Maize kernel color depends on the interaction between hardness and carotenoid concentration

https://doi.org/10.1016/j.jcs.2019.102901Get rights and content

Highlights

  • Endosperm vitreousness affects the optical expression of kernel color.

  • Maize kernel hardness impacts color regardless of total carotenoid concentration.

  • Assuming carotenoid concentration based only on kernel color may be misleading.

  • Higher carotenoid concentrations were found in harder kernels.

Abstract

Maize kernel color and carotenoid concentration are traits valued by the food industry to ensure the quality of their products. Correlations between color and carotenoid concentration have been extensively reported. Based on the concept that chromaticity is modified differently by opaque and translucent materials, we tested the hypothesis that maize kernel color is not only the result of total carotenoid concentration but also a consequence of kernel hardness. Kernel hardness (test weight, vitreousness, and floaters percentage), carotenoid concentration, and color (HunterLab) were measured in thirteen commercial hybrids. Genotypes showed significant differences in all analyzed kernel hardness traits, carotenoid concentration (24.7–39.4 mg kg−1), and HunterLab color dimensions. Kernel color values and kernel hardness were correlated. Genotype differences in b (yellowness) were observed in kernels with similar total carotenoid concentration but contrasting hardness. For a similar carotenoid concentration harder genotypes always showed lower b values. When whole kernels were milled and color was measured on the resulting flour, genotype differences in yellowness disappeared, further supporting that the kernel vitreous structure affects kernel color. Our results sustain the notion that the genotype capacity to form larger proportions of vitreous endosperm impacts color regardless of total carotenoid concentration.

Introduction

Maize (Zea mays L.) is one of the most prevalent cereal crops, along with rice and wheat (Ranum et al., 2014). It accounts for part of the staple diet of millions of people in Latin America, Asia, and Africa (Ranum et al., 2014). In addition to being used as a minimally processed food and feed source at the household level, maize is feedstock to processes yielding ingredients and products that include flour, cornmeal, grits, starch, snacks, tortillas, and breakfast cereals, among many others. The increasing interest in functional and healthy foods has drawn research focus into bioactive compounds derived from maize and their health properties (Luo and Wang, 2012). Maize is not only a source of macronutrients, it also contributes to the diet with various phytochemicals, such as phenolic compounds, phytosterols, and carotenoids (Nuss and Tanumihardjo, 2010).

Carotenoids, the most widespread organic pigments in nature, are well-known for their outstanding nutritional value. In humans, some carotenoids (β-carotene, β-cryptoxanthin, and α-carotene) are precursors of retinol or vitamin A, which is important in multiple biological processes (Venado et al., 2017). Because of their antioxidant activity, carotenoids are major contributors to the reduction of free radicals. This characteristic results in positive effects on consumer health and chemical and sensory quality food improvement by preventing lipid oxidation (Žilić et al., 2012).

Total carotenoid concentration is highly variable among yellow and orange maize types, and commonly ranges from 16 to 30 mg kg−1 (dry weight basis; Kljak and Grbeša, 2015). Breeding efforts towards the development of high carotenoid maize genotypes are currently taking place in countries where maize is an important staple food crop (Menkir et al., 2017). Within the maize kernel, more than 70% of total carotenoids are located in the vitreous endosperm (the hard and translucent endosperm fraction), and the rest is distributed among floury endosperm (the soft and chalky endosperm fraction), germ, and bran fractions (Blessin et al., 1963). Higher proportions of vitreous endosperm found in hard kernels are commonly associated with an improved ability to store a high concentration of carotenoids. However, information connecting kernel hardness and total carotenoid concentration in maize is not available.

In addition to the aforementioned benefits, carotenoids play a major role in yellow and orange pigmentation of the maize kernel. Several studies linked carotenoid concentration with reflectance color measurements. Results showed consistent correlations between color saturation and carotenoid concentration, suggesting that carotenoids determine color intensity in orange and yellow maize kernels (Kljak et al., 2014; Lozano-Alejo et al., 2007). The color of intact kernels and their derived products can be objectively measured with colorimeters. These instruments can define color in terms of the HunterLab three-dimensional (L, a, and b) color space. These coordinates determine lightness (L) and chromaticity (a defines red/greenness and b defines yellow/blueness) of samples (Choudhury, 2010). Measuring color using a three-dimensional color space is widely accepted throughout the agricultural industry for the assessment of visual quality (Dowell, 1998). From these coordinates, chroma (C, a measurement of color saturation) and hue angle (h, a representation of the actual perceived color) can be calculated.

The optical perception of color is related to the amount and the spectrum of the light reflected from an object into the human eye (Hutchings, 1999). Besides pigment concentration, geometric features of the object modulate its color attributes and appearance (Hutchings, 1999). Chromaticity is modified differently by opaque and translucent materials; their differences in physical structure impact the way that light is scattered through them (Hunter and Harold, 1987). Maize kernel translucency is associated with the amount of vitreous (hard) endosperm relative to the amount of floury (soft) endosperm. Harder kernels present a greater proportion of vitreous endosperm, and are characterized by higher vitreousness, higher kernel density, higher test weight, and lower flotation indices (Abdala et al., 2018; Caballero-Rothar et al., 2018). Vitreous endosperm is translucent because of its compacity, which results from starch granules being tightly packed within a thick and continuous protein matrix. The floury endosperm, where the protein matrix is thinner, has numerous air-filled spaces that prevent the light from passing through. These air pockets reduce translucency, resulting in an opaque appearance (Robutti et al., 1974). We hypothesize that in addition to the total carotenoid concentration kernel hardness has an impact on kernel color.

Today, there is no information regarding the effect of kernel hardness over maize kernel color. We are interested in understanding the relationship between maize kernel color, total carotenoid concentration, and kernel hardness. Our objective was to test how genotype differences in kernel hardness and total carotenoid concentration impact kernel color. We hypothesize that maize kernel color is not only the result of total carotenoid concentration, but also a consequence of kernel physical properties. To test this hypothesis, we grew a number of current commercial genotypes differing in kernel hardness, total carotenoid concentration, and color in different environmental conditions.

Section snippets

Crop management

Two field experiments were conducted at Campo Experimental Villarino, Facultad de Ciencias Agrarias, Universidad Nacional de Rosario in Zavalla, Santa Fe, Argentina (33° 1′ S, 60° 53’ W), during the 2017/2018 growing season. Experiments were planted on October 5 and December 27, 2017, two contrasting planting dates commonly used by farmers in the region (Abdala et al., 2018). These different planting dates allowed us to test all genotypes under contrasting growing environments. In both cases

Crop yield, kernel weight, and size

Significant yield differences were evident for genotypes and environments (p < 0.001; Table 1). These factors together accounted for 75% of the total explored yield variation. When averaged across environments, genotype yield ranged from 8434 to 12,221 kg ha−1 (Table 1). Higher yields were observed in the earlier environment (October 5 planting date) and the interaction genotype x environment was not significant (p > 0.05).

Individual kernel weight showed significant genotype and environment

Discussion

Our results showed that kernel hardness plays an important role on color appearance. It was partly associated with an improved ability to store a higher concentration of carotenoids, where genotypes with harder kernels had on average higher carotenoid concentrations than the softer ones. However, and most importantly, at similar total carotenoid levels, significant b differences existed among kernels with contrasting kernel hardness (Fig. 1A). All color parameters correlated positively with

Conclusions

Our results demonstrate that kernel hardness impacts maize kernel color regardless of total carotenoid concentration. This was tested in a set of commercial genotypes showing a range of kernel hardness and color attributes. HunterLab color dimensions were negatively correlated with vitreousness. Different colors were observed in kernels with the same total carotenoid concentration but contrasting hardness. This indicates that assumptions of total carotenoid concentration solely based on kernel

CRediT authorship contribution statement

Ezequiel Saenz: Conceptualization, Visualization, Formal analysis, Writing - original draft. Lucas J. Abdala: Data curation, Writing - review & editing. Lucas Borrás: Funding acquisition, Writing - review & editing. José A. Gerde: Formal analysis, Funding acquisition, Supervision, Writing - review & editing.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgements

Authors wish to thank GR Rodriguez for lending the colorimeter. The study was funded by the Ministerio de Educación, Cultura, Ciencia y Tecnología from Argentina (PICT 2016-0956) and CONICET (Scientific Research Council of Argentina, PUE 22920160100043).

References (31)

  • C.W. Blessin et al.

    Carotenoids of corn and sorghum. 5. Distribution of xanthophylls and carotenes in hand-dissected and dry-milled fractions of yellow dent corn

    Cereal Chem.

    (1963)
  • N.N. Caballero-Rothar et al.

    Role of yield genetic progress on the biochemical determinants of maize kernel hardness

    J. Cereal Sci.

    (2018)
  • A.K.R. Choudhury

    Scales for communicating colours

  • M.A. Dombrink-Kurtzman et al.

    Zein composition in hard and soft endosperm of maize

    Cereal Chem.

    (1993)
  • F.E. Dowell

    Automated color classification of single wheat kernels using visible and near-infrared reflectance

    Cereal Chem.

    (1998)
  • Cited by (15)

    • Unraveling the difference in physicochemical properties, sensory, and volatile profiles of dry chili sauce and traditional fresh dry chili sauce fermented by Lactobacillus plantarum PC8 using electronic nose and HS-SPME-GC-MS

      2022, Food Bioscience
      Citation Excerpt :

      The amino nitrogen content was obtained by following formula: Amino nitrogen (%) = [(V–V0) * c * 0.0014 * 100] / (m * 20) * 100, where c represents the concentration of NaOH (mol/L), m means the sample mass and 0.0014 represents the molar mass of nitrogen (g/mmol). The method of color analysis referred to the previous research (Saenz, Abdala, Borrás, & Gerde, 2020). Briefly, the color of chili sauce was analyzed with a WSC-Y automatic colorimetric aberration meter with 30 mm aperture diameter and 30 mm condenser diameter.

    View all citing articles on Scopus
    View full text