Amylose content and micromorphology of popcorn progenies with different popping expansion volumes

Teores de amilose e micromorfologia de progênies de milho-pipoca com diferentes volumes de expansão

Ana Izabella Freire Elisa de Melo Castro Ariana Mota Pereira Renata Ranielly Pedroza Cruz Filipe Bittencourt Machado de Souza Wantuir Filipe Teixeira Chagas João Cândido de Souza About the authors

ABSTRACT:

Popcorn (Zea mays var. everta) has a higher commercial value than common maize, in addition to being a popular food among consumers. Today, there is a constant search for cultivars with superior performance for several traits of interest in the case of popcorn, yield and popping expansion. On this basis, this project proposes to characterize progenies of popcorn with different values ​​of expansion capacity regarding chemical composition and micromorphology. Kernels from the fifth cycle (C5) of intrapopulation recurrent selection were evaluated. The progenies were selected based on the popping expansion volume of their kernels. The kernels were quantified for amylose and analyzed for starch granule arrangement and pericarp thickness by scanning electron microscopy. Progenies with low popping expansion volume (0 and 7 mL g-1) showed amylose contents of 21.24 and 20.18%, respectively; a less compact endosperm, with individual starch granules interspaced with empty spaces; and pericarp thickness between 40.94 and 38.99 µm, respectively. By contrast, progenies with high popping expansion volume (30 and 35 mL g-1) showed amylose contents of 23.92 and 26.10%; a vitreous endosperm; more-compact starch granules without empty spaces in between; and pericarp thickness between 107.66 and 107.84 µm. Progenies with higher popping expansion volume exhibited a thicker pericarp, a high amylose percentage and a more-compact endosperm, whereas those with the lower expansion volumes showed a thinner pericarp, a lower amylose percentage and individual starch granules.

Key words:
Zea mays var. everta; endosperm; pericarp; starch granule; thickness.

RESUMO:

O milho-pipoca (Zea mays var. everta) tem um valor comercial mais elevado do que o milho comum e é um alimento popular entre os consumidores. Hoje, existe uma constante busca por cultivares com desempenho superior para os diversos caracteres de interesse, no caso da pipoca, produtividade e capacidade de expansão. O trabalho propõe caracterizar progênies de milho-pipoca com diferentes valores de capacidade de expansão quanto à composição química e micromorfologia. Foram estudados grãos do quinto ciclo (C5) de seleção recorrente intrapopulacional. As progênies foram selecionadas com base nos valores de capacidade de expansão de seus grãos. Os grãos foram quantificados para amilose e analisados quanto ao arranjo dos grânulos de amido e espessura do pericarpo por microscopia eletrônica de varredura. Progênies com baixa capacidade de expansão (0 e 7 mL g-1) apresentaram teor de amilose de 21,24 e 20,18% respectivamente; endosperma menos compacto, com grânulos de amido individualizados e intercalados com espaços vazios; e espessura do pericarpo medindo entre 40,94 e 38,99 µm, respectivamente. Por outro lado, progênies com alta capacidade de expansão (30 e 35 mL g-1) apresentaram teor de amilose de 23,92 e 26,10%, endosperma vítreo, sendo os grânulos de amido mais compactos e sem espaços vazios entre eles; e espessura do pericarpo medindo entre 107,66 e 107,84 µm. Progênies com altos valores de capacidade de expansão apresentam maior espessura de pericarpo, alta porcentagem de amilose e endosperma mais compactado, enquanto as com menores valores de expansão, apresentam menor espessura do pericarpo, menor porcentagem de amilose e grânulos de amido individualizados.

Palavras-chave:
Zea mays var. everta; endosperma; pericarpo; grânulos de amido; espessura.

INTRODUCTION:

Popcorn (Zea mays var. everta) has a higher commercial value than common maize and is a popular food among consumers all over the world. Popcorn consumption has increased over the years, which has made it an economically attractive crop to producers (RIBEIRO et al., 2016RIBEIRO, R.M. et al. History of northern corn leaf blight disease in the seventh cycle of recurrent selection of an UENF-14 popcorn population. Acta Scientiarum. Agronomy, v.38, p.447-455,2016. Available from: <Available from: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1807-86212016000400447 >. Accessed: Aug. 16, 2018. doi: 10.4025/actasciagron.v38i4.30573.
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). The development and release of new popcorn cultivars are essential to reducing the dependence on genotypes from foreign countries and meeting the demand of producers and consumers by offering them cultivars and products with the traits of interest (GUIMARÃES et al., 2018GUIMARÃES, A. G. et al. Genetic gains and selection advances of the uenf-14 popcorn population. Revista Caatinga, v.31, n.2, p.271-278, 2018. Available from: <Available from: http://www.scielo.br/scielo.php?pid=S1983-21252019000300625&script=sci_arttext >. Accessed: Set. 15, 2018. doi: 10.1590/1983-21252018v31n202rc.
http://www.scielo.br/scielo.php?pid=S198...
).

Popping expansion is the main feature that indicates quality in popcorn (ARNHOLD et al., 2010ARNHOLD, E. et al. Seleção de linhagens S5 de milho-pipoca com base em desempenho e divergência genética. Acta Scientiarum. Agronomy, v.32, p.279-283, 2010. Available from: <Available from: http://www.scielo.br/pdf/asagr/v32n2/a14v32n2.pdf >. Accessed: Sep. 05, 2018. doi: 10.4025/actasciagron.v32i2.3886.
http://www.scielo.br/pdf/asagr/v32n2/a14...
). This parameter is defined as the ratio between the volume of popped popcorn and the weight of the kernel used for popping (VIEIRA et al., 2009VIEIRA, R. A. et al. Agronomic performance of new popcorn hybrids in northwestern Paraná State. Acta Scientiarum. Agronomy, v.31, p.29-36, 2009. Available from: <Available from: http://www.redalyc.org/html/3030/303026584006/ >. Accessed: Jan. 23, 2019. doi: 10.4025/actasciagron.v31i1.4142.
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).

Breeding programs constantly search for cultivars with superior performance for the various traits of interest to meet the demand of both producers and consumers; in the case of popcorn, these traits are yield and popping expansion, respectively. The recurrent selection methodology is used repeatedly, whereby the best genotypes in each cycle are selected to obtain gains at the end of each selection process (HALLAUER et al., 2010HALLAUER, A. R. et al. Handbook of plant breeding: Quantitative genetics in maize breeding, New York: Springer, 2010. v.6, Cap.8, p.383-423.).

In popcorn, the endosperm alone represents more than 80% of the dry kernel and is composed of two regions: one hard and crystalline (vitreous endosperm) and one soft and opaque (floury endosperm), each having specific physicochemical features (PIOVESAN et al., 2011PIOVESAN, V. et al. Corns with different endosperm textures and alpha-amylase added in the piglet diet. Ciência Rural, v.41, p.2014-2019, 2011. Available from: <Available from: http://www.scielo.br/scielo.php?script=sci_abstract&pid=S010384782011001100027&lng=pt&nrm=iso&tlng=en >. Accessed: Aug. 23, 2018. doi: 10.1590/S0103-84782011005000134.
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). The importance of evaluating the endosperm of popcorn kernels lies in that there seems to be a strong correlation between popping expansion and kernel density, which in turn is correlated with the amount of hard starch in the kernel (TANDJUNG et al., 2005TANDJUNG, A. S. et al. Role of the pericarp cellulose matrix as a moisture barrier in microwaveable popcorn. Biomacromolecules, v.6, p.1654 -1660, 2005. Available from: <Available from: https://pubs.acs.org/doi/abs/10.1021/bm049220l >. Accessed: Jul. 23, 2018. doi: 10.1021/bm049220l.
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).

In addition to representing a protection for the embryo and the endosperm, pericarp thickness plays important roles in determining grain quality, especially for popcorn varieties (DONG et al., 2015DONG, Y. et al. Dynamic proteomic characteristics and network integration revealing key proteins for two kernel tissue developments in popcorn. Plos One, v.10, e 0143181, 2015. Available from: <Available from: https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0143181#citedHeader >. Accessed: Mar. 03, 2019. doi: 10.1371/journal.pone.0143181.
https://journals.plos.org/plosone/articl...
). When the pericarp ruptures, the superheated water within it vaporizes and rapidly diffuses through nucleation sites, creating a driving force for popcorn expansion (SWELEY et al., 2013SWELEY, J. C. et al. Quality traits and popping performance considerations for popcorn (Zea mays Everta). Food reviews international, v.29, n.2, p.157-177, 2013. Available from: <Available from: https://www.tandfonline.com/doi/abs/10.1080/87559129.2012.714435 >. Accessed: Nov. 18, 2018. doi: 10.1080/87559129.2012.714435.
https://www.tandfonline.com/doi/abs/10.1...
). Pericarp thickness has been positively correlated with expansion volume (MOHAMED et al., 1993MOHAMED, A. A. et al. Pericarp thickness and other kernel physical characteristics relate to microwave popping quality of popcorn. Journal of Food Science, v.58, p.342-346, 1993. Accessed: Dec. 09, 2018. doi: 10.1111/j.1365-2621.1993.tb04271.x.
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).

Popping expansion may be affected by several factors. Several studies have reported the effects of genotype (PHUMELELE et al., 2014PHUMELELE, J. et al. Assessment of popping ability of new tropical popcorn hybrids. Australian Journal of Crop Science, v.8, p.831-839, 2014. Available from: <Available from: https://search.informit.com.au/documentSummary;dn=477516971039757;res=IELHSS >. Accessed: Jul. 03, 19.
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), starch composition and structure (BORRAS et al., 2006BORRAS, F. et al. Relationship between popcorn composition and expansion volume and discrimination of corn types by using zein properties. Cereal Chemistry, v.83, p.86-92, 2006. Available from: <Available from: https://onlinelibrary.wiley.com/doi/abs/10.1094/CC-83-0086 >. Accessed: Nov. 19, 2018. doi: 10.1094/CC-83-0086.
https://onlinelibrary.wiley.com/doi/abs/...
), physical properties (PORDESIMO et al., 1990PORDESIMO, L.O. et al. Physical properties as indicators of popping characteristics of microwave popcorn. Journal of Food Science. v.55, p.1352-1355, 1990. Available from: <Available from: https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.13652621.1990.tb03934.x#accessDenialLayout >. Accessed: Jun. 03, 19. doi: 10.1111/j.1365-2621.1990.tb03934.x.
https://onlinelibrary.wiley.com/doi/pdf/...
), yield components (LI et al., 2008LI, Y.L. et al. The genetic relationship between popping expansion volume and two yield components in popcorn using unconditional and conditional QTL analysis. Euphytica, v.162, p.345-351, 2008. Available from: <Available from: https://link.springer.com/content/pdf/10.1007%2Fs10681-007-9513-4.pdf >. Accessed: Apr. 17, 2019. doi: 10.1007/s10681-007-9513-4.
https://link.springer.com/content/pdf/10...
), chemical composition (PARAGINSKI et al., 2017PARAGINSKI, R.T. et al. Properties of popcorn starch expanded in a microwave, with and without the presence of vegetable oil. Journal of Food Processing and Preservation, v.41, e13142, 2017. Available from: <Available from: https://onlinelibrary.wiley.com/doi/full/10.1111/jfpp.13142 >. Accessed: Mar. 17, 2019. doi: 10.1111/jfpp.13142.
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), agronomic characteristics (SOUSA et al., 2016SOUSA, H.M.V. et al. Agronomic performance of popcorn maize genotypes in northeastern Pará state. Revista Brasileira de Milho e Sorgo, v.15, p.305-317, 2016. Available from: <Available from: http://rbms.cnpms.embrapa.br/index.php/ojs/article/view/514 >. Accessed: Jul. 17, 2019. doi: 10.18512/1980-6477/rbms.v15n2p305-317.
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) and kernel water content. However, none of those studies investigated the potential relationship between micromorphology and popping expansion in popcorn.

The use of scanning electron microscopy has been proven to be of great importance to characterize the microstructure of various materials. On this basis, this project proposes to characterize progenies of popcorn with different values ​​of expansion capacity regarding chemical composition and micromorphology.

MATERIALS AND METHODS:

Sampling

Ten half-sib popcorn progenies from the fifth cycle (C5) of intra-population recurrent selection with different expansion volumes (0, 3, 7, 10, 20, 23, 30, 31, 33 and 35 mL g-1) were sampled. These values were calculated as the ratio between the volume of popped popcorn and kernel weight. For popping, 10 g samples with 13% moisture were collected from each treatment and put into a hot-air popcorn popper for 10 s (ARNHOLD et al., 2009ARNHOLD, E. et al. Eficiências relativas de métodos de seleção de famílias endogâmicas em milho-pipoca. Acta Scientiarum. Agronomy, v.31, p.203-207, 2009. Available from: <Available from: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1807-86212009000200003 >. Acessed: Set. 05, 2018. doi: 10.1590/S1807-86212009000200003.
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).

Kernel amylose content

Amylose content was determined in all ten progenies, by the iodine colorimetric method (MCGRANCE et al., 1998MCGRANCE, S.J. et al. A simple and rapid colorimetric method for the determination of amylose in starch products. Starch/Stärke, v.50, p.158-163, 1998. Available from: <Available from: https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1521379X(199804)50:4%3C158::AID-STAR158%3E3.0.CO;2-7 >. Accessed: Aug. 23, 2018. doi: 10.1002/(SICI)1521-379X(199804)50:4<158::AID-STAR158>3.0.CO;2-7.
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). Approximately 20 mg of defatted starch were added to 8 mL of 90% dimethyl sulfoxide (DMSO), shaken for 20 min and subjected to a water bath at 85 °C for 2 h. After cooling, the content was transferred to a volumetric flask, completed to 25 mL with distilled water and homogenized. A 1-mL aliquot of the solution was added to 5 mL of I2/KI solution (0.0025 mol L-1of I2and 0.0065 mol L-1of KI) and the volume was completed to 50 mL. The resulting solution was homogenized and left to rest for 15 min, after which time its absorbance was read at 600 nm. To obtain the standard curve, 20 mg of pure potato amylose (Sigma-Aldrich) were subjected to the same process described above, and aliquots of 0.2, 0.4, 0.6, 0.8 and 1.0 mL had their UV - absorbances determined using a UV-1601 spectrophotometer (Shimadzu, Kyoto, Japan).

Kernel micromorphology

For the micromorphological analysis of starch granule arrangement, kernels were sampled from progenies with expansion volumes of 0 and 7 (low popping expansion) and 30 and 35 mL g-1 (high popping expansion). For pericarp thickness measurement, kernels from progenies with expansion volumes of 0, 7, 10, 20, 30 and 35 mL g-1 were sampled. Cross-sections were obtained from the mid region of kernels, mounted on aluminum stubs with a double-sided adhesive carbon tape and sputter-coated with gold (sputter coated equipment model SCD 050). Sampling was standardized so that the micrographs were obtained from the same region in all kernels. Photographic documentation was carried out using a scanning electron microscope (LEO Evo 40) at the Laboratory of Electron Microscopy and Ultrastructural Analysis at Federal University of Lavras, Brazil.

Statistical analysis

A randomized block design with five replicates was used for amylose percentage quantification and a completely randomized design with five replicates was used for pericarp thickness assessment. Data were subjected to analysis of variance and means were grouped by the Scott and Knott test (P<0.05). Subsequently, the Spearman correlation (STEEL et al., 1980STEEL, R. G., TORRIE, J. K. Principles and procedures of statistics: a biometrical approach. Tokyo: McGraw-Hill, 1980, 2v.) between popping expansion and amylose content was estimated (This comment was answered in the reviewer’s word document 1). Data were analyzed using packages of SAS (SAS Institute, 2000SAS INSTITUTE. SAS software: user’s guide: version 8.2. Cary, 2000. 291p.) and Sisvar (FERREIRA 2011FERREIRA, D. F. Sisvar: a computer statistical analysis system. Ciência e Agrotecnologia, v.35, p.1039-1042, 2011. Available from: <Available from: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1413-70542011000600001 >. Accessed: Dec. 20, 2018. doi: 10.1590/S1413-70542011000600001.
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) software.

RESULTS AND DISCUSSION:

Amylose content

The studied popcorn progenies differed significantly for amylose content in the kernel endosperm. Kernels form progenies with high popping expansion volumes (30 and 35 mL g-1) showed higher amylose percentages: 23.92% and 26.10%, respectively (Table 1). Kernels from progenies with popping expansion volumes of 20 and 23 mL g-1 showed intermediate amylose percentages (Table 1). Kernels from progenies with lower popping expansion volumes (0, 3, 7 and 10 mL g-1) showed the lowest amylose percentages (Table 1).

The amylose content of starch granules may determine endosperm hardness (DOMBRINK-KURTZMAN et al.,, 1997DOMBRINK-KURTZMAN, M. A.; KNUTSON, C. A A study of maize endosperm hardness in relation to amylose content and susceptibility to damage. Cereal Chemistry, v.74, p.776-780,1997. Available from: <Available from: https://onlinelibrary.wiley.com/doi/abs/10.1094/CCHEM.1997.74.6.776 >. Accessed: Aug. 20, 2018. doi: 10.1094/CCHEM.1997.74.6.776.
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). In hard endosperms, because the peripheral amorphous region of starch is larger, compression in this endosperm type results in a higher amylose content (BEMILLER et al., 2011BEMILLER, J. N. Pasting, paste, and gel properties of starch-hydrocolloid combinations. Carbohydrate polymers, v.86, n.2, p.386-423, 2011. Available from: <Available from: https://www.sciencedirect.com/science/article/pii/S0144861711004449 >. Acessed: Out. 19, 2018. doi: 10.1016/j.carbpol.2011.05.064.
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). Regarding the chemical basis of kernels with a hard endosperm, variations may exist in starch chemical composition, structural organization of starch granule components, structure of the protein layer among granules, presence of intercellular spaces and degree of compaction of starch granules (LEE et al., 2014LEE, K. M. et al. Application of raman spectroscopy for qualitative and quantitative analysis of aflatoxins in ground maize samples. Journal of Cereal Science, v.59, n.1, p.70-78, 2014. Available from: <Available from: https://www.sciencedirect.com/science/article/pii/S073352101300165 >. Accessed: Aug. 23, 2018. doi: 10.1016/j.jcs.2013.10.004.
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).

Table 1
Amylose content and pericarp thickness in kernels of popcorn progenies with different popping expansion volumes.

Conversely, starch granules in the floury endosperm are subjected to less compression and have a higher proportion of amylopectin. SWELEY et al (2012SWELEY, J. et al. Effects of hybrid, environment, oil addition, and microwave wattage on popped popcorn morphology. Journal of Cereal Science, v.56, p.276-281, 2012. Available from: <Available from: https://nebraska.pure.elsevier.com/en/publications/effects-of-hybrid-environment-oil-addition-and-microwave-wattage- >. Accessed: Dec. 23, 2018. doi: 10.1016/j.jcs.2012.05.009.
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) studied three hybrids from three different locations in the USA and reported a negative correlation between the percentage of amylopectin and kernel popping expansion. In our study on the popcorn kernel, the estimated correlation between popping expansion and amylose percentage was 0.95 (P ≤ 0.01), which thus reveals that higher popping expansion is associated with higher amylose percentages.

Starch granule arrangement

Kernels of progenies having low popping expansion volumes (0 and 7 mL g-1) showed a less compact endosperm (Figure 1A and B) and (Figure 2A and B). In the endosperm, an individual arrangement of granules interspersed with voids is observed. In this endosperm type, starch granules are dispersed and not surrounded by a protein matrix, which results in the formation of voids during the grain-drying process from spaces that had been previously filled with water throughout grain development (PAES et al., 2011PAES, M. C. D. et al. Características físicas e químicas do grão de milho. In: BOREM, A.; RIOS, S. De A.. Milho Biofortificado. 1.ed.Visconde do Rio Branco: Suprema, 2011, 3, p.45-86.).

Figure 1
Scanning electron micrographs of the endosperm in kernels of popcorn progenies. (A) and (B) Progenies with low popping expansion volumes (0 and 7, respectively) showing individual starch granules (is). (C) and (D) Progenies with high popping expansion volumes (30 and 35, respectively) showing compact starch granules (cs). Bars= 10 µm.

Figure 2
Scanning electron micrographs of the endosperm in kernels of popcorn progenies. (A) and (B) Progenies with low popping expansion volumes (0 and 7, respectively) showing individual starch granules (is). (C) and (D) Progenies with high popping expansion volumes (30 and 35, respectively) showing compact starch granules (cs). Bars= 20 µm. AL= aleurone layer, E= endosperm.

The progenies with high popping expansion volumes (30 and 35 mL g-1); conversely, had more compact endosperms with aggregated starch granules without voids interspersed among them (Figure 1C and D). In the vitreous endosperm, there is a dense protein matrix with well-structured protein bodies, without voids among them (PAES et al., 2011PAES, M. C. D. et al. Características físicas e químicas do grão de milho. In: BOREM, A.; RIOS, S. De A.. Milho Biofortificado. 1.ed.Visconde do Rio Branco: Suprema, 2011, 3, p.45-86.). The kernels of popcorn cultivars with hard and semi-hard endosperm have higher density values, more compact starch granule arrangement, endosperm cell walls with firmer appearance and polygon-shaped starch granules (PEREIRA et al., 2008PEREIRA, RC. et al. Relationship between structural and biochemical characteristics and texture of corn grains. Genetic and Molecular Research, v.2, p.498-508, 2008. Available from <Available from https://pdfs.semanticscholar.org/0c48/1d7352d0d2dc8e715f640c70a0ce4cb2384c.pdf >. Accessed: Nov. 23, 2018.
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). It is possible to observe the more angular shape of the granules (Figure 2C and D), which is characteristic of vitreous endosperm.

The ideal popcorn kernel has a smaller amount of floury endosperm and a higher proportion of vitreous endosperm, as the latter endosperm type is the one that most contributes to kernel expansion during popping (BORRAS et al., 2006BORRAS, F. et al. Relationship between popcorn composition and expansion volume and discrimination of corn types by using zein properties. Cereal Chemistry, v.83, p.86-92, 2006. Available from: <Available from: https://onlinelibrary.wiley.com/doi/abs/10.1094/CC-83-0086 >. Accessed: Nov. 19, 2018. doi: 10.1094/CC-83-0086.
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). As a result of increasing vapor pressure and temperature within the kernel, water vapor in the translucent endosperm is forced into the starch granules (VAN DER SMAN et al., 2017VAN DER SMAN, R. G. M. et al. Critical factors in microwave expansion of starchy snacks. Journal of Food Engineering, p.69-84, 2017. Available from: <Available from: https://www.sciencedirect.com/science/article/abs/pii/S0260877417301991 >. Accessed: Aug. 01, 2018. doi: 10.1016/j.jfoodeng.2017.05.001.
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), which soften and become a cohesive, viscoelastic mass. Pereira et al. (2008PEREIRA, RC. et al. Relationship between structural and biochemical characteristics and texture of corn grains. Genetic and Molecular Research, v.2, p.498-508, 2008. Available from <Available from https://pdfs.semanticscholar.org/0c48/1d7352d0d2dc8e715f640c70a0ce4cb2384c.pdf >. Accessed: Nov. 23, 2018.
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) stated that starch granules of the translucent endosperm undergo a process of gelatinization and crystalline melting. Conversely to what occurs in translucent endosperm during heating, in the opaque endosperm, water vapor apparently does not penetrate the starch granules, but rather enters the numerous voids among them. As a result, starch granules in the opaque endosperm do not undergo gelatinization during popping (VAN DER SMAN et al., 2017VAN DER SMAN, R. G. M. et al. Critical factors in microwave expansion of starchy snacks. Journal of Food Engineering, p.69-84, 2017. Available from: <Available from: https://www.sciencedirect.com/science/article/abs/pii/S0260877417301991 >. Accessed: Aug. 01, 2018. doi: 10.1016/j.jfoodeng.2017.05.001.
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). While starch granules in the translucent endosperm are highly expanded and responsible for flake formation, starch granules in the opaque endosperm appear to undergo little change during popping other than moving apart from each other (PEREIRA et al., 2008PEREIRA, RC. et al. Relationship between structural and biochemical characteristics and texture of corn grains. Genetic and Molecular Research, v.2, p.498-508, 2008. Available from <Available from https://pdfs.semanticscholar.org/0c48/1d7352d0d2dc8e715f640c70a0ce4cb2384c.pdf >. Accessed: Nov. 23, 2018.
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). Floury endosperms contribute little to the bulk of the starch foam (PARKER et al., 1999PARKER, M. et al. Effects of popping on the endosperm cell walls of sorghum and maize. Journal of Cereal Science, v.30, p.209-216, 1999. Available from: <Available from: https://www.sciencedirect.com/science/article/pii/S0733521099902813 >. Accessed: Sep. 23, 2018. doi: 10.1006/jcrs.1999.0281.
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). The endosperm contributes to expansion volume (SANTOS et al., 2008SANTOS, F.S. et al. Genetic gain prediction of the third recurrent selection cycle in a popcorn population. Acta Scientiarum. Agronomy, v.30, p.651-655, 2008. Available from: <Available from: http://www.scielo.br/scielo.php?pid=S180786212008000500008&script=sci_arttext&tlng=es >. Accessed: Sep. 23, 2018. doi: 10.1590/S1807-86212008000500008.
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). Higher amounts of opaque endosperm decrease expansion volume (WILLIER & BRUNSON et al. 1927WILLIER, J.; BRUNSON, A. Factors affecting the popping quality of popcorn. Journal of Agricultural Research, v.35, p.615, 1927. Available from: <Available from: https://naldc.nal.usda.gov/download/IND43967466/PDF >. Accessed: Jan. 16, 2019.
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).

Pericarp thickness

Significant differences in pericarp thickness were observed between the analyzed popcorn progenies (Table 1). The progenies with high popping expansion volumes (30 and 35 mL g-1) showed higher pericarp thickness (107.66 and 107.84 µm, respectively) (Figure 3C and D), while the progenies with low popping expansion volumes (0 and 7 mL g-1) showed lower values of this variable (40.94 and 38.99 µm, respectively) (Figure 3A and B).

Figure 3
Scanning electron micrographs of the endosperm of the pericarp, in a longitudinal section, in kernels of popcorn progenies. (A) and (B) Progenies with low popping expansion volumes (0 and 7, respectively). (C) and (D) Progenies with high popping expansion volumes (30 and 35, respectively). Bars= 20 µm. AL= aleurone layer, P= pericarp.

Some of the phenotypic traits known to affect popped volume include physical characteristics such as pericarp thickness and kernel size, shape, density and hardness (BORRAS et al., 2006BORRAS, F. et al. Relationship between popcorn composition and expansion volume and discrimination of corn types by using zein properties. Cereal Chemistry, v.83, p.86-92, 2006. Available from: <Available from: https://onlinelibrary.wiley.com/doi/abs/10.1094/CC-83-0086 >. Accessed: Nov. 19, 2018. doi: 10.1094/CC-83-0086.
https://onlinelibrary.wiley.com/doi/abs/...
). Popcorn kernels with high popping expansion volumes usually have not only a predominantly vitreous endosperm, but also a thicker pericarp (WILLIER et al., 1927WILLIER, J.; BRUNSON, A. Factors affecting the popping quality of popcorn. Journal of Agricultural Research, v.35, p.615, 1927. Available from: <Available from: https://naldc.nal.usda.gov/download/IND43967466/PDF >. Accessed: Jan. 16, 2019.
https://naldc.nal.usda.gov/download/IND4...
).

The popcorn pericarp thickness varies from 40 to 120 µm depending on the maize variety, location of kernel on the ear and position of the pericarp on the kernel (PEREIRA et al., 2008PEREIRA, RC. et al. Relationship between structural and biochemical characteristics and texture of corn grains. Genetic and Molecular Research, v.2, p.498-508, 2008. Available from <Available from https://pdfs.semanticscholar.org/0c48/1d7352d0d2dc8e715f640c70a0ce4cb2384c.pdf >. Accessed: Nov. 23, 2018.
https://pdfs.semanticscholar.org/0c48/1d...
). This fact highlighted the importance of standardizing the region in studies evaluating pericarp thickness, as was done in the present work.

Pericarp rupture occurs when the inner pressure in the kernel exceeds the combined burst pressure of the pericarp and atmospheric pressure. The popcorn kernel pericarp not only provides protection and containment to the endosperm, but also acts as a pressure vessel during heating, which gives popcorn its characteristic popping ability (PEREIRA et al., 2008PEREIRA, RC. et al. Relationship between structural and biochemical characteristics and texture of corn grains. Genetic and Molecular Research, v.2, p.498-508, 2008. Available from <Available from https://pdfs.semanticscholar.org/0c48/1d7352d0d2dc8e715f640c70a0ce4cb2384c.pdf >. Accessed: Nov. 23, 2018.
https://pdfs.semanticscholar.org/0c48/1d...
). As heat is applied to popcorn, the pericarp limits kernel moisture loss and facilitates vapor pressure build-up and water superheating within the kernels (RAKESH et al., 2013RAKESH, V. et al. Transport in deformable hygroscopic porous media during microwave puffing. AIChE Journal, v.59, n.1, p.33-45, 2013. Available from: <Available from: https://onlinelibrary.wiley.com/doi/abs/10.1002/aic.13793 >. Accessed: Nov. 23, 2018. doi: 10.1002/aic.13793.
https://onlinelibrary.wiley.com/doi/abs/...
). SWELEY et al. (2013SWELEY, J. C. et al. Quality traits and popping performance considerations for popcorn (Zea mays Everta). Food reviews international, v.29, n.2, p.157-177, 2013. Available from: <Available from: https://www.tandfonline.com/doi/abs/10.1080/87559129.2012.714435 >. Accessed: Nov. 18, 2018. doi: 10.1080/87559129.2012.714435.
https://www.tandfonline.com/doi/abs/10.1...
) suggested the use of pericarp thickness as an indicator of popping quality.

It should be emphasized that the use of complementary techniques is very important for breeding programs, since their adoption increases selection accuracy and reliability. In this sense, by using micromorphology studies in progenies of popcorn, the correlation between amylose content, pericarp thickness and starch granule density in the endosperm with popping expansion was reported to be of great importance for the development of cultivars in popcorn breeding programs.

CONCLUSION:

The kernels of popcorn progenies with higher popping expansion volumes have a thicker pericarp, higher amylose percentage and a more compact endosperm, whereas progenies with lower popping expansion volumes showed a thinner pericarp, lower amylose percentage and individual starch granules.

ACKNOWLEDGMENTS

The authors thank FAPEMIG (Foundation for Supporting Research of the State of Minas Gerais), National Council for Scientific and Technological Development (CNPq) (Processos números:142274/2016-0 e 142265/2016-0) and was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) (Processo número 88882.349320/2019-1), Brasil - Finance code 001.

REFERENCES

  • CR-2018-0962.R2

Publication Dates

  • Publication in this collection
    10 Feb 2020
  • Date of issue
    2020

History

  • Received
    20 Nov 2018
  • Accepted
    03 Dec 2019
  • Reviewed
    27 Jan 2020
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