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Article

Storage Temperature and Grain Moisture Effects on Market and End Use Properties of Red Lentil

by
Bhawana Bhattarai
1,2,*,
Cassandra K. Walker
1,2,
Ashley J. Wallace
1,
James G. Nuttall
1,2,
Graham Hepworth
3,
Joe F. Panozzo
2,
Debra L. Partington
4 and
Glenn J. Fitzgerald
1,2
1
Agriculture Victoria, 110 Natimuk Road, Horsham, VIC 3400, Australia
2
Centre for Agricultural Innovation, The University of Melbourne, Parkville, VIC 3010, Australia
3
Statistical Consulting Centre, The University of Melbourne, Parkville, VIC 3010, Australia
4
EPA Victoria, 200 Victoria Street, Carlton, VIC 3053, Australia
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(9), 2261; https://doi.org/10.3390/agronomy13092261
Submission received: 18 July 2023 / Revised: 23 August 2023 / Accepted: 26 August 2023 / Published: 28 August 2023
(This article belongs to the Section Horticultural and Floricultural Crops)
Browse Figures
Versions Notes

Abstract

:
Storing lentil is a strategy used by growers to manage price volatility. However, studies investigating the impact of storage conditions on the market and end use properties of lentil are limited. This study examined the effects of storage temperature (4, 15, 25, and 35 °C) and grain moisture (10 and 14%, w/w) on traits related to market (seed coat colour), viability (germination capacity), and end use properties (hydration capacity, milling efficiency, and cooking quality) in four red lentil cultivars (PBA Bolt, PBA Hallmark, PBA Hurricane, PBA Jumbo2) over 360 days. Storing lentil at 14% moisture content and 35 °C significantly (p = 0.05) darkened seed coat after 30 days, caused complete loss of viability within 180 days and reduced cooking quality (cooked firmness) after 120 days across all tested cultivars. Storing lentil at 10% moisture content and 35 °C reduced hydration capacity after 30 days, and milling efficiency after 120 days across all cultivars tested. PBA Jumbo2 exhibited a higher rate of degradation in hydration capacity and cooking quality, and a lower rate of degradation in the other traits studied. Storing lentil at ≤15 °C prevented degradation of all quality traits. These findings will support improved lentil storage protocols to maintain quality and improve economic outcomes for the pulse industry.

1. Introduction

Lentil are traded based on a broad spectrum of grain quality traits that play a significant role in market value and end-use properties. Seed coat colour is an important determinant of market value, as consumers prefer grains with a bright seed coat that are free from blemishes [1]. End-use properties are essential to ensure the efficient and effective processing of lentil grain [1]. These properties include milling efficiency (measured by assessing milling yield and milling by-products), hydration capacity, and cooking quality (measured by assessing cooked firmness). The cooked firmness of lentil refers to the maximum force required to crush a specific weight of cooked grains [2], where low cooked firmness values indicate optimum cooking quality. End-use properties impact the sensory experience, nutritional value, and overall satisfaction of consumers [3], affecting grain value.
Lentil prices are subject to significant volatility due to changes in supply, demand and market interventions such as tariffs [4]. Storing grain on-farm for extended periods is a practice used by growers to manage the effects of volatile prices. However, storing grain for extended periods of time has been shown to increase the risk of deterioration in grain quality traits related to market, viability, and end-use properties in pulses. Degradation in quality traits during storage can be impacted by a range of factors including storage temperature, grain moisture content, humidity and storage time, as observed in chickpea [5], pinto bean [6] and wheat [7].
Seed coat brightness influences grain marketability, and, as a result, grain traders visually assess seed coat colour when determining the market value of lentil. In Australia, premium grade, classified as ‘Grade 1’, refers to lentil that contains less than 1% (by weight) of poor coloured seed [8] and attracts the highest market price. However, storing pulse grains over extended periods with temperatures above 20 °C has been reported to increases the risk of seed coat darkening in faba bean [9], pinto bean [6] and cow pea [10]. Seed coat darkening in pulses has been reportedly linked to the breakdown of phenolic compounds in faba bean [9] and green lentil [11]; however, there has been insufficient investigation into the relationship between storage conditions and the seed coat colour of lentil.
Grain viability is an important grain quality trait for growers and is used to determine the establishment of subsequent crops [12]. For lentil grains, a germination capacity less than 90% leads to poor emergence [13], requiring increased seeding rates or input costs, as reported in soybean [14]. Studies conducted in pinto bean [6], wheat [7], red lentil [15] and white bean [16] demonstrated that when storage temperatures exceed 20 °C and grain moisture content exceeds 10%, the germination capacity can be significantly compromised by extended storage periods. High storage temperature and grain moisture content can accelerate the metabolic and enzymatic activities in grain [17], thereby increasing seed respiration and producing toxic compounds such as acetaldehyde and ethanol [18], in turn leading to reduced germination capacity. The impact of storage conditions on the germination capacity of red lentil in Canada has been studied by Sravanthi et al. [15]. However, studies investigating the impact of storage conditions across different lentil cultivars are limited.
Low hydration capacity has been reportedly linked to sub-optimal cooking quality in black gram [19] and can indicate adverse processing traits such as low milling efficiency which further impacts on the end-use properties. Hydration capacity has also been linked to grain softening, with implications for other food manufacturing processes such as fermentation of bean [20]. Furthermore, high hydration capacity has also been shown to eliminate antinutritional properties in lupin [21]. High milling efficiency translates to higher milling yield (splits and dehulled whole grain) by maintaining minimal breakages as observed for chickpeas [22]. Furthermore, cooking qualities such as low cooked firmness values are more desirable by consumers, as reported in black bean [19] and adzuki bean [3]. High hydration capacity was found to be correlated with low cooked firmness values in black bean [23] and resulted in quicker cooking times in field pea [24]. Storing pulse grains above 20 °C increased the risk of degradation in end use properties (hydration capacity, milling efficiency and cooking quality–cooked firmness) in chickpea [5], rice [25], and faba bean [26]. Such changes have been correlated with degradation of physicochemical properties and water absorption capacity, as observed in black bean [27]. This degradation was characterized by hard shell formation, reduced permeability, and increased adhesive chemistry between cotyledons in adzuki bean [3] and in black gram [19]. The economic benefits of the lentil grain and food processing industries rely on the production of uniform and quick-to-cook products, as these qualities are in high demand among consumers. However, there is a lack of knowledge regarding whether the end-use properties of lentil are compromised if stored under a range of environmental conditions after harvest.
This study investigated the impact of storage conditions on the quality traits of Australian red lentil cultivars by undertaking a 360-day laboratory incubation experiment assessing a range of storage temperatures and grain moisture contents. This study tested the hypothesis that quality traits including seed coat colour, germination capacity, hydration capacity, milling efficiency and cooking quality (cooked firmness) will decline over time when stored at high temperatures and high grain moisture content. The primary objective of this study is to identify the relationship between different storage conditions and traits associated with market, viability, and end-use properties. This new knowledge will inform storage management practices for growers in order to minimise the risk of deterioration in market and end use properties and assist in maintaining profitability when storing lentil prior to sale.

2. Materials and Methods

2.1. Sample Preparation

Four commercial red lentil cultivars (PBA Hurricane, PBA Hallmark, PBA Bolt and PBA Jumbo2) classified as ‘Grade 1′ were sourced from a commercial trader at Horsham VIC, Australia, in December at the end of the 2019 growing season. Cultivars were chosen to represent a range of seed sizes; PBA Hurricane representing a small grain cultivar (seed size index 4.1 mm), PBA Hallmark and PBA Bolt representing medium grain cultivars (seed size index 4.3 and 4.6 mm) and PBA Jumbo2 representing larger grain cultivars (seed size index 4.7 mm). Grains were collected at 10% (w/w) moisture content, half of the grain was tempered with distilled water at room temperature using a Chopin mixture (Chopin Technologies, Villeneuve-la-Garenne, France) to create a high (14%, w/w) moisture level. The 10% moisture content represents typical harvest moisture levels in the field, while 14% represents the highest recommended harvest moisture content. Four different storage temperatures (4, 15, 25 and 35 °C) were selected to represent a range of conditions that may be experienced in semi-arid climates, with 4 °C as the control temperature treatment.

2.2. Storage Conditions

Incubation chambers (Carel, Brugine, Italy) were used to maintain the 4 and 15 °C treatments, and two ovens (Memmert oven, model number 100-800, Schwabach, Germany and Clayson oven, model number OM1000, Melbourne, VIC, Australia) were used to maintain temperature for the 25 and 35 °C treatments. A 1:2 (w/v) solution of sodium chloride (NaCl) in distilled water was placed in sealed plastic tubs, which regulated humidity at a constant 75%, maintaining the grain at 14% (w/w) moisture, as described by Young [28], for the high moisture treatment.

2.3. Experimental Design

A factorial design incubation with 3 replicates was conducted for 360 days. The four treatment factors were temperature (4, 15, 25 and 35 °C), grain moisture content (10 and 14%), cultivar (PBA Hurricane, PBA Hallmark, PBA Bolt and PBA Jumbo2) and storage time. One kilogram of lentil seed from each grain moisture content and cultivar was placed in a 2-L plastic container with a perforated lid to allow for aeration. The grain containers were contained within larger plastic tubs and stored in a corresponding temperature-controlled environment.

2.4. Sampling

A time series sampling method was carried out in order to monitor changes in quality traits over time. Grains stored in plastic containers were mixed evenly to produce a homogeneous mixture and avoid sample bias. Fifty grams of these homogenized grains were collected every 30 days to assess seed coat colour, hydration, and germination capacity. Additionally, samples were collected every 120 days to assess milling efficiency, cooking quality (cooked firmness) and cotyledon colour. Milling efficiency, cooking quality (cooked firmness) and cotyledon colour were assessed less frequently compared to other traits, as these traits were more labour intensive and required larger quantities of grain.

2.5. Quality Assessment Traits

2.5.1. Seed Coat and Cotyledon Colour

Seed coat and cotyledon colour were quantified as change in the colour index value (CIE ∆E*ab) as described by Wrolstad et al. [29]. Seed coat and cotyledon colour was measured using a Colour Minolta Spectrophotometer (CM5, Hamburg, Germany) with Commission Internationale de l’Elcairage (CIE) values L*, a* and b*; where L* represents a scale of brightness, a* represents a scale of greenness to redness and b* represents a scale of blueness to yellowness. An increase in the L* value signifies a brighter grain, and a decrease in value signifies darker grain. A positive increase in both a* and b* values indicates a more brown/yellow grain, while negative values of a* and b* denote a more green/blue grain. The colour measurements were taken three times per sample and the average values were calculated. The seed coat colour index was calculated as described in Formula (1):
Seed coat colour index (CIE ∆E*ab): [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2
where ∆L* = L*0 − L*t, ∆a∗ = a∗0− a∗t and ∆b* = b*0 − b*t
Initial L*, a* and b* values (subscript by 0) and values at each storage time interval (subscript by t) were used to calculate CIE ∆E*ab values, which, in turn, were used to compare colour changes in samples. Increases in the value of CIE ∆E*ab represents darkening of seed coat and cotyledon colour.

2.5.2. Image Acquisition

Images of the grain and cotyledon samples were acquired for colour, separately using a Tagarno microscope (Tagarano, A/S, Horsens, Denmark) at 4.3× magnification with a resolution of 1920 × 1080 pixels and illuminated with a white LED ring light fixed to the camera lens as described by Assadzadeh et al. [30]. The instrument was calibrated as per the manufacturer’s requirements (Tagarano, A/S, Horsens, Denmark).

2.5.3. Germination Capacity

One hundred seeds were placed on a filter paper (Whatman No.1) in a 90 mm diameter Petri dish saturated with 8 mL of distilled water. The plates were incubated at 20 °C and 60–70% humidity for 72 h. The percentage of germinated seeds after a 72 h incubation period was measured as the germination capacity.

2.5.4. Hydration Capacity

Hydration capacity was measured for 100 seeds from each sample. One hundred seeds were weighed (w1) before being hydrated in 60 mL of water (w/v) for 8 h at 20 °C. After 8 h, seeds were drained and dried in double layered tissue paper for 15 min. Excess water was removed, dried seeds were weighed (w2) again and hydration capacity was calculated as described in Formula (2):
Hydration capacity (%): [(W2 − W1)/W1] × 100

2.5.5. Milling Efficiency

Fifty grams samples from each treatment were tempered at 14% moisture, eight hours prior to milling for optimal milling efficiency. Tempered grain samples were dehulled and split through two horizontal round stones as described by Erskine et al. [31]. Samples were aspirated to remove the hulls and sorted for the splits and dehulled whole grains; each portion was weighed. Milling efficiency was calculated from the initial weight of the sample before being milled (Wi) and the sum of the final dehulled and split weight (Wf), as described in Formula (3):
Milling efficiency (%): [(Wi + Wf)/Wi] × 100

2.5.6. Cooking Quality (Cooked Firmness)

Cooked firmness was measured as the force (N) required to shear one gram of cooked split lentil using a texture analyser (TA.XT.plus, Hamilton, USA). Splits were cooked for five minutes in a stainless-steel beaker with 800 mL of distilled water. The cooked samples were drained and cooled in water for 90 s. A portion of the sample was weighed (7.5 ± 0.5 g) in a Kramer Shear Cell and passed through the TA.XT texture analyser to shear the cooked sample. The result was expressed as the cooked firmness of the sample, which is the maximum shear force per gram of cooked sample. High cooked firmness of the cooked product indicates poor cooking quality. Cooked firmness was calculated as described in Formula (4):
Cooked firmness: Maximum shear force (N)/Weight of cooked splits (g)

2.6. Statistical Analysis

The effects of cultivar, grain moisture and temperature on key quality traits were analysed using analysis of variance (ANOVA) with storage time modelled as a repeated measures factor. The analysis was conducted using GenStat (22nd edition) software [32]. Repeated measurement was used to identify the trend and rate of change in quality traits to understand the impact of treatments over time. To compare factor combination means over the storage period, least significant difference (LSD) was calculated at the 0.05 level of significance.

3. Results

3.1. Seed Coat Colour

Seed coat colour was significantly (p ≤ 0.01) affected by a four-way interaction of storage time, grain moisture content, temperature, and cultivar. Irrespective of the cultivar, the rate of darkening significantly increased at high storage temperatures and grain moisture (Figure 1 and Figure 2). The highest rate of darkening was observed at a moisture content of 14% and 35 °C.
All tested cultivars exhibited significant increases in seed coat darkening after 30 days of storage at 14% moisture content and 35 °C, and after 60 days of storage at 10% moisture content and 35 °C (Figure 2). At 14% moisture content and 35 °C, the rate of darkening between PBA Bolt and PBA Hallmark was not significantly different throughout the study period; however, PBA Bolt was observed to have highest rate of darkening.
At 14% moisture content and 35 °C, the linear rate of darkening for PBA Bolt (2.336 × 10−2 CIE ∆E*ab unit per day) was significantly higher after 90 days of storage compared to PBA Hurricane and PBA Jumbo2 (average rate of 1.996 × 10−2 CIE ∆E*ab unit per day). Similarly at 10% moisture content and 35 °C, the rate of darkening for PBA Bolt (0.794 × 10−2 CIE ∆E*ab unit per day) was significantly higher after 150 days of storage compared to PBA Hurricane and PBA Jumbo2 (average rate of 0.473 × 10−2 CIE ∆E*ab unit per day). Storing grain at 10% moisture content slowed the rate of darkening; the rates of darkening for all cultivars stored at 10% moisture content and 35 °C was one third of the rates observed for cultivars stored at 14% moisture content and 35 °C.
When storing lentil at 14% moisture content and 25 °C, significant darkening was observed after 60 days of storage regardless of cultivar. However, the rate of darkening for PBA Bolt over 360 days was significantly higher (1.090 × 10−2 CIE ∆E*ab unit per day) after 150 days compared to all other cultivars (average rate of 0.705 090 × 10−2 CIE ∆E*ab unit per day). Cultivars stored at 25 °C and 10% moisture content were observed to have no significant darkening (average rate of 0.270 × 10−2 CIE ∆E*ab unit per day).
Storing lentil at 10% moisture content and ≤25 °C or at 14% moisture content and ≤15 °C resulted in no significant change in seed coat colour throughout the study period irrespective of cultivar.

3.2. Cotyledon Colour

Cotyledon colour was significantly (p ≤ 0.01) affected by a four-way interaction of storage time, grain moisture content, temperature, and cultivar. Significant darkening of the cotyledon was observed for all cultivars stored at temperatures at ≥25 °C, regardless of moisture content (Figure 3 and Figure 4). All cultivars stored at 35 °C were significantly darkened after 120 days regardless of moisture content, where no significant difference in the rate of darkening was observed between cultivars. Over 360 days, PBA Bolt at 14% moisture content and 35 °C exhibited the highest linear rate of darkening (1.120 × 10−2 CIE ∆E*ab per day) compared to all other cultivars (average rate of 1.071 × 10−2 CIE ∆E*ab per day).
Rate of darkening was slower at low grain moisture content and temperature (Figure 3). Significant darkening was not observed for all cultivars until 240 days when stored at 14% moisture content and 25 °C. At the same temperature (25 °C), significant darkening was not observed until 360 days when stored at 10% moisture content. When stored at 14% moisture content and 25 °C, there was no significant difference in the rate of darkening between cultivars. After 240 days of storage at 10% moisture content and 25 °C, the rate of darkening for PBA Bolt (0.592 × 10−2 CIE ∆E*ab per day) was significantly higher compared to all other cultivars (average rate of 0.334 × 10−2 CIE ∆E*ab per day).
No significant cotyledon darkening was observed for cultivars stored at 14% moisture content and ≤15 °C throughout the study period, similar to observations for all cultivars stored at 10% moisture content and ≤25 °C.

3.3. Germination Capacity

Germination capacity was significantly (p ≤ 0.01) affected by a four-way interaction of storage time, grain moisture content, temperature, and cultivar. Grain viability reduced over time, with higher reductions in germination capacity observed for all cultivars stored at a combination of high moisture content (14%) and high temperatures (≥25 °C) (Figure 5). Grain viability was significantly reduced for all cultivars regardless of moisture content when stored at 35 °C; however, greater detrimental effects were observed at 14% moisture content.
The highest rate of reduction was observed for all cultivars stored at 14% moisture content and 35 °C, resulting in a complete loss of viability within 180 days of storage, by which time germination capacity had reduced to zero (Figure 5). While all cultivars lost their viability within 180 days, PBA Hallmark exhibited a significantly higher linear rate of reduction in viability (43.703 × 10−2% reduction per day) after 60 days of storage compared to all other cultivars while PBA Jumbo2 exhibited the lowest rate of reduction (16.296 × 10−2% reduction per day). Conversely, lentil grains remained viable (>80%) over 360 days when stored at 10% moisture content and 35 °C regardless of cultivar. PBA Jumbo2, stored at 14% moisture content, maintained over 96% viability at 25 °C throughout the study period. All other cultivars maintained more than 90% viability when stored at 14% moisture content and 25 °C until 240 days, then gradually decreased to less than 70%, with PBA Hallmark exhibiting a higher rate of reduction than others (13.611 × 10−2% reduction per day).
All cultivars stored at 10% moisture content and ≤25 °C or stored at ≤15 °C irrespective of moisture content remained viable (≥95%) throughout the study period.

3.4. Hydration Capacity

Hydration capacity was significantly (p ≤ 0.01) affected by a four-way interaction of storage time, grain moisture content, temperature, and cultivar. Hydration capacity of all cultivars reduced significantly over time across treatments (Figure 6). Regardless of grain moisture content and cultivar, a significant reduction was observed after 30 days of storage at high temperatures (≥25 °C). However, storing lentil at 10% moisture content and 35 °C exhibited the highest rate of reduction. A significant difference in the rate of reduction between cultivars was observed when storing lentil at 10% moisture content and 35 °C, where PBA Jumbo2 exhibited the highest linear rate of reduction (19.694 × 10−2% per day) after 90 days of storage, compared to all other cultivars (average rate of 9.752 × 10−2% per day). On the other hand, when stored below 35 °C at 10% moisture content or stored at 14% moisture content regardless of storage temperatures, PBA Jumbo2 exhibited a lower rate of reduction. In addition, there was no significant difference in the rate of reduction between cultivars stored at 14% moisture content and 35 °C.
The rate of reduction in hydration capacity was lower for all cultivars at the high grain moisture content (14%) (Figure 6). When stored at 10% moisture content and 35 °C, the rate of reduction was almost twice as high for PBA Bolt, PBA Hurricane, and PBA Jumbo2 compared to 14% moisture content and the same temperature (35 °C). However, the rate of reduction for PBA Hallmark was not as pronounced as in the other cultivars.
Lower temperatures (4 and 15 °C) resulted in a minimal reduction in hydration capacity for all cultivars over time, regardless of grain moisture content.

3.5. Milling Efficiency

Milling efficiency was significantly (p ≤ 0.01) affected by a four-way interaction of storage time, grain moisture content, temperature, and cultivar. Milling efficiency was significantly reduced for all cultivars stored at 10% moisture content and ≥25 °C as well as at 14% moisture content and 35 °C (Figure 7). Storing lentil at 10% moisture content and ≥25 °C resulted in a significant reduction in milling efficiency after 120 days of storage. However, the highest rate of reduction was observed at 35 °C and 10% moisture content, and a significant difference in the rate of reduction between cultivars was observed. PBA Hurricane exhibited the highest linear rate of reduction (4.870 × 10−2% reduction per day) after 120 days of storage compared to all other cultivars (average rate of 3.966 × 10−2% reduction per day).
Storing lentil at a moisture content of 14% resulted in the least impact on milling efficiency across cultivars irrespective of storage temperatures (Figure 7). Only PBA Bolt and PBA Jumbo2 showed a significant reduction after 240 days of storage at 35 °C and 14% moisture content; however, the rate of reduction in these cultivars was minimal (no more than 1.574 × 10−2% reduction per day).
When storing lentil at low temperatures (4 and 15 °C), it was observed that there were minimal reductions in milling efficiency over time regardless of grain moisture content.

3.6. Cooking Quality (Cooked Firmness)

Cooking quality was significantly (p ≤ 0.01) affected by a four-way interaction of storage time, grain moisture content, temperature, and cultivar. The cooking quality of all cultivars reduced over time, as indicated by an increase in cooked firmness at high temperatures (≥25 °C) regardless of moisture content (Figure 8). Cooked firmness significantly increased after 120 days of storage at 35 °C, irrespective of grain moisture content and cultivar. However, it was observed that the rate of increase was higher at 14% moisture content.
When storing lentil at 14% moisture content and 35 °C, a significant increase in the rate of cooked firmness was observed across cultivars. PBA Jumbo2 exhibited the highest linear rate of increase in firmness (9.722 × 10−2 N/g increase per day) after 240 days compared to PBA Hallmark and after 330 days compared to PBA Hurricane and PBA Bolt.
Storing lentil grains at 10% moisture content slowed the rate of increase in cooked firmness (Figure 8). Storing lentil at 10% moisture content and 35 °C reduced the cooked firmness by more than 2 times in all cultivars compared to storing at 14% grain moisture content at 35 °C. Storing PBA Jumbo2 at 10% moisture content and 35 °C resulted in significantly higher rates of increase in firmness (4.148 × 10−2 N/g increase per day) after 240 days compared to all other cultivars, which exhibited similar rates of increase in firmness (averaging a rate of 2.848 × 10−2 N/g increase per day).
Storing lentil at 25 °C, regardless of grain moisture content, resulted in a significant increase in cooked firmness for all cultivars after 120 days of storage. Under these conditions, all cultivars exhibited similar rates of increase in cooked firmness throughout the study period. All cultivars stored at or below 25 °C were observed to maintain minimal reduction in cooking quality regardless of grain moisture content.

4. Discussion

4.1. Storing Red Lentil at Temperatures above 15 °C Darkens the Seed Coat and Cotyledon

This study found that seed coat and cotyledon colour can be maintained over time if lentil grain is stored at or below 15 °C, regardless of cultivar. Darkening accelerates when grain is stored at high temperatures (≥25 °C); however, the degree of darkening increases at 14% grain moisture content. These findings are similar to studies in faba bean where increases in seed coat darkening were minimal for grains stored at ≤ 10% grain moisture contents and at low temperatures, ≤25 °C [9], and darkening was accelerated above these conditions. Similar observations were reported in studies of pinto bean [6] and chickpea [5], where minimal darkening was observed when grains were stored at ≤12% moisture content and at ≤20 °C, whereas darkening accelerated at high grain moisture and temperatures. The darkening of seed coat may relate to the biochemical breakdown of complex phenolic compounds and the polymerization of proanthocyanidins, as detected and linked to darker seed coat in green lentil [11] and faba bean [9].
Factors such as storage temperature, humidity, grain moisture content at harvest, and storage practices can result in increased risk of seed coat darkening below the optimal market grade threshold. Changes in seed coat colour can be detected by the human eye at levels as low as 0.5–1.0 when observing CIE ∆E*ab values [33,34]. The least significant difference (LSD) detected in this study for lentil colour (CIE ∆E*ab) was 0.62 and would be considered a change in colour that can be detected by the human eye. Therefore, changes in seed coat color, as subjectively assessed by grain operators at receival sites, may occur in as little as 30 days from harvest if harvested grains are stored at high temperatures, irrespective of grain moisture content and cultivars. This presents a challenge for growers, as storing lentil grain beyond 30 days under sub-optimal conditions (high temperature and high moisture) may result in darkening of the seed coat, impacting grain value when classified by grain traders. This study suggests that storing ‘Grade1′ lentil at a grain moisture content of 10% and temperatures below 25 °C is necessary to retain grain value.

4.2. Grain Viability of Red Lentil Can Be Retained by Storing Temperatures at or below 15 °C

The grain viability of lentil can be retained above 97% regardless of cultivar for extended periods by storing at or below 15 °C. Grain viability reduces at high grain moisture content (14%) and temperatures (≥25 °C). In this study, grain viability remained above 85% even at high storage temperature (35 °C) when grain moisture content was low (10%). Previous studies have also demonstrated improved retention of grain viability at low temperatures (<20 °C) and grain moisture contents below 12% in pinto bean [6] and red lentil [15]. In addition, studies in wheat reports similar findings where viability was retained in stored grain at 15% moisture content and 10 °C [7]. However, at high grain moisture content and storage temperature, the viability of wheat was observed to decrease. Retention of grain viability when storing at lower temperatures is likely to be related with prevention of aging by lowering seed respiration rates [35].
Storing lentil at high temperatures (35 °C) and high grain moisture content (14%) could potentially lead to a complete failure of crop establishment if seeds were used in the subsequent growing season. Storage conditions at or above 25 °C and 14% moisture content can have negative impacts on grain viability, resulting in compromised (<80%) germination capacity for PBA Bolt, PBA Hallmark and PBA Hurricane. In this case, increased seeding rates would be recommended to compensate for the reduced germination capacity, leading to additional input costs for the grower [14].

4.3. Storing Red Lentil at Temperatures above 15 °C Results in Sub Optimal end Use Properties

Storing red lentil at or below 15 °C can help to maintain the end-use properties (hydration capacity, milling efficiency and cooking quality) of grain regardless of cultivar during extended storage periods; storing lentil above 15 °C can result in significant reductions in these quality traits. This study reported that the reduction in hydration capacity and milling efficiency accelerates at low grain moisture content (10%) and longer storage time. Meanwhile, reduction in cooking quality, as indicated by an increase in the cooked firmness, was observed when lentil was stored at high grain moisture content (14%).
Higher rates of reduction in hydration capacity were observed at low grain moisture content (10%) and high storage temperature conditions (≥25 °C); this was consistent with findings in soybean [36]. In soybean, an increased rate of reduction in hydration capacity was observed when stored at low grain moisture content (9%) and 30 °C compared to soybean stored at 13% moisture content and 30 °C [36]. The observed link between lower storage temperature and prevention of grain hardening (which is linked to sub-optimal milling efficiency) and associated reductions in cooking quality has been reported in faba bean [26]. Storing faba bean at 5 °C was reported to prevent grain hardening and reductions in cooking quality compared to 50 °C [26]. Similarly, black bean hardening by the formation of hard shells was observed when black bean were stored at elevated temperatures [27,37].
The rate of reduction in hydration capacity was expected to be higher for grains stored at 14% moisture; however, it was not observed in this study. Higher reductions in hydration capacity over time at low moisture content and high temperatures were likely to be associated with the change in physical properties of the lentil grain. These changes include the formation of a hard seed coat which can reduce permeability, thereby creating a water-repellent condition in grain, preventing water from moving through the seed coat into the cotyledon, as observed in black bean [27] and faba bean [26]. Over a prolonged periods of exposure to high temperatures and low grain moisture content, the micropyle membrane of lentil may become blocked due to the phenomenon of seed drying, and a residual membrane may have formed in the cotyledon. These factors have been shown to slow water uptake in various genotypes of Phaseolus bean [38]. A loss in hydration capacity may also be attributed to an increase in cotyledon thickness over time. It has been reported that an increase in cotyledon thickness, resulting from the reduced extractability of phenolic compounds, leads to a loss in hydration capacity in faba beans [26] and black soybean [36]. In addition, higher rates of reduction in hydration capacity in PBA Jumbo2 (larger grain) may be related to the larger seed size, as larger grain size in adzuki bean has been reported to be correlated with lower hydration capacity [3,39,40].
Reduction in cooking quality (higher cooked firmness) observed in this study was expected to be related with low hydration capacity; however, it was not observed in this study. Therefore, the decline in cooking quality may be attributed to biochemical changes, such as phytase activity in the cotyledon, which has been shown to hinder pectin dissolution during cooking [36]. Consequently, cooking quality was observed to be reduced by pectin dissolution, as this lead to decreased cell separation in the cotyledon, even though grain hydration was not affected in soybean [36]. In addition, it was observed that darker grains exhibited lower cooking quality, similar to observations in adzuki bean [3]. Lower cooking quality in lentil for darkened grain may be attributed to increases in cotyledon thickness due to the reduction of tannins and polyphenols, as observed in faba bean [26]. Similarly, increasing lignin content was also reported to lower cooking quality for pulse grain stored at high temperature as observed for adzuki bean [39] and cowpea [41].
The effects of storage conditions on milling efficiency varied with cultivar, and this is likely to be related to grain size. Previous studies reported that smaller grain size was associated with reduced milling efficiency in chickpea [22,42]. It is therefore likely that the reduction in milling efficiency observed with PBA Hurricane (small grain) compared with PBA Jumbo2 (larger grain) may have been impacted by the smaller seed size. Sub-optimal milling efficiency for grains with lower hydration capacity may be due to the formation of a hard shell [27] which may hinder the dehulling process during milling. The adhesion chemistry between cotyledon and seed coat, as well as between the cotyledons, where minerals bind strongly with phenolic and carboxylic groups to make cotyledons harder as reported in chickpea [43], may also play a role in the decreased milling efficiency observed in this study. This chemistry may alter depending on how phenolic compounds change over time in storage [11].
Findings in this study indicate that end-use properties begin to degrade as early as 30 days after harvest if harvested grain are stored at high temperatures. To maintain the desired end-use properties of lentil, it is essential for grain growers and handlers to implement cooling strategies that prevent grain degradation. Specifically, reducing the storage temperature to 15 °C within 30 days of harvesting can ensure that end-use properties are maintained. Although there was a significant difference in response between cultivars for all of the end-use properties assessed in this study, all tested cultivars were shown to significantly degrade in properties at the same time points across the treatments and traits. Therefore, storage management protocols could be applied uniformly to the four cultivars, maintaining processing quality over time.
In semi-arid regions such as those found in south-eastern Australia, lentil is typically harvested during late spring and early summer and is often harvested when daily maximum temperatures exceed 30 °C for around 180 days. This study has shown that grain quality traits (market trait, viability, and end-use properties) of red lentil begin to degrade as early as 30 days after harvest when stored at higher temperatures. Given the high temperatures experienced during harvest, it is recommended that the storage temperature of the harvested grain be lowered as soon as practical. Active cooling measures within grain storage systems may help to reduce grain temperatures and mitigate the risk of quality degradation. Similarly, a grain moisture content of 14% is currently the threshold for exporting grains as ‘Grade 1’ in countries like Australia [8]; however, this study demonstrates that the risk of downgrading increases when red lentil is stored at this moisture content combined with suboptimal higher temperature conditions. Therefore, grain storage systems that dry or aerate the grain to 10% moisture content may prevent quality degradation.
Growers can benefit from regular monitoring of the storage environment using sensors, particularly for temperature. With this knowledge, growers can manage the storage environment, which will lead to more informed decisions on the rate and length of cooling that maintain quality traits of lentil grains.
Implementing a cooling system and utilizing sensor-based monitoring for the storage environment can present challenges to growers, both in terms of financial investment and technical requirements. It is crucial to investigate these challenges, including conducting an economic analysis to assess the feasibility and cost-effectiveness of such measures. In addition, further study is required to fully understand how quality changes in practise as a result of the rate of diurnal variation in temperature and grain moisture content over time, as this study only examined the effects of two constant moisture contents and four temperatures.
The response of the tested cultivars varied across the traits studied. PBA Jumbo2 in particular exhibited a lower rate of reduction in seed coat darkening and grain viability compared to other cultivars while displaying a higher rate of reduction in the traits studied for end-use properties. This observed response could be attributed to genetics or potentially a batch effect. Further research is required to investigate genetic factors underlying these variations. Moreover, conducting further research involving the same cultivar sourced from different locations is essential to validate and confirm the observed differences in the cultivars.

5. Future Direction

While this study has identified the importance of storage conditions on the grain quality of red lentil, it was conducted in a small-scale incubation environment. It is therefore imperative that this be followed up with commercial scale studies under external conditions in order to validate the assessed effects of temperature and grain moisture dynamics at scale. This would confirm the impact of temperatures and grain moisture contents on quality traits in a commercial operation, with implications for establishing protocols that maintain lentil quality traits. Additionally, this research supports the groundwork for creating a model that can predict changes in lentil quality during prolonged storage at different grain moisture contents and storage temperatures. Growers would gain benefit from such models by making informed managerial and financial decisions, which would ultimately improve economic returns.

Author Contributions

B.B.: investigation, data collection, data analysis, writing—original draft; C.K.W. and J.G.N.: supervision, conceptualization, and review and editing; A.J.W. and G.J.F.: supervision, review and editing; D.L.P. and G.H.: methodology, statistical analysis and review; J.F.P.: conceptualization, review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Victorian Grains Innovation Partnership as a part of the “Capturing the Value of Grains On-Farm” project in a collaboration between Agriculture Victoria and the Australian Grains Research Development Corporation (GRDC), grant ID GRDC 8049295.

Data Availability Statement

All data generated or analysed during this study are presented in this article.

Acknowledgments

This research was funded through the Victorian Grains Innovation Partnership, a collaboration between Agriculture Victoria and the Australian Grains Research Development Corporation (GRDC). The authors would like to acknowledge the Centre for Agricultural Innovation (CAI), a joint initiative between Agriculture Victoria and The University of Melbourne supporting the PhD program. The authors are also grateful to the Crop Quality and Crop Agronomy teams at the Grains Innovation Park for providing the necessary technological and technical support.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Seed coat colour of red lentil (PBA Bolt) (a) before storage and after 360 days of storage at 14% grain moisture content and temperatures of (b) 25 °C and (c) 35 °C.
Figure 1. Seed coat colour of red lentil (PBA Bolt) (a) before storage and after 360 days of storage at 14% grain moisture content and temperatures of (b) 25 °C and (c) 35 °C.
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Figure 2. Impact of storage conditions on the seed coat colour of red lentil cultivars stored at four temperature and two moisture levels, measured every 30 days over a period of 360 days. Vertical bar represents LSD = 0.62, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
Figure 2. Impact of storage conditions on the seed coat colour of red lentil cultivars stored at four temperature and two moisture levels, measured every 30 days over a period of 360 days. Vertical bar represents LSD = 0.62, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
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Figure 3. Impact of storage conditions on the cotyledon colour of red lentil cultivars stored at four temperature and two moisture levels, measured every 120 days over a period of 360 days. Vertical bar represents LSD = 1, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
Figure 3. Impact of storage conditions on the cotyledon colour of red lentil cultivars stored at four temperature and two moisture levels, measured every 120 days over a period of 360 days. Vertical bar represents LSD = 1, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
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Figure 4. Cotyledon colour of red lentil (PBA Bolt) (a) before storage, and after 360 days of storage at 14% grain moisture content and temperatures of (b) 25 °C and (c) 35 °C.
Figure 4. Cotyledon colour of red lentil (PBA Bolt) (a) before storage, and after 360 days of storage at 14% grain moisture content and temperatures of (b) 25 °C and (c) 35 °C.
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Figure 5. Impact of storage conditions on germination capacity of red lentil cultivars stored at four temperature and two moisture levels, measured every 30 days over a period of 360 days. Vertical bar represents the LSD = 2.8, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
Figure 5. Impact of storage conditions on germination capacity of red lentil cultivars stored at four temperature and two moisture levels, measured every 30 days over a period of 360 days. Vertical bar represents the LSD = 2.8, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
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Figure 6. Impact of storage conditions on the hydration capacity of red lentil cultivars stored at four temperature and two moisture levels, measured every 30 days over a period of 360 days. Vertical bar represents LSD = 3.4, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
Figure 6. Impact of storage conditions on the hydration capacity of red lentil cultivars stored at four temperature and two moisture levels, measured every 30 days over a period of 360 days. Vertical bar represents LSD = 3.4, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
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Figure 7. Impact of storage conditions on the milling efficiency of red lentil cultivars stored at four temperature and two moisture levels, measured every 120 days over a period of 360 days. Vertical bar represents LSD = 2.53, (p = 0.05) for the interaction of temperature and grain moisture content over the storage period.
Figure 7. Impact of storage conditions on the milling efficiency of red lentil cultivars stored at four temperature and two moisture levels, measured every 120 days over a period of 360 days. Vertical bar represents LSD = 2.53, (p = 0.05) for the interaction of temperature and grain moisture content over the storage period.
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Figure 8. Impact of storage conditions on the cooking quality (cooked firmness) of red lentil cultivars stored at four temperature and two moisture levels, measured every 120 days over a period of 360 days. Vertical bar represents LSD = 1.8, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
Figure 8. Impact of storage conditions on the cooking quality (cooked firmness) of red lentil cultivars stored at four temperature and two moisture levels, measured every 120 days over a period of 360 days. Vertical bar represents LSD = 1.8, (p = 0.05) for the interaction of temperature, grain moisture content and cultivar over the storage period.
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Bhattarai, B.; Walker, C.K.; Wallace, A.J.; Nuttall, J.G.; Hepworth, G.; Panozzo, J.F.; Partington, D.L.; Fitzgerald, G.J. Storage Temperature and Grain Moisture Effects on Market and End Use Properties of Red Lentil. Agronomy 2023, 13, 2261. https://doi.org/10.3390/agronomy13092261

AMA Style

Bhattarai B, Walker CK, Wallace AJ, Nuttall JG, Hepworth G, Panozzo JF, Partington DL, Fitzgerald GJ. Storage Temperature and Grain Moisture Effects on Market and End Use Properties of Red Lentil. Agronomy. 2023; 13(9):2261. https://doi.org/10.3390/agronomy13092261

Chicago/Turabian Style

Bhattarai, Bhawana, Cassandra K. Walker, Ashley J. Wallace, James G. Nuttall, Graham Hepworth, Joe F. Panozzo, Debra L. Partington, and Glenn J. Fitzgerald. 2023. "Storage Temperature and Grain Moisture Effects on Market and End Use Properties of Red Lentil" Agronomy 13, no. 9: 2261. https://doi.org/10.3390/agronomy13092261

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