Glass transition and caking properties of amorphous carbohydrate blend and maca (Lepidium meyenii Walpers) powders

概要

Doctoral Thesis

Glass transition and caking properties of
amorphous carbohydrate blend and maca
(Lepidium meyenii Walpers) powders
（Summary）

ALVINO GRANADOS ALEX EDUARDO

Graduate School of Biosphere Science
Hiroshima University
March 2021

Maca (Lepidium meyenii Walp.) is a carbohydrate-rich vegetable root of the Brassicaceae
family, native to the central Andes of Peru. Consumption of maca as functional food has been
encouraged due to its bioactive compounds including glucosinolates, macaenes and macamides.
The majority of maca roots are dehydrated and milled into a powder.
Commonly, food powders are at least partially amorphous, showing glass-to-rubber transition
(glass transition) upon changes on temperature and water content. The glass transition of
amorphous powder is characterized by the glass transition temperature (Tg). Amorphous
powders are physically stable in the glassy state (T < Tg) because their macroscopic molecular
mobility is very low. In contrast, amorphous powders are physically unstable in the rubbery
state (T > Tg). The Tg of hydrophilic amorphous powders decreases with increasing water
content because of the water plasticizing effect. Consequently, glass transition can also occur
by a change of water content or water activity (aw), even at constant temperatures.
Caking is a physical deterioration in which free-flowing powders are agglomerated into lumps,
due to deformation and bridging of sticky particles as a result of plasticization and decrease of
surface viscosity. Due to the fact that glassy powders are free-flowing, but rubbery powders are
sticky and susceptible to agglomeration, the effect of water content on the Tg (Tg-curve) is useful
to predict the caking of amorphous food powders.
Although maca is an outstanding food material, there has been little effort to understand its
physical properties in comparison with its chemical and physiological properties. In particular,
the glass transition and caking properties of maca powder have not been reported. Therefore,
the purpose of this study was to understand the glass transition and caking properties of maca
powders. In addition, amorphous carbohydrate blend powders were employed as food powder
models, and predictive approaches for the caking of food powders were proposed.
In chapter 1, introduction and purpose of this thesis were shown as mentioned above. In
chapter 2, fundamentals for the experiments were explained.
In Chapter 3, a commercially available maca powder was employed, and X-ray diffraction,

starch gelatinization, water sorption, glass transition, and caking properties of the maca powder
were investigated. From the X-ray diffraction pattern and enthalpy change for starch
gelatinization, it was suggested that starch in the maca powder became largely amorphous
during production of the maca powder. Effect of water content on the Tg of the maca powder
was investigated using a differential scanning calorimetry (DSC). The maca powder showed a
broad glass transition behavior reflecting a continuously distributed glass transition. From the
Tg-curve, critical water content (wc) was evaluated as the water content at Tg = 25 ºC.
Furthermore, the wc was converted to critical water activity (awc) through the water sorption
isotherm (equilibrium water content versus aw). As expected, there was negligible caking in the
glassy state (aw < awc). In the rubbery state (aw > awc), the degree of caking and hardness of cake
of maca powder gradually increased with increase in aw. Since maca powder showed a
continuously distributed glass transition, the molecular mobility required for caking will have
been provided incrementally by the increase in aw above awc.
In Chapter 4, browning, starch gelatinization, water sorption, glass transition, and caking
properties of the freeze-dried maca powders were investigated, and the results were compared
with those for the commercial maca powder. The freeze-dried maca powders had lower
browning and starch gelatinization than the commercial maca. There was a minor difference in
the anhydrous Tg (79.5~80.2 ºC) and in awc among the samples. The degree of caking could be
described uniformly as a function of aw/awc, and the behavior was characterized by a stretching
exponential function. This equation is mathematically equivalent to the Avrami equation. The
Avrami model describes effect of annealing time on the degree of crystallization at a constant
temperature. Given that crystallization is an orderly aggregation of molecules, the Avrami
equation is analogically applicable for the caking (agglomeration) of powders. A novel
modification of the proposed equation is that the Avrami equation was changed from “timedependency” of crystallization to “aw-dependency” of caking.

In Chapter 5, freeze-dried water-soluble MD and plasticizers (glucose, maltose, and sorbitol)
blend powders were employed as model food powders, and glass transition and caking
behaviors were investigated. In order to characterize the difference in the dependence of
viscosity on aw in the rubbery state, the Tg-range (temperature-difference between onset and
offset of glass transition) was evaluated from DSC thermograms. In addition, the Tg-range was
corresponded to awc-range (aw-difference between onset of and offset of awc). The Tg-range and
awc-range increased by the addition of plasticizers because of the distributed molecular mobility
in the rubbery state. In addition, it was confirmed that the distributed molecular mobility
affected the degree of caking. The Tg-range was converted to the dependence of viscosity on aw,
and a predictive model for the caking of water-soluble amorphous powders was proposed based
on the Tg-range.
In Chapter 6, the effect of cellulose content on the Tg-range, mechanical relaxation, and degree
of caking of MD-glucose blend powder was investigated, and the degree of caking was analyzed
by three approaches (aw/awc, predicted η, and isothermal mechanical relaxation). Firstly, the
degree of caking was described as a function of aw/awc according to Chapter 4, and characterized
by a stretched exponential function depending on the water-insoluble dispersion content.
Secondly, the degree of caking was described as a function of log predicted η according to
Chapter 5, and characterized by a linear function depending on the water-insoluble dispersion
content. Thirdly, the degree of caking was described as a function of degree of isothermal
mechanical relaxation (ΔF), and characterized by a linear function independent of aw and
cellulose content. Among the three approaches, the characterization based on ΔF will have been
a practical and useful approach to predict caking behavior, as F can be rapidly evaluated. In
addition, maca powders partially obeyed the linear function.
In Chapter 7, general conclusion and future subjects were explained. It was concluded that the
characterization based on ΔF will be a practical and useful approach to predict caking behavior
of amorphous food powders. This approach, however, is somewhat phenomenological, as the

F value will depend on the experimental conditions more or less (measurement time and initial
compression). For the application of this approach to practical food powders, it is necessary to
clarify the physical meaning of F. In addition, the effect of powder composition on the
relationship between the degree of caking and F should be understood in more detail.
Amorphous part of maca will have been constructed by protein, partially gelatinized starch, and
sugar. In particular, protein is an electrically charged polymer, and thus its electrostatic
repulsive effects will affect the caking behavior. For the better understanding of the caking
behavior of amorphous food powders, these will be important subjects.