649

1 INTRODUCTION

Soybeans are one of the most important crops,

covering about 6% of the world's arable land. It has

been observed since the 1970s, that the soybean

production area has higher growth rate than any other

major crop. Hartman et al. [13] highlighted a number

of important abiotic and biotic factors, which may be

The Influence of a Static, Homogeneous Magnetic Field

on the

Sorption Properties of Soybean Meal During

aritime Transport

Ocieczek, A. Kaizer & Z. Otremba

Gdynia Maritime University, Gdynia, Poland

ABSTRACT: Providing safety and maintaining high quality of dry cargo, transported by sea, is associated with

the necessity of taking into consideration their

specific property, which is hygroscopicity. Susceptibility to the

impact of water, which also occurs in vapour state, concerns mainly dry cargo, which are organic matter,

containing carbohydrates and protein in their composition. This is because these substances have strong

connection with water. The example of a bulk cargo often transported by sea is soybean meal, which is mainly

produced in the USA, Brazil and Argentina. Due to its economic importance, the quality of soybean meal, which

is globally used in the animal nutrition (poultry and swine), remains an important research issue. This product

is obtained by subjecting the soybeans to cracking and dehulling processes, in order to facilitate the extraction of

the oil.

Water absorption of soybean meal cau

ses reactions taking place in it, which leads to the changes in its chemical

composition and, consequently, also in its nutritional values. Moreover, increasing the water content, leading to

the increase of water activity, may significantly deteriorate the microbiological safety of the meal. Therefore, the

research was undertaken to determine whether the sorption properties of soybean meal will change due to the

influence of a static, homogeneous magnetic field.

This aim has been achieved by determining and comparing the water vapour adsorption isotherms. The

comparison of the isotherms determined under normal conditions and under the influence of a static,

homogeneous magnetic field with an induction of 10 mT has been made on the basis of empirical data.

Furthermore, using the Brunauer, Emett and Teller equation (BET), the monolayer and the energy constant of

the sorption process have been estimated. The isotherms were determined at 20°C. The study lasted 9 days.

Desiccators with aqueous supersaturated solutions of substances and a generator of a static magnetic field were

used in the research.

The obtained results have indicated that the influence of the magnetic field is a factor that causes the

differentiation between the sorption properties of soybean meal expressed in the volume of the monolayer and

the energy associated with the sorption phenomenon. The inferred findings show, that the magnetic field has an

impact on the course of the sorption phenomenon in organic samples, and may determine the stability of the

cargo during long-term maritime transport.

http://www.transnav.eu

the

International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 15

Number 3

September 2021

DOI: 10.12716/1001.15.03.20

650

critical elements that potentially endanger the

soybean production by direct impact on reduction of

crop yields and / or quality of soybean seeds. Critical

abiotic factors include extremely unfavorable soil

properties, temperature fluctuations and the

availability of water for agricultural production,

which may limit the production volume, but also

could influence the quality of the raw material

produced. On the other hand, critical biotic factors are

usually geographically and environmentally

determined by influencing the type of specific and

usually dominant pathogens and pests. However, due

to the lack of advanced technologies, a wide range of

appropriate soybean management strategies has not

been widely available yet, the development in this

area is in progress. For example, new soybean

varieties enable to cultivate them, even in soil

conditions which have been considered as unsuitable

for long [23]. This currently occurs in Argentina and

Brazil, in areas which had been formerly inadequate

for soybean cultivation. Previously, it had been

primarily American soybean farmers, who had

responded to the growing global demand for protein

feed for the meat and poultry sector [6]. Taking the

above into considerations, it can be assumed that in

some time perspective Europe may join China and

Asian countries, which have also contributed to the

development of the global soybean market. This,

particularly, should translate into the promotion of

sustainable development, in which a significant role

may be played by the reduction of the necessity of

bulk transport of this raw material or its products [21].

In conclusion, it is likely that both the crop yields of

soybean and the area under its cultivation will

increase, and therefore there will be the, associated

with these, significant increase in the production of

soybean meal and oil [24].

Soybean can be, and is, consumed by humans as a

food. For example, in China it is now the main

ingredient for the production of food products such as

tofu and soy milk. However, it should be emphasized,

that its consumption by humans is still low, in

comparison to the level of its use in the production of

fodders, which actually determines the value of

international trade in this raw material. At the same

time, it should be noticed that it is the rapid growth of

world population and, consequently, of many

economies, especially in Asia, that has led to an

increased demand for animal protein and vegetable

oil. Therefore, it can be assumed that the international

soybean trade will continue to develop, at least in the

near term.

Global trade in oilseeds includes many raw

materials, such as: rapeseed, sunflower and cotton,

which could potentially be an alternative to soybeans.

However, the specific nutritional quality requirements

for protein meal, for edible vegetable oil, and for oil

used for a production of biofuel, determine the ratio

of various oilseeds in the market, depending on the

demand for products made from these seeds.

Therefore, trade, but not only it, in particular oily

feedstock, is conditioned primarily by the

development of industry. International trade has been

expanding rapidly, since modern economies became

highly industrialized. This is because of the fact, that

there is a positive correlation between trade and

development [14]. Nevertheless, globalization has not

always improved development, taking into account

social variables such as employment and poverty [15].

An important problem is also, how the trade in

oilseeds affects food security both in the countries of

their origin and outside them [25]. In addition, the

impact of the technical and natural effects of oilseed

trade on climate change, is also increasingly taken into

account [35]. In this context, the issue of maintaining

the safety and good quality of bulk cargo, which are

both soybeans and soybean meal, is especially

important. The case when the reduction of the safety

and/or quality level, as well as the destruction of the

raw material and the product, happens, should be

considered not only as a direct loss, but also as the

environmental burden connected with their

production [3, 37].

2 THREATS TO THE SAFETY AND QUALITY OF

SOYBEAN MEAL IN TRANSPORT

Soybean meal is one of the most commonly used

ingredients in compound feed. The growing demand

for this product, determines more requirements for

the safety during transport processes. In transport

processes, soybean meal occurs in the form of a solid

bulk cargo. Due to its inherent properties, it is

classified as a special cargo. It is categorized,

according to the IMDG code, as a dangerous good of

class 4.2 [7].

The influence of external factors on changes in the

quality characteristics of soybean meal has an

important role in appearance of threats during the

transport processes of this type of cargo. Maintaining

the safety and quality of soybean meal during

transport depends on the conditions of storage,

handling and transport. In studies, which concerns the

changes in the quality of soybean meal during

maritime transport, the main external factor taken into

account in the research, is the ambient humidity [7].

This parameter is an important element of the

environment, and moreover it is an important factor

of the sensitivity of the cargo, which is a soybean

meal. Due to the hygroscopicity of soybean meal, it

absorbs water vapor from the environment, and hence

the transport and technological quality features of its

change. These properties are for example: kinetic

angle of repose, size of molecule, bulk density,

porosity. Therefore, the absorption of water vapor by

soybean meal, changes the essential characteristics of

this cargo at particular stages of the transport chain.

Fortunately, the correlation between the changes in

the transport-technological qualities of soybean meal

and the occurring under the influence of air humidity

changes of water content in soybean meal, has been

identified. In relation to the above, it has been

discovered that under the influence of surrounding

the cargo humid air, in which the transport processes

take place, and depending on the degree of

comminution of soybean meal grains, there is a

significant differentiation in transport and

technological qualitative characteristics [7].

Weighing all above considerations, it could be

assumed, that searching for innovative methods,

which will enable to stabilize the microbiological

safety and the quality characteristics of soybean meal

651

as a cargo under the conditions of its exposure to

water vapor, is a reasonable performance. The

phenomenon of water vapor sorption on the surface

of organic samples, e.g. soybean meal, may have

different kinetics (different directions and different

dynamics), which depends on their properties related

to the chemical composition and physical structure

[7]. It is also connected with the difference between

the water activity in the sample and the relative vapor

pressure of the surrounding air, and also it results

from the nature of the phenomenon taking the form of

adsorption or desorption [12, 17]. The sorption

phenomenon is also related to the extent of ordering

of the matrix in the solid matter subjected to the

interaction with water vapor.

Substances characterized by a relatively high

homogeneity (e.g. starch) do not undergo any changes

in the sorption mechanism due to the water vapor

present in the surrounding atmosphere [22]. On the

other hand, amorphous substances (e.g. powdered

milk) are prone to a change in the sorption mechanism

associated with structural changes in components,

which includes imbibition, growth of the mobility of

protein chains, and revealing new sorption centers

[27]. As a result of the structural changes of the

components, the sorption rate is constantly high or

even increases periodically. This is because of the fact,

that sorption involves an increasing number of active

centers available for water molecules. In addition,

after absorbing a certain amount of water, specific for

each type of substance, amorphous components can

turn into a crystal [31], which, in the range of average

values of relative humidity of the atmosphere, is able

to absorb and retain only small amounts of water. The

transformation of the amorphous component into a

crystalline solid causes a modification of the sorption

mechanism.

3 THE ADVANCEMENT OF THE RESEARCH ON

THE INFLUENCE OF THE MAGNETIC FIELD

ON THE SURFACE PHENOMENA

The influence of the magnetic field on various types of

biological and physicochemical processes has been

studied so far in terms of its usefulness in

environmental engineering, for example in the

crystallization of calcium carbonate [9], in the water

purification [1, 2], in the coagulation and

sedimentation of colloid particles [16, 36], and in the

wastewater treatment [18, 38]. The research results

have also indicated, that the magnetic field affects the

dynamics of some reactions occurring in food during

its storage [20]. However, the most spectacular are the

reports predicating that the magnetic field influences

the proper functioning of living organisms [10].

The studies on the impact of the magnetic field on

the course of surface phenomena are definitely rare. In

the paper prepared by Ocieczek and Otremba [28], the

existence of the influence of the heterogeneous, static

magnetic field on the course of the phenomenon of

water vapor desorption, induced by an increase in

ambient temperature, from the surface of starch

particles, and the practical consequences associated

with it, was demonstrated. It was proven that the

magnetic field increases the value and rate of water

desorption from the starch surface. Although the

mechanism of the course of the desorption process in

the presence of a magnetic field had not been fully

recognized yet, the results of this work have begun

the research aiming for creation of the base for

forecasting the changes occurring in food under

different conditions, even different from those on

Earth. This, in consequence, may be significant in the

future, in consideration of the developing exploration

of extraterrestrial space. What is more, this work

highlighted that the phenomenon of desorption

should be investigated in terms of its role in the

effectiveness of the devices created for dewatering

samples, which are sensitive to high temperatures. On

the other hand, the work of Ocieczek and Otremba

[30] includes an assessment of the effect of a

homogeneous, static magnetic field on the

phenomenon of water vapor sorption, in this case

induced by the influence of the environment with a

humidity of 7% and 75%. It has been found that the

static and homogeneous magnetic field increases the

rate of water adsorption by the tested organic

substances, especially in the initial phase of this

process. In addition, it has been discovered that the

magnetic field affects the sample-atmosphere dynamic

equilibrium, namely with the impact of magnetic field

it is higher. On the other hand, the course of water

desorption from organic substances in the magnetic

field is similar to this process in the absence of the

field. At the same time, no grounds have been found

to conclude, that the magnetic field is not involved in

this process. It has been indicated, that it is more

likely that the interference of the magnetic field in the

used research model, may take place through

kinetically mutually competing processes. The above

mentioned work also contained elements of the

research on the statics of the sorption phenomenon,

which has become the inspiration for the studies

presented in this article. In the next paper [29] the

same authors discussed the effect of a homogeneous,

static magnetic field on the direction, dynamics and

scope of adsorption and desorption of water vapor

through the solid matrix at two different

temperatures. These studies concerned the kinetics of

the sorption phenomenon and the aim of the system

to reach the dynamic equilibrium. The adopted

research model was an extension of the assumptions

of the work described above [30] and enabled to find

that the impact of the magnetic field on the kinetics

and the scope of the surface phenomenon is

statistically significant, and this statement may be

rudimental for the organization and management of

technological processes and storage.

Therefore, the aim of this work is to develop the

knowledge concerning the influence of the magnetic

field, this time, on the statics of the sorption

phenomenon. The essence of the study was to verify

the assumption of the existence of an influence of a

homogeneous, static magnetic field with an induction

of 10 mT on the equilibrium state of the tested system

at a constant temperature 20°C. The study was carried

out parallelly: under control conditions and under the

influence of a homogeneous, static magnetic field. As

a result of the research, a state of dynamic equilibrium

between the tested samples of soybean meal and the

external environment with a humidity ranging from 7

to 98% was identified. In accordance with that, it was

hypothesized that the influence of a homogeneous,

652

static magnetic field changes the state of dynamic

equilibrium of the examined system, in relation to its

aiming to reach maximum disorder and minimum

energy. The study included the determination and

comparison of water vapor sorption isotherms, and

the indication of the parameters of the Brunauer,

Emmett and Teller equation by adjusting them to the

set of empirical data, describing sorption isotherms.

4 RESEARCH MATERIAL, RESEARCH METHODS

AND ORGANIZATION OF THE EXPERIMENT

The research material was soybean meal, which is a

source of protein and valuable amino acids, used as

animal feed. The research material was characterized

by determining the parameters critical for stability

during transport and storage, such as water content

and water activity. In addition, bulk and tapped

density of soybean meal was determined. The main

element of the research was the determination of

water vapor sorption isotherms describing the state of

dynamic equilibrium, and the indication of the

parameters of the BET model.

The water content of the tested material was

determined by drying. About 1 g of the test product

was weighed into the weighed weighing vessels with

an accuracy of 0.0001 g. They were then placed in a

dryer at 105°C and dried to obtain constant weight.

The determination was carried out in three parallel

attempts. The water content were calculated in grams

per 100 grams of dry matter.

Water activity (aw) at 20°C was determined with

an accuracy of ±0.003 in the AquaLab apparatus (ver.

AS4 2,14.0 2017, Series 4TE and 4TEV Decagon

Devices, Inc., Pullman, WA, USA).

Density, which defines the typical of bulk

materials ratio of the mass per unit volume, was

determined in two variants. Determination of the bulk

density consisted in weighing a sample with a mass of

100 g and transferring it into a measuring cylinder in

order to define the volume occupied by this mass. The

essence of the test was to insert dry matter into the

cylinder in such a way that it would not be

compacted. On the other hand, the determination of

the tapped density, consisted in subjecting the sample,

which was prepared for measuring the bulk density,

to mechanical compaction. It was achieved by

exposing the cylinder with the sample to the

vibrations resulting from gentle hitting on a flexible

surface for 60 seconds. As a result of the 60-second

vibration influence on the soybean meal sample

placed in the cylinder, it became denser, which

manifested itself in a reduction in the volume

occupied by the sample, which was read on the

cylinder scale. The tapped density was always

characterized by a higher value than the bulk density.

The determination was carried out in three parallel

attempts.

Sorption isotherms were determined by the

referenced static-desiccator method. The time for the

samples to reach the state of dynamic equilibrium was

set at 9 days, on the basis of a previously conducted

reconnaissance experiment, which had determined the

minimum time necessary to reach the dynamic

equilibrium between the environment and the surface

of soybean meal [30]. 1 g samples of soybean meal,

weighed with an accuracy of 0.0001 g, were placed in

11 desiccators containing supersaturated solutions of

the substance (NaOH - 0.0698; LiCl - 0.1114;

3COOK - 0.2310; MgCl2 -0.3303; K2CO3 - 0.4400;

2Cr2O7 - 0.5480; KJ - 0.6986; NaCl - 0.7542; KCl -

0.8513; KNO

3 - 0.9320; K2Cr2O7 – 0,98) with a water

activity from 0.07 to 0.98 [11]. In desiccators with an

activity higher than 0.6, to eliminate the risk of mold

growth, thymol was placed. Measurements of the

mass and water activity of the tested samples were

carried out each time after the end of time of their

exposure to the environment. The experiment was

conducted in a climatic chamber with a temperature at

20°C ± 1°C. The determination in each of the

desiccators was carried out in three parallel attempts.

Then the procedure was analogous, but the

desiccators with the samples of soybean meal were

placed in a homogeneous, static magnetic field with

an induction of 10 mT (Fig. 1.).

Figure 1. Experimental setup: desiccator with reference

samples in geomagnetic field 0.05 mT (left), desiccator

between Helmholtz coils, which generated static

homogeneous magnetic field 10 mT (right)

The static MF generator has been designed and

constructed at the Physics Department of Gdynia

Maritime University. It consists of two identical

Helmholtz coils arranged parallel to each other. The

electric current in the coils (14 amps) required to

generate static MF 10 mT is provided by two

laboratory switching mode power supplies HCS-3602

(Manson, China). The heat from the coils is discharged

by means of circulating water through the flow cooler

(Titan 4000, Aqua Medic, Germany). Generator is

equipped with AC gaussmeter GM-2 (AlphaLab Inc.,

USA) that measures and adjusts the value of magnetic

induction.

The significance of the differences between the

average water content and water activity in the tested

samples after their incubation under control

conditions and under the influence of a homogeneous,

static magnetic field, was determined using the

Student's t-test. Earlier, the variability assessment

(variance estimation) in the compared groups had

been performed, using the Fisher’s exact test.

In order to determine the differences in the

sorption properties of the tested samples of soybean

meal, on the basis of empirical results, their

transformation has been carried out so as to identify

and compare the parameters of the BET model as:

(1 )[1 ( 1) ]

v Ca

a Ca

− +−

(1)

where:

w – water activity (–); v – equilibrium water content (g

2O/100g d.m.); vm – water content in the monolayer

(g H

2O/100g d.m.); C – energy constant [11, 31].

653

The parameters of the BET equation were

determined numerically, using non-linear regression

and the Monte Carlo algorithm. The minimization of

the sum of squared residuals was used as the objective

function. The calculations were made in the Excel

2010 spreadsheet. The values of the standard errors of

the determined parameters were estimated using the

SolverAid macro command based on the Hessian

matrix [39].

5 RESULTS AND DISCUSSION

Skimmed soybean meal is the material remaining after

the extraction of oil from soybean flakes. Soybean

meal is considered as a dry material (12% d.m.),

because it is in the form of solid particles [26].

Soybean meal is also an heterogeneous mixture of

particles, which differ in size and shape. These

particles are subject to intermolecular interactions

which determine the cohesion of the material, and the

cohesion of the material plays an important role in the

transport process [7].

The parameters for describing cohesion are bulk

density and tapped density. As a result of the

research, it was found that the soybean meal has a

bulk density of 0.606 ± 0.010 g / cm

and a tapped

density of 0.666 ± 0.021 g / cm

. This means that under

the influence of vibration, soybean meal becomes

denser, which increases its density by more than 9%.

This fact is important, influencing both its mechanical

and thermal properties as a cargo. In the studies by

Drzewieniecka et al. [7] it has also been defined that

water content is a factor that significantly influences

the bulk density of soybean meal. In the case of

soybean meal with 10% water content, the bulk

density was 0.593 g / cm

and it was higher in

comparison to dry meal (at 0% of water), in which the

bulk density was 0.584 g / cm

. The growth of the

density of soybean meal due to the increase of water

content and vibrations may lead to self-heating or

even to self-ignition.

Soybean meal is a hygroscopic cargo. Ambient air

humidity associated with daily and seasonal

fluctuations affects the changes in the water content in

soybean meal. This concerns especially the maritime

transport, where modifications in these parameters

are additionally caused by changes of climatic zones.

Therefore, another important parameter

characterizing the tested soybean meal was the water

content and water activity. It was found that the water

content was at 11.8567 ± 0.1076 g / 100 d.m. This value

can be considered as relatively low and appropriate to

ensure its storage stability in the transport process,

provided that the risk associated with the absorption

of water from the environment is eliminated [26].

Equally important factor is the water activity, which

determines changes in dry mass and their dynamics

during storage. This parameter is mainly determined

by the water content, but its level is also influenced by

interactions between the surface of the object and

water, by condensation of water vapor in capillaries,

and by the concentration and type of water-soluble

substances [5]. The water activity was found to be at

0.4755 ± 0.0018. This value can be considered as

relatively low, because it ensures the microbiological

stability of the meal [34]. This statement results from

the fact, that microbes have the ability to reproduce if

the water activity is higher than 0.6 [32]. Therefore,

the water present in the examined meal cannot

increase the level of microbial contamination.

In homogeneous bodies, such as liquids, the

intermolecular forces inside the phase balance each

other. Only particles on the surface of the material are

subject to unbalanced forces, which leads to the

induction of sorption. In inhomogeneous bodies such

as soybean meal, no balance of forces should be

expected, even inside the mass. However, the energy

state of the surfaces of individual particles is always

higher than the energy state of the interior [8].

Products with a developed surface, such as meal, are

characterized by high surface energy, which

significantly affects its interaction with the

environment. On the surface of the product there are

processes leading to the saturation of unbalanced

forces and reaching the state of minimum energy.

This, thus, leads to a densification of the particles of

the phase being in contact with the surface.

Although the phenomenon of water vapor

sorption on the surface of organic samples goes on

with different kinetics, it leads to a state of dynamic

equilibrium, depending on the ambient temperature.

Therefore, the range of the sorption process is

reflected in the static description of this phenomenon,

which is the sorption isotherm. Each point on the

sorption isotherm is reached as a result of a dynamic

process with different specificities related to more or

less complex mechanisms [17]. Most often, the water

vapor sorption isotherms determined for biological

material have a sigmoidal shape with two

characteristic inflection points [31]. First of them is in

the range of low water activities and corresponds to

the formation of the monomolecular layer. The second

one is in the range of high water activities and

indicates the initiation of the capillary condensation

phenomenon. Table 1 summarizes the empirically

obtained results, which describe the characteristic for

the sorption isotherm dependencies between the

water content and water activity in the entire range.

Identification of the significance of the influence of

the magnetic field on the values of water content and

water activity in the tested samples was made using

the Student's t-test, which was preceded with an

assessment of the variability in the compared groups

with the use of Fisher's exact test. The results are

summarized in Table 2.

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Table 1. Isotherms of the water vapor sorption on the surface of soybean meal particles determined under control conditions

and under the influence of a homogeneous, static magnetic field with an induction of 10 mT

__________________________________________________________________________________________________

L.p. Control conditions Conditions with the influence of a magnetic field

Water activity±SD Water content±SD Water activity±SD Water content±SD

__________________________________________________________________________________________________

1. 0.0486 ± 0.0073 5.2863 ± 0.0152 0.0760 ± 0.0016 5.8567 ± 0.3749

2. 0.1210 ± 0.0010 7.8056 ± 0.0604 0.1197 ± 0.0017 7.3297 ± 0.0973

3. 0.2348 ± 0.0014 10.0782 ± 0.0158 0.2361 ± 0.0005 10.0702 ± 0.0467

4. 0.3382 ± 0.0023 11.1321 ± 0.0440 0.3310 ± 0.0012 10.9741 ± 0.0471

5. 0.4155 ± 0.0008 12.3417 ± 0.0403 0.4238 ± 0.0017 12.4136 ± 0.0414

6. 0.5059 ± 0.0025 13.3563 ± 0.0618 0.5376 ± 0.0007 13.3052 ± 0.0323

7. 0.6701 ± 0.0023 14.6235 ± 0.1754 0.5934 ± 0.0019 13.6217 ± 0.0333

8. 0.7383 ± 0.0006 16.2498 ± 0.1170 0.7445 ± 0.0022 16.3769 ± 0.1800

9. 0.8153 ± 0.0028 19.9768 ± 0.3502 0.8227 ± 0.0033 20.2717 ± 0.2417

10. 0.9228 ± 0.0049 30.7746 ± 0.9779 0.9444 ± 0.0009 33.2165 ± 0.5210

11. 0.9485 ± 0.0024 35.7387 ± 0.3564 0.9535 ± 0.0030 37.1627 ± 1.5803

__________________________________________________________________________________________________

Statistical evaluation of the equilibrium water

content and water activity in soybean meal samples

subjected to the influence of water vapor under the

control conditions, and under the influence of the

magnetic field, indicated the existence of the impact of

this field on the amount and condition of water in the

tested samples. Desiccators with a statistically

significant effect of the field on water content and

water activity are marked in bold. At the same time, it

should be noted that the influence of the magnetic

field had a greater impact on the condition of water

expressed by its activity than on the water content.

Therefore, it can be assumed that the performed

observation was the result of the fact that the magnetic

field influenced not only the condition of water

contained in the soybean meal but also that which was

contained in supersaturated solutions of substances in

desiccators. Thus, the achieving of the equilibrium

state, examined with the use of sorption isotherms,

was the result of phenomena that could strengthen or

weaken each other. Ocieczek and Otremba [29, 30]

had reached similar conclusions in their research on

the kinetics of the sorption process.

Table 2. The results of the significance of the influence of the

magnetic field on the water content and water activity in the

state of dynamic equilibrium with the environment

_______________________________________________

L.p. Water content n=3 Water activity n=3

F-test P-value F-test P-value

_______________________________________________

1. 0.0033 0.0580 0.0904 0.0031

2. 0.5568 0.0020 0.5342 0.3312

3. 0.2044 0.7927 0.2161 0.1750

4. 0.9308 0.0132 0.4216 0.0093

5. 0.9745 0.0975 0.3433 0.0017

6. 0.4286 0.0027 0.1570 2.8424E-05

7. 0.0695 0.0002 0.8250 1.4488E-06

8. 0.5941 0.3632 0.1161 0.0093

9. 0.6455 0.2962 0.8367 0.0404

10. 0.4421 0.0188 0.0620 0.0017

11. 0.0968 0.2025 0.7901 0.0867

_______________________________________________

The obtained results (Table 1) have indicated that

the course of both isotherms was typical and have

allowed to classify them as type II in the Brunauer’s

classification. The first part of the sigmoidal isotherm

corresponds to the low values of water activity

present in the atmosphere. This area is consistent with

the coverage of energetically different hydrophilic

groups on the surface of the particles by water

molecules. Covering all available hydrophilic groups

by water molecules is equivalent to formation of a

monolayer. This value is specific to each product and

depends on its chemical composition and physical

structure. The second area on the sorption isotherm

corresponds to the average values of water activity

present in the environment, which is in equilibrium

with the tested product. This part of the isotherm

describes the state in which a multilayer is formed,

and this is associated with the appearance of water-

water interactions. These bonds are weaker than the

water-matrix, which are characteristic of the first zone

of the sorption isotherm. The third area of the

sorption isotherm, which corresponds to the high

values of water activity present in the atmosphere,

starts the phenomenon of capillary condensation in

the microcapillaries, which are filled first. Then the

mesocapillaries and macrocapillaries fill up.

The course of the sorption process under the

influence of the increasing saturation of the air with

water vapor was expressed in the theory of multilayer

adsorption [4] and capillary condensation [40]. Minor

coverage of the adsorbent surface with water

molecules (the first area on the sorption isotherm)

causes water-matrix interactions, which determine the

energy state of the adsorbent surface. However, each

adsorbed water molecule changes the sorption

conditions. After coverage of all available, although

usually energetically different, active centers, the

intermolecular forces become homogeneous,

represented mainly by water-water interactions (the

second sorption area on the sorption isotherm). This

type of interaction at an appropriate high water

content in the environment leads to the filling of some

capillaries. This phenomenon is known as capillary

condensation, which corresponds to the third area on

the sorption isotherm.

Lewicki et al. [22] claimed that the existence of the

connection between the rate of sorption and the stage

of the process described by particular areas on the

sorption isotherm entitles to determine such

parameters, based on sorption isotherms, as:

monolayer capacity and water content required for

initiation of capillary condensation in the tested

material. For the analysis of the sorption

phenomenon, various mathematical models of

sorption isotherms has been used, through exploring

empirical data. The BET model deserves attention.

This model is one of the most widely used theoretical

models. It is based on the assumptions defined by

Langmuir in the multimolecular idea of sorption. The

BET model makes the transformation of empirical

data and the determination of the monolayer capacity

(vm), and the energy constant (C) possible. The BET

equation is the most commonly used model to study

surface phenomena occurring in food. It is used to

655

interpret multilayer sorption isotherms, especially

those of types II and III. It has been found to be an

effective method of estimating the amount of water

associated with the polar molecules of dehydrated

food. This model assumes that the sigmoidal shape of

the isotherm results from multilayer adsorption.

Therefore, this approach acknowledges that each

adsorbed molecule becomes the adsorption site for the

next molecule of adsorbate. The forces involved in the

formation of successive layers are analogous to the

forces causing the condensation of vapor into a liquid.

On the other hand, it should be emphasized that the

BET equation, regardless of the chosen method for

identifying its parameters (analytical or numerical),

describes well the course of the sorption phenomenon

only in a limited range of water activity (0.05 ÷ 0.5)

[33].

Taking into account the limitations of the BET

model, its parameters were estimated in the range

indicated as properly corresponding to empirical

data. The values of the sums of squared deviations

and the error values with which they were

determined, demonstrate that the BET model

describes the process of water vapor adsorption on

the surface of soybean meal properly (Table 3).

Table 3. Parameters of the BET model of isotherms of

soybean meal determined at 20°C under the control

conditions and under the influence of a homogeneous, static

magnetic field with an induction of 10 mT

_______________________________________________

Parameter Control conditions Conditions with the

influence of a

magnetic field

_______________________________________________

Value Standard Value Standard

error error

_______________________________________________

vm - monolayer 5.5892 2.3716 7.0856 3.1486

[g H

2O·100

-1

d.m.]

w - water activity 0.0573 0.1125

corresponding to

the monolayer [-]

C - energy 1.1670 0.3545 0.9596 0.3122

constant [-]

SS - sums of 5.7904 1.3893 6.6157 1.4850

squared

deviations

_______________________________________________

The water content corresponding to the coverage

of the surface of the meal’s particles with a monolayer

of water is the minimum, and at the same time, the

optimal water content. This water content determines

the durability of the dry product, because it signifies

low water activity. This water does not freeze at -

40°C, it cannot act as a solvent, it has no plasticizing

properties, it does not cause hydrolytic reactions, but

at the same time, it protects against oxidative

reactions. In practice, the knowledge of this parameter

is used to determine the end point of the drying

process. It is worth mentioning, that this action is

intended to optimize the costs of technological

processes, and it is also important in the storage and

transport of food. According to Karel [19], most food

products have a monomolecular capacity from 4 to 11

kg per 100 kg of dry substance. The monolayer values

determined with the use of the BET model were in the

range of 5.59 ÷ 7.09 g H

2O per 100 g of dry matter. The

monolayer volume (v

m) estimated using the BET

model indicates the availability of polar sites for water

vapor. The volume of monolayer is determined by the

components that are donors of hydrophilic groups

and by their physical state conditioned by, for

example, the degree of comminution [27]. The amount

of water in the product greater than the capacity of the

monomolecular layer leads to the reaching of a critical

water content, the exceeding of which causes various

undesirable changes in the quality of the product. The

differentiation of the monolayer values of the tested

samples of soybean meal may indicate the different

state of water molecules as a result of the influence of

the magnetic field. At the same time, taking into

consideration everything that has been discovered

about the volume of the monolayer so far, it can be

stated that soybean meal under the influence of the

magnetic field is less sensitive to higher concentration

of water vapor in the environment. A higher value of

the monolayer indicates that the meal, going to reach

equilibrium with the environment, can absorb more

water, which will be strongly bound to its matrix.

Therefore, changes of a hydrolytic nature will not be

intensified, while changes of an oxidative nature

should be limited. This assumption has been made on

the basis of previous findings, which indicated that

hydrolytic changes require adequately high water

activity. Moreover, the higher water content of

relatively low water activity, should limit the access of

oxygen to the larger surface of the soybean meal

particles. What is worth mentioning is the fact, that

this should restrain the contact of oxygen with the

lipid fraction, reducing the risk associated with their

oxidation.

The second parameter of the BET model is the

energy constant. The energy constant C reflects the

difference between the monolayer desorption

enthalpy and the enthalpy of vaporization of the

liquid adsorbent. Its values in both cases were very

low, what indicates the physical nature of the

researched phenomenon. Therefore, it can be

concluded that the influence of the magnetic field

does not change the nature of the water vapor

sorption process, which is still fully reversible. This

statement is an additional argument in undertaking

further research on the role of the magnetic field in

stabilizing the quality of dry bulk cargo during

transport, especially maritime.

6 CONCLUSIONS

Maintaining low water content, and consequently low

water activity, is the primary factor in preserving the

quality of a dry bulk cargo, such as soybean meal.

Furthermore, the low water content also ensures

microbiological safety. On the other hand, the low

water content facilitates the dusting of the cargo and

the increase of the risk of explosion during cargo

handling operations.

The elementary method of stabilizing the quality of

soybean meal is its deep dehydration, although it

should be emphasized that this method has reached

the limits of its usefulness. This is because of the

challenge to maintain the low water content, which is

problematic. Therefore, it is reasonable to search for

new, also unconventional, methods allowing the

stabilization of water content and, especially, water

activity.

656

Currently, the methods facilitating the

optimization of procedures related to preparation and

handling of bulk cargo for transport and storage are

being searched. In reference to that, the obtained

results indicate that soybean meal, which is a highly

hygroscopic cargo, is also sensitive to water, in the

form of vapor. Moreover, it was found that the

influence of a homogeneous, static magnetic field is a

factor that differentiates the water content and water

activity in the soybean meal. The exploration of

empirical results with the use of BET model has

indicated that the influence of the magnetic field is a

factor that differentiates also the sorption properties

of soybean meal. Under the impact of magnetic field,

the volume of the monolayer is greater, which makes

the soybean meal less sensitive to the surrounding

water vapor. At the same time, the energy connected

with the sorption phenomenon under the influence of

the magnetic field does not change. Therefore, the

magnetic field does not change the nature of the

sorption process. Consequently, there are indications

that the presence of a strong magnetic field may

determine the stability of the cargo during long-term

transport or storage.

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