Dielectric properties of papaya seeds from 75 kHz to 5 MHz
ture content, relative permittivity, seed bulk density INTRODUCTION The papaya tree (Carica papaya L. is a typically tropical fruit tree with a probable centre of origin between the southern region of Mexico and Costa Rica (Kim et al. 2002) However, this tree has become widely distributed in several areas of the world, with reports of its production in virtually all tropical countries and warm climate subtropical regions (Fuentes and Santamaria, 2014) The fruit is considered relatively exotic in temperate countries, whereas papaya is an important vitamin source in countries with tropical climate. In addition, proteinases extracted from the latex of green fruits are widely used in the pharmaceutical and food industries (Carvalho and Renner, 2012) The annual global production of papaya is greater than 12 million t, with a continuous growth rate of 367 000 t per year in the last 15 years (FAO, 2017) In Brazil alone, with annual production of 1.6 million t (FAO, 2017) the estimated use of papaya seeds is approximately 5 000 kg per year with a market value of US$ 2 200 kg for the Calimosa hybrid papaya seeds and US$ 3 500 kg for the Tainung hybrid papaya seeds (Carlesso, 2009) The values mentioned above demonstrate the importance of an in depth study of the steps involved in the processing of papaya seeds, including their handling, drying and storage, all of which involve the accurate determination of the moisture content of the seeds. Thus, it is pertinent to include the study of the dielectric properties of papaya seeds in the research aimed at determining the on line moisture content of agricultural commodities of greater commercial expression, such as cereal grains, oilseeds and coffee. P.A. BERBERT et al. 186 Although papaya seeds exhibit variable physiological behaviours in relation to storage conditions (Sun and Liang, 2001) papaya seeds were initially classified as orthodox by Ellis et al. (1985) However, this classification was re evaluated by Ellis et al. (1991) when stating that the behaviour of papaya seeds in storage should be redefined as an intermediate between those observed in orthodox and recalcitrant seeds. Orthodox and intermediate seeds are generally stored for extended periods of time with reduced moisture contents (5 to 8% (values of the percentage moisture content are expressed on a wet basis (w.b. throughout the paper) at low temperatures. Moura et al. (2013) emphasized the importance of the on line moisture content sensors installed in the storage environment that would function as a method of quality control of the product in storage, without the need to destroy the samples, as is the case with the oven drying method. The on line moisture content meters of granular agricultural materials are classified as indirect measurement methods because they are based on physical properties that can be correlated with the volumetric concentration of water in the seed sample. Currently, meters that use dielectric properties as parameters for estimating the moisture content are the most widely studied. According to Trabelsi et al. (2013) it is necessary to analyse in detail the relative permittivity (e' and the loss factor (e" as a function of frequency, along with bulk density (r) and the temperature of seed samples contained inside the capacitive sensors to correlate dielectric properties with the seed moisture content. This procedure predicts which frequencies and ranges of the moisture content and bulk density may integrate dielectric models that shall be incorporated into electronic measurement systems. Therefore, in order to determine the moisture content with accuracy, the effects of both bulk density and temperature must be made explicit through additional measurements and compensation, or eliminated by the identification of functions that are independent of bulk density and insensitive to temperature variations (Trabelsi et al. 2013) Most researchers looking for solutions to reduce the effect of bulk density on the on line determination of the moisture content of agricultural products have concentrated their efforts on the development of meters operating at microwave frequencies (Digman et al. 2012; Lisovsky, 2007) This focus is partly due to the decrease or elimination of problems related to ionic conductivity, which allows the study of a wider range of moisture contents. However, researchers have also demonstrated that it is possible to indirectly measure the seed moisture content (generally up to 23% regardless of the bulk density variations of the sample, using radiofrequencies (Funk et al. 2007; Moura et al. 2013) This is due to the fact that, with the appropriate choice of the size and shape of electrodes, it is possible to measure the moisture content using a considerable volume of seeds, resulting in a more representative measurement. To design a capacitive sensor that can integrate an online moisture content meter for agricultural purposes, and that is as comprehensive as possible, it is first necessary to determine the dielectric properties of various types of products. The investigation of the variation of e' and e" as a function of the moisture content (M) along with bulk density (r) and the electric field oscillation frequency (F) makes it possible to determine the ranges or values of these variables, which will compose the dielectric model M f [e'(F) e"(F) that is independent of the variation of r. In fact, studies on the dielectric properties of seeds, such as rice (Prasad and Singh, 2007) maize (Cseresnyes et al. 2013; Sacilik and Colak, 2010) sorghum (Moura et al. 2013) and wheat (Berbert and Stenning, 1996) oilseeds, such as peanuts (Boldor et al. 2004) flax seed (Sacilik et al. 2006) and soybean (Trabelsi and Nelson, 2006) legumes, such as beans (Berbert et al. 2002) coffee (Berbert et al. 2001) and wheat flour mixed with oat meal (Luczycka et al. 2013) have been performed over the last decades, using both microwave frequencies and radiofrequencies. Thus, when designing a capacitive sensor that can integrate the electronic circuit of an on line moisture content meter prototype of granular agricultural products, there should be new dielectric functions that would allow the estimation of the moisture content of as many products as possible, to make it more comprehensive. The inclusion of new dielectric functions is also useful for calibrating the meter as new varieties or cultivars enter the market (Trabelsi and Nelson, 2006) Considering the above, the objective of the present study was to determine the dielectric properties of papaya seeds of the Golden cultivar, and the Tainung and Calimosa hybrids for the moisture content values ranging from 6 to 23% at four levels of bulk density, for frequencies between 75 kHz and 5 MHz, using automatic data collection methods through software specifically developed for this purpose. In this study, the effect of temperature on dielectric properties was not evaluated, constituting the subject to be studied in a forthcoming paper on dielectric models for moisture estimation. MATERIAL AND METHODS The papaya fruits (Carica papaya L. of the Solo group, cv. Golden, from the F2 generation of the Calimosa hybrid and the Tainung hybrid were harvested at maturation stage 2 in a commercial crop of the company CalimanAgricola S/A located in the municipality of Linhares, State of Espirito Santo, Brazil (19deg23'28"S latitude, 40deg04'20"W longitude, altitude: 33 m) This stage of maturation is characterised by the fact that the fruit peel surface has 15 to 25% of a yellowish colour (CEAGESP, 2015) Fruits are harvested in early stages of maturation to reduce the loss of firmness during the subsequent handling and transportation. Besides, papaya harvested at maturation stage 2 had DIELECTRIC PROPERTIES OF PAPAYA SEEDS 187 received higher scores in sensory evaluation, mainly flavour and appearance, by the time it reached market shelves, which generally coincides with full ripeness (Bron and Jacomino, 2006) We collected 45 fruits of each genotype. Subsequently, the fruits were transported to the Laboratory of Agricultural Engineering of the Centre for Agricultural Sciences and Technologies of the State University of Northern Rio de Janeiro, located in the municipality of Campos dos Goytacazes, State of Rio de Janeiro, Brazil (21deg45'15"S latitude, 41deg19'28"W longitude, altitude: 13 m).When they arrived at the laboratory, the fruits were placed on a bench and remained at ambient conditions for approximately 10 days until reaching maturation stage 5, which is the stage in which the fruits are fully ripe, with more than 75% of the yellowish peel surface (CEAGESP, 2015) Seeds obtained from fruits at maturation stage 5 have higher germination and vigour, as compared to seeds from unripe fruits (Martins et al. 2006) The fruits were cut longitudinally to remove the seeds and placental remnants, which were then transferred to a steel mesh sieve. Cleaning was performed by friction against the mesh of the sieve in running water until the placenta and sarcotesta were completely removed. The seeds were exposed to laboratory ambient conditions for approximately 3 h over a wire sieve until the surface water evaporated. The pre drying procedure reduced the moisture content of the seeds to approximately 40% Subsequently, the seeds were dried at 40degC in a prototype fixed bed dryer with tangential air flow until the moisture content was reduced to approximately 23% The seed lot was then divided into sub lots that were packed in sealed 1 750 ml glass containers and stored in a BOD incubator at 10degC. To obtain sub lots with the desired moisture content values, namely between 23 and 6% in each experiment, one of the stored containers was removed from the refrigerated environment and remained on the bench until it was in thermal equilibrium with the ambient air. Afterwards, the seeds were arranged on four rectangular trays with
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