Correlation between morphology, hydration kinetics and mathematical models on Andean lupin (Lupinus mutabilis Sweet) grains

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Highlights

  • The Andean lupin hydration kinetics was firstly studied, described and modeled.

  • The seed coat was demonstrated as the main cause of its sigmoidal behavior.

  • Microstructural study was correlated with hydration kinetics.

  • The water influx pathway into the grain was established.

  • The hydration of grains were characterized by diffusion and/or capillarity.

Abstract

This work describes the hydration kinetics of Andean lupin (Lupinus mutabilis Sweet) grains, correlating its morphology with mathematical models in order to explain the process. Microstructural analysis of the grains was performed using a scanning electron microscope and the hydration kinetics was determined between 23 °C and 60 °C. The hydration kinetics showed the sigmoidal behavior, which was fitted with two sigmoidal models. Further, this behavior was explained by the grain morphology, demonstrating that the seed coat was the cause of this behavior, related to the slow initial water intake. It was demonstrated that the water entered to the grain by diffusion through the seed coat, and by capillarity through the hilar fissure. Besides, an increase in the process temperature resulted in higher water absorption rate, smaller hydration time and higher final moisture content, enhancing the process. Each model's parameter (equilibrium moisture, water absorption rate and lag phase time) was then modeled as function of the temperature. Finally, two general models were obtained with good agreement, which can be used to predict the moisture of the grain as function of both time and temperature.

Introduction

The Andean lupin, also called Chocho or Tarwi (NRC-US, 1989) is a legume from the Andean region, of South America, which is widely used by the local population as food and natural medicine (Jacobsen & Mujica, 2006). It is known by its high nutritional value, with a high content of proteins (44.3 g/100 g) and unsaturated fatty acids (40.4 g/100 g of omega-9, 37.1 g/100 g of omega-6 and 2.9 g/100 g of omega-3, with respect to the total fat content – 16.5 g/100 g) (Jacobsen & Mujica, 2006). It is used mainly as a protein source in human and animal nutrition in various parts of the world, and its consumption has increased in recent years (Güémes-Vera, Peña-Bautista, Jiménez-Martínez, Dávila-Ortiz, & Calderón-Domínguez, 2008). This grain is being considered an internationally promising crop, especially in Peru, where its production is growing since it started to be incentivized (Brigas Céspedes, 2014, Mohme Seminario, 2014).

The hydration process in grains is a prior step to different processes such as cooking, extraction, germination and wet milling, since it prepares the grains for processing. In most cases, this stage is a batch unit operation, with a long duration (between 4 and 18 h on average). Many studies have already been conducted about the hydration of different grains, such as adzuki beans (Oliveira et al., 2013), chickpeas (Gowen et al., 2007, Ibarz et al., 2004, Yildirim et al., 2011), white lupin (Solomon, 2009), red kidney beans (Abu-Ghannam and McKenna, 1997a, Abu-Ghannam and McKenna, 1997b), sesame seeds (Khazaei & Mohammadi, 2009), and so on. However, most of these works just evaluated the grain hydration using simple kinetics models, neglecting the initial lag phase that some grains have. The few studies that considered sigmoidal hydration kinetics for grains neither ensure the cause of lag phase nor explain the process morphologically giving only suppositions. Further, there is not any work in the literature studying the hydration kinetics of Andean lupin (Lupinus mutabilis Sweet), despite that it is an important stage because it increases the water content of the grain and enhances the alkaloids extraction in the subsequent stages (Carvajal-Larenas, Nout, van Boekel, Koziol, & Linnemann, 2013).

The present work correlated the morphology, hydration behavior and mathematical models in order to explain and predict the hydration kinetics of Andean lupin (L. mutabilis Sweet) grains.

Section snippets

Water uptake behavior

Andean lupin grains (L. mutabilis Sweet) (9.08 ± 1.44 g/100 g d.b moisture, 9.98 ± 0.64 mm length, 8.39 ± 0.41 mm width and 6.02 ± 0.35 mm thick) were purchased in a local market of Trujillo – Perú. The Andean lupin used was breed in the north Andean region of Perú (La Libertad ∼3200 masl (meters above the sea level)). After harvest, the grains were stored for 2 months before being sold. The grains were selected by eliminating those that were not intact and stained. The grains were stored in a

Andean lupin hydration kinetics

The effect of temperature on the moisture content of Andean lupin grains is shown in Fig. 1. The moisture increased with the duration of soaking and a lag phase at the first part of the curve can be clearly seen, where the water uptake rate was low at all evaluated temperatures. Thus, the grain hydration can be described by a sigmoidal behavior, with an initial lag phase followed by a higher absorption rate phase and, finally, by a stationary phase.

This behavior is similar to different grains

Conclusions

The Andean lupin grains (L. mutabilis Sweet) hydration follows a sigmoidal behavior, which was proven to be due to the seed coat and the first distribution of water into it. Since seed coat has the function of controlling water intake in the grain by waterproofing, it reduces the water absorption rate almost six times and allows higher water holding capacity. Besides, it was demonstrated that the water entrance into the grain is carried out both by capillarity through the hilar fissure, and

Acknowledgments

The authors thank the “Ministerio de Educación del Perú” for the A.C. Miano scholarship granted by the program “Programa Nacional de Becas y Crédito Educativo” (PRONABEC). The authors are grateful to the “Núcleo de Apoio à Pesquisa em Microscopia Eletrônica Aplicada à Pesquisa Agropecuária” (NAP/MEPA-ESALQ/USP) for the support and facilities of Electronic Microscopy.

Nomenclature

a
Linear model parameter (slope) (Equation (3)) [Different unit]
b
Linear model parameter (ordinate axis intercept) (Equation (3)) [Different unit]
kIA
Ibarz–Augusto's kinetics parameter related to the lag phase and water absorption rate (Equations (5), (9))) [g/100 g d.b.−1 min−1]
kK
Kaptso et al. kinetics parameter related to water absorption rate (Equations (4), (8))) [g/100 g d.b.−1]
Meq
Equilibrium moisture content (Equations (4), (5), (6), (7)) [g/100 g d.b.]
Mexperimental
Experimental moisture

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