i.e., the
effective baking temperature, and a discrete one, i.e., themode
in which the oven operates (NC, FC, and SFC).
These parameters are presented in Table 2. It is important to
notice that both k and t1/2 significantly depend on the baking
effective temperature. Regarding the mode of operation influence,
we again see that there are no significant differences
between FC and SFC.
The dependence of kinetic constant k with effective temperature
follows a behavior that adjusts to Arrhenius equation,
defined by an activation heat Ea and a pre-exponential factor
k0. The Ea (kilojoule per mole) is equal to 23.08±7.49 and
27.09±4.08 for NC and FC/SFC conditions, respectively. The
pre-exponential factor k0 (per second) is equal to 2.51±0.69
and 8.41±0.08 for NC and FC/SFC conditions, respectively.
The Ea values are within the range found by other authors
regarding color evolution: Zanoni et al. (1995)) have analyzed surface color evolution during bread baking following the ÄE
parameter, a process characterized by an activation energy of
64.15 kJ/mol; Broyart et al. (1998)) analyzed lightness parameter
L* evolution in crackers finding an activation energy
within the 70–90 kJ/mol range; Sakin-Yilmazer et al. (2013)
found that surface color evolution in muffins, monitored
through acrylamide concentration present an activation energy
of 36.35 kJ/mol.
On the other hand, the kinetic parameter t1/2, presenting
values within the 1,980–685, 1,463–435, and 1,565–550 range for NC, FC, and SFC, respectively, decrease with the
increase of Teff, following an exponential dependence with
effective temperature, (R2>0.93, Fig. 2b). This behavior is
similar to that observed for minimal baking times; therefore,
there are no significant differences between FC and SFC
conditions.