SELECTED PROPERTIES OF YELLOW LUPINE SEEDS FORMED BY HYDRATION

Dariusz Andrejko¹, Agnieszka Kamińska^2
1 Department of Food Engineering and Machinery, Agricultural University of Lublin, Poland
² Department of Applied Mathematics, Agricultural University of Lublin, Poland

ABSTRACT

The paper presents results of studies on the changes of physical properties of yellow lupine seeds, Radames cv. under the effect of hydration. The dynamics of changes of the water content in lupine seeds depending on the time of their hydration varied; in the first phase of the process a very fast increase of the water content in the seeds was observed, in the second the rate of water absorption decreased, while in the third the seeds achieved full capacity of hydration. Peleg’s model was applied for the mathematical description of the phenomenon. Intensive absorption of the water by lupine seeds was manifested by their increased size. The studies found out a 72% increase of the seeds’ width, a 45% increase of the thickness and a 46.2% increase of the length. The effect of hydration was lowered resistance of lupine seeds to pressing, especially visible for the first 2 hours of the process. A further increase of the hydration time did not cause any significant changes of this volume.

Key words: Peleg’s model, lupine seeds, pressing force, geometric properties, water content.

INTRODUCTION

The process of hydration is commonly used in the processing of cereal grain and the seeds of the pulses. A number of important changes in the structure of the raw materials take place in its course and they are mainly associated with increased water content [2, 10, 13]. Water is bound in the seeds by various chemical and physical forces and its content changes together with the changes occurring in the environment. In the process of hydration, and especially in its initial stage, hydration of the colloids contained in the seeds and manifested by the seeds’ swelling is the most important phenomenon affecting a big initial rate of water absorption. This phenomenon is called imbibition and it is characterized by the occurrence of considerable pressures of water suction, which even reach the value of a few hundred MPa [12]. Parrish and Leopold [15] state that the greatest imbibition potential was observed after 10 minutes’ hydration, after wetting the cover and releasing the adsorbed gases. The rate of imbibition suction of the water decreases in time and is dependent on the kind of colloids, their capacity, temperature and pH. Imbibition is physicochemical in nature and it is not connected with any living processes. Both “alive” and “dead” seeds absorb water equally well in the initial stage of hydration [20]. In the swollen seeds proper conditions appear for increasing the living processes and then osmosis stars to play a decisive role in water distribution [8].

During the process of hydration water gets in contact with the seeds on the whole surface of the cover. There are, however, zones of different permeability in it. An important role in the mechanism of water absorption in the seeds is played by the stamp, the stamp slit and mikropyle [17]. The same authors [17], while analyzing the process of water absorption in the soybean seeds, distinguished three stages. In the first one (the first 5 hours of moistening), water is absorbed relatively fast thanks to the forces of imbibition. The second stage (between the 5^th and the 12^th hours) is characterized by a decreased rate of water absorption. In this time the seeds reach almost full capacity of hydration. In the third stage (over 12 hours) the seeds reach the full capacity of hydration. In this time osmotic forces take over a decisive role in the inner distribution of water.

PURPOSE OF STUDIES

Water absorption by the seeds, which – in consequence – brings about their greater size, that is swelling, is one of the underestimated but interesting phenomena. It should be noticed that increased moisture of the seeds is followed by changes in their physical properties, which have to be considered while designing technological processes. Hence, the purpose of studies was an attempt at a mathematical description of changes in the physical properties (hydration, geometric properties and resistance) on yellow lupine seeds, Radames cv. depending on the period of moistening.

MATERIALS AND METHODS

Material. The studies referred to yellow lupine, Radames cv. The seeds of the same cultivar came from the harvest of 2004. The basic physical properties of the seeds and the chemical composition of the seed leaf and the seed coat are presented in tables 1 and 2.

Hydration of the seeds. Lupine seeds (water content before hydration was 0.133 kg∙kg_d.m.^-1) were moistened using the immersion method. Seed samples of the weight 1000 g were immersed in distilled water at the temperature of 20°C. In the course of the process at definite intervals (0.25; 0.5; 1.0; 2.0; 3.0; 4.0; 6.0; 8.0; 10.0 h) weighed portions of 50 g were sampled, dried on a tissue and their physical properties were marked.

Physical properties. The following physical properties were marked:

• water content, PN-86/A-74011,

• geometrical dimensions, measurements were performed for 30 randomly chosen seeds, using the computer system of picture analysis SVISTMET. The system was installed in computer PC-AT. A camera CCD was used to record the pictures. The picture was displayed on the screen of a colour monitor NEC connected to a specialist vision card. The second monitor served the user’s menu. In order to get a picture for analysis, a unit object was isolated (one seed) and placed in a special frame within the vision field of CCD camera and lit from four sides by means of dispersed light with the aim of minimizing the shadow. Next, the picture obtained from the camera and displayed on the control monitor was recorded in the form of a bit-map in the mass memory of SVISTMET system. After initial treatment of the seeds’ picture the procedures of SVISTMET program were started, making it possible to mark the geometrical dimensions, after former scaling of the system [5].

• pressing force, the measurements were carried out in 30 repetitions according to the methods presented by Andrejko [3]. The applied method consisted in using the test of axial pressing of singular seeds between the parallel plates of a resistance machine Intron 4302. During the testing, the speed of the movable plate was permanent and it was 10 mm/min. Single lupine seeds were placed with the seed leaves parallel to the surface of the immobile plate, next they were pressed by means of the movable plate. The axis of the pressing force ran along the diameter of the cross-section of the seeds. The measurement was conducted till the moment when the seed broke, registering the value of the pressing force when it happened.

A description of water absorption. Peleg’s model was applied to describe the process of water absorption by lupine seeds during their hydration:

(1)

where:
u(τ) – water content in seeds after time τ, [kg∙kg_d.m.^-1],
u₀ – initial water content in seeds, [kg∙kg_d.m.^-1],
K₁ – constant value, [h/kg∙kg_d.m.^-1],
K₂ – constant value, [l/kg∙kg_d.m.^-1].

The advantage of this equation is its simplicity as compared to others [11]. Aiming at the mathematical description of the phenomenon of water absorption, the expression was positively verified for a few species of cereals and pulse crops [1, 9, 16, 18, 19].

Statistical analysis. The statistical analysis of the data used the analysis of variance and linear and non-linear regression [4, 14]. The iterative method of Gauss-Newton’s linearization with the function of loss in the form of the least squares was applied in the curve-line regression for approximation of unknown parameters. Besides, verification of the obtained regression models was performed and the analysis of the rests was made in order to verify the assumptions. Selected results of the analyses were presented on the example of Peleg’s model. Statistical analysis of the experimental data was conducted by means of Statistica 6.0.

RESULTS AND DISCUSSION

Changes of the seeds’ hydration. The process of water absorption by lupine seeds can be divided into three phases (fig. 1). In the first phase (first four hours of moistening) the seeds absorbed the water relatively fast, the effect of which was a considerable increase of their water content. In the second phase (from 4^th to 12^th hours) the rate of water absorption dropped and the seeds reached almost full capacity of hydration. The third phase (from 12^th to 48^th hours, not presented in figure 1). was characterized by slight changes in the water content in the seeds. In this phase constant exchange of water with the environment takes place; however, the main causative mechanisms of these phenomena are not imbibition forces but osmosis.

The calculations provided an equation describing the interrelations between the hydration time and the content of water in yellow lupine seeds, Radames cv. (2):

R2 = 98.89%.

(2)

The suggested model (within the hydration time range from 0 to 12 hours) explains 98.9% variability of the water content in lupine seeds. Estimation of the results of regression presented in tab. 3-4 confirms both high significance of the whole model and its parameters. In order to verify the assumption of regression the analysis of the residuals was conducted and it did not show any serious deviations. To give an example, the normal plot of the residuals (fig. 2) and the histogram of the residuals (fig. 3) were presented, which confirmed the thesis that there was no basis to reject the hypothesis of the normal distribution of the residuals.

Changes of geometrical properties of lupine seeds. Intensive water absorption by yellow lupine seeds, Radames cv. was manifested in their greater size. Both the values and the dynamics of that increase were different on different levels. The seeds’ width increased by about 72%, their length – by about 46.5% and the thickness – by about 45%. The dependence of the width and the length of the seeds on the water content was described by means of a logarithm function of regression (fig. 4 and 5), while the relation between the thickness and the water content was described using a multinomial function of regression of third degree (fig. 6).

Changes of the seeds’ resistance to pressing. In the process of hydration the seeds undergo damage as a result of inner strains caused by high gradients of the water potential appearing in the process of mass exchange [21]. As stated by Grundas et al. [7], in the case of caryopses the greatest intensity of this type of mechanical injuries of the endosperm is observed as early as after 3 hours of moistening. However, it should be stressed that after 12 hours of hydration the changes in the physical structure of the endosperm are followed by changes of its biochemical properties [6]. The effects of these changed manifested by decreased resistance of the seeds to the effect of the pressing forces are presented in figure 7. A decrease of the resistance of the yellow lupine seeds Radames cv. was visible during the first 2 hours of hydration, i.e. until the seeds reached the water content of about 0.6 kg∙kg_d.m^-1. A further increase of the hydration time did not cause any significant changes of this value. Variance analysis confirmed this. Changes of the value of the pressing force under the effect of increased water content during the first two hours of hydration were described in the exponential function of regression (fig. 7).and the percentage of explained variance reached 97.3%.

CONCLUSIONS

1. Three phases can be distinguished in the process of water absorption by the seeds of yellow lupine Radames cv.: the first phase characterized by intensive absorption of water by the seeds, the second shown in a considerable decrease of the rate of absorbing the water, and the third, when then seeds reach the full capacity of hydration.

2. Peleg’s model can be successfully applied for a mathematical description of the process of water absorption by yellow lupine seeds Radames cv.

3. Increased water content in the seeds of yellow lupine, Radames cv., which is the effect of the hydration process, caused the following changes of the physical properties:

• increased geometrical dimensions of the seeds. The relation between the width and length of the water content in the seeds is well described by the logarithm function of regression, while the dynamics of the increase of the thickness in the function of seed moisture is reflected by the multinomial function of third degree.

• decreased value of the pressing force. These changes are especially visible during the first 2 hours of the moistening. Their course is described by the multinomial function of regression.

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