Water–solids interactions, matrix structural properties
and the rate of non-enzymatic browning
, C. Schebor, M.P. Buera
Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria (1428),
Ciudad de Buenos Aires, Argentina
Received 20 February 2005; accepted 15 August 2005
Available online 13 October 2005
The kinetics of non-enzymatic browning (NEB) was studied in freeze-dried model and food systems in a wide range of relative humid-
ity (R.H.) values.
PVP, lactose, lactose–starch solutions and food (milk, cabbage, apple, potato, and chicken meat) systems were freeze-dried, equili-
brated at 11–85% of R.H. and incubated at 70 °C. Thermal transitions were determined by DSC. The kinetics of NEB development
was analyzed. In PVP systems the maximum rate occurred at 33% R.H., at which T
was close to the storage temperature. Above
33% R.H. the samples presented a ﬂuid aspect at 70 °C and the NEB rate decreased when increasing R.H. In tissues containing struc-
turing water insoluble biopolymers and presenting an intermediate degree of collapse, the maximum rate of NEB occurred at relative
humidities in a range of 50–80%, when the samples were well above T
at the storage temperature. In the lactose systems the maximum
rate occurred at R.H. close to 40%, at conditions at which lactose was highly crystalline.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Non-enzymatic browning; Kinetics; Glass transition; Relative humidity
The non-enzymatic browning (NEB) reaction is one of
the most prevalent and studied chemical reactions that oc-
curs in foods during heating and storage. The NEB rate is
known to be aﬀected by several physico-chemical factors,
being the most studied: concentration, ratio and chemical
nature of the reactants (type of amine and carbonyl groups
involved); pH; relative humidity; temperature and time of
heating (Labuza & Baisier, 1992).
In ﬂuid liquid systems, NEB rate diminishes continu-
ously as relative humidity (R.H.) increases, mainly due to
the fact that water is a product of the reaction (Eichner
& Karel, 1972; Labuza & Saltmarch, 1981). On the other
side, in solid or quasi-solid systems, in which NEB reac-
tants are submitted to mobility restrictions, a maximum
rate of NEB is observed at a given intermediate R.H. value.
The decreasing browning rate at high water content/rela-
tive humidity has also been related to the dilution of reac-
tants at constant reactant mass (Loncin, Jackmain,
Tutundjian Provost, Lenges, & Bimbenet, 1965; White &
Bell, 1999). Thus, the existence of a maximum in the plot
NEB rate versus water content (or R.H.) is a consequence
of the low reaction rates due to mobility limitations of reac-
tants at low water contents (Buera & Karel, 1995; Karmas,
Buera, & Karel, 1992) and inhibition by product/dilution
of reactants at high water contents (Karmas & Karel,
1994; van Boekel, 2001; White & Bell, 1999). It was gener-
ally accepted that the maximum NEB rate in food systems
is in the range of R.H. between 60% and 80% and that it
depends on the type of food (Eichner & Karel, 1972; Labuza
& Saltmarch, 1981). The kinetics of NEB has also been
related to the glass transition phenomenon (Bell, Touma,
White, & Chen, 1998; Bell, White, & Chen, 1998; Karmas
et al., 1992). Although the formation of amorphous glassy
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Corresponding author. Tel./fax: +54 11 4576 3366.
E-mail address: email@example.com (N. Acevedo).
Journal of Food Engineering 77 (2006) 1108–1115