Effects of extrusion conditions and nitrogen injection on physical, mechanical, and microstructural properties of red lentil puffed snacks

Journal Pre-proof Effects of extrusion conditions and nitrogen injection on physical, mechanical, and microstructural properties of red lentil puffed snacks Siwen Luo, Elyssa Chan, Mustafa Tugrul Masatcioglu, Chyngyz Erkinbaev, Jitendra Paliwal, Filiz Koksel

PII:

S0960-3085(19)30743-6

DOI:

https://doi.org/10.1016/j.fbp.2020.02.002

Reference:

FBP 1223

To appear in:

Food and Bioproducts Processing

Received Date:

7 August 2019

Revised Date:

19 December 2019

Accepted Date:

3 February 2020

Please cite this article as: Luo S, Chan E, Masatcioglu MT, Erkinbaev C, Paliwal J, Koksel F, Effects of extrusion conditions and nitrogen injection on physical, mechanical, and microstructural properties of red lentil puffed snacks, Food and Bioproducts Processing (2020), doi: https://doi.org/10.1016/j.fbp.2020.02.002

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Title: Effects of extrusion conditions and nitrogen injection on physical, mechanical, and microstructural properties of red lentil puffed snacks

Authors: Siwen Luo1, Elyssa Chan1, Mustafa Tugrul Masatcioglu2, Chyngyz Erkinbaev3, Jitendra Paliwal3, Filiz Koksel1, *

Food and Human Nutritional Sciences Department, University of Manitoba, 250 Ellis

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Building, 13 Freedman Crescent, Winnipeg, R3T 2N2, MB, Canada 2

Food Engineering Department, Hatay Mustafa Kemal University, Tayfur Sokmen Campus,

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31034, Antakya, Hatay, Turkey

Department of Biosystems Engineering, University of Manitoba, E2-376, EITC, 75A

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Chancellor's Circle, Winnipeg, R3T 2N2, MB, Canada

*Corresponding author, e-mail address: [email protected]

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Highlights

An increase of screw speed from 150 rpm to 200 rpm caused a significant increase in expansion.

N2 injection resulted in a more uniform cell structure in extrudates.

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Extrudates produced with 300 kPa N2 injection had the maximum expansion.

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Increased feed moisture content caused a significant reduction in expansion and

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crispiness.

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Abstract Purpose: Red lentils are rich in proteins and fibres, and thus have great potential to be used as healthy ingredients in extruded snacks. However, at high protein and fibre concentrations, the level of expansion, microstructure uniformity and textural appeal are impaired. Using physical blowing agents during extrusion can overcome these quality issues. This study investigates the effects of moisture content (18%-22%), screw speed (150 rpm-200 rpm), and

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blowing agent (i.e., N2) injection pressure (0-500 kPa) on physical, mechanical and microstructural properties of puffed red lentil snacks. Principle results:

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Compared to conventional extrusion, N2 injection at 300 kPa decreased extrudate density from 0.25-0.41 kg/m3 to 0.12-0.26 kg/m3 (across all feed moistures and screw speeds

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studied), and substantially altered the extrudate’s microstructure. Extrudates with N2 injection

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had numerous small cells that were more evenly distributed. Moreover, an increase in feed moisture from 18% to 22% increased extrudate hardness and decreased crispiness. Major conclusions:

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N2 injection during extrusion has a great potential in manipulating extrudate expansion, microstructure, texture, and colour of red lentil extrudates. Extrudates produced with 300 kPa N2 injection pressure had the maximum expansion compared to other N2

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injection pressures studied. Keywords:

Extrusion; Red lentil; Nitrogen injection; Microstructure; Texture; Colour

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1. Introduction Healthy snack foods is a rapidly growing food sector given the increasing interest in nutritionally-dense and texturally appealing foods that are also convenient to consume onthe-go (Brennan et al., 2013; Harper, 1979). However, most common puffed snacks available in the market are cereal flour based and relatively high in carbohydrate, low in protein (e.g., Cheetos® made with corn flour has ~7% protein), and not healthy lifestyle oriented. Red lentil, as the most commonly consumed lentil, presents great potential to be used as

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wholesome and nutritious ingredients in snack foods, since they are generally low in fat, and high in protein (20 to 30%) and dietary fibre (10 to 20%) (Singh et al., 2016). However, this nutritious crop is not widely consumed in the Western hemisphere, probably due to the

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limited knowledge on its cooking properties and hence its incorporation to our daily diet (Szedljak et al., 2019). For example, Canada is one of the largest lentil producers globally,

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but exports most of the production without adding value (Bekkering, 2015). Therefore, novel

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technologies for producing pulse-based foods that are compatible with the eating habits of western cultures are needed to increase the consumption of these nutritionally dense crops. Extrusion cooking is a versatile, low cost, energy efficient, and environmentally-

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friendly processing technique to manufacture cereal and pulse-based puffed foods (e.g., breakfast cereals, expanded snacks) with consumer appeal. The final product quality (e.g., radial expansion index, density, microstructure, texture and colour) of extruded foods is

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greatly associated with extrusion parameters (e.g., feed material composition, feed moisture content and screw speed). For puffed snacks, high expansion, low density, uniform microstructure and crunchy texture are desired and play important roles in product development (Alam et al., 2016; Meng, et al., 2010; Philipp et al., 2017; Trater et al., 2005). Radial expansion index, density, microstructure and texture properties are associated with consumer appeal while colour acts both as an appearance attribute, but also an indicator for

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degree of cooking (Taverna et al., 2012). In puffed foods like expanded snacks, colour can also provide information on porous microstructure due to the incident light being partially reflected at the surface, entering into the extrudate cells, and again being reflected at cell walls, i.e., due to diffuse reflection. Among different extrusion process parameters, high screw speed and low feed moisture content have previously been reported to result in extrudates with better expansion and more uniform cell structure (Meng et al., 2010). The use of physical blowing agents (e.g.,

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various gases like N2, CO2, etc.) can also improve extrudate expansion and microstructure uniformity. Blowing agents have been shown to enhance consumer appeal in terms of physical, textural and structural characteristics of extrudates (Chan et al., 2019; Freeman et

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al., 2008), especially of those that are rich in proteins and fibres. In their gaseous form, physical blowing agents can be introduced into the extruder barrel during processing through

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an injection valve so as to provide additional bubble nucleation sites, substantially affecting

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extrudate expanded structure. The plasticizing effect of blowing agents, such as supercritical CO2, is also proven to play an important role in manipulating final product quality (Chen and Rizvi, 2006; Panak-Balentić et al., 2017; Paraman et al., 2012). As a low molecular weight

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molecule, nitrogen might act as a plasticizer and change the rheological properties of food material during extrusion, and therefore affect product quality (Cho and Rizvi, 2010; Di Maio et al., 2005).

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Despite their nutritional benefits, there are many challenges that need to be tackled if pulses are to be more frequently incorporated into puffed foods. The objective of this study was to investigate the effects of feed moisture content, screw speed and physical blowing agent (i.e., N2) injection pressure on the physical (i.e., expansion index, extrudate density, colour), mechanical (i.e., texture) and microstructural properties of extrudates made from red lentil flour.

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2. Materials and methods 2.1. Materials and proximate composition Red lentil flour was supplied by Ingredion Incorporated (IL, USA), with the proximate composition (dry basis) of 25.1% protein, 0.7% fat, and 2.8% ash and 71.5% total carbohydrates by difference (with 8.83% dietary fibre), in line with previous studies (Dogan

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et al., 2013). The dietary fibre content was measured according to AACC method 32-07.01 with minor modifications (60 ml crucible, 0.5 g sample, and 1.1 g Celite were used in this study), while protein, fat and ash content were provided by the company. A nitrogen t-tank

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(Innovair Group, Winnipeg, MB, Canada) was used during nitrogen assisted extrusion. Distilled water was used in all extrusion runs.

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2.2. Extrusion process

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A lab-scale, co-rotating twin screw extruder (MPF19, APV Baker Ltd., Peterborough, UK) with screw length-to-diameter ratio of 25:1, a circular die with diameter of 2.3 mm and five temperature control zones, set at 60, 90, 120, 140, and 150 oC from feed-to-die sections

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was used to extrude red lentil flour. The screw configuration was kept constant following the configuration reported by Koksel and Masatcioglu (2018), as shown in Figure 1, and red lentil flour was fed into the extruder at a constant feed rate of 3 kg/h. The two screw speeds

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used were 150 rpm and 200 rpm. Water injection was calibrated to control the moisture content at 18, 20, and 22%. For conventional extrusion, no physical blowing agent was used during the process, while for physical blowing agent assisted extrusion, N2 gas at three different pressure levels (300, 400, and 500 kPa) was injected into the barrel. Specific screw elements (i.e., reverse paddles) were used to control the flow direction of N2 and prevent the back flow of N2. N2 injection pressure used in the study was selected based on preliminary

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extrusion runs, where injecting below 300 kPa did not result in significant visual changes in extrudates and injecting over 500 kPa gave a significantly longer time for the extruder to reach steady-state conditions and therefore was not practical. At the highest N2 injection pressure, no N2 back flow was observed visually. All extrusion runs under different treatments (i.e. feed moisture, screw speed, and N2 injection pressure) were performed in duplicates. Extrudates were collected after reaching steady-state conditions. Parameters including

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torque and die pressure were recorded. The specific mechanical energy (SME) for each extrusion run was calculated based on the parameters recorded using Eq. (1) (Meng et al., 2010): 𝑎𝑐𝑡𝑢𝑎𝑙 𝑠𝑐𝑟𝑒𝑤 𝑠𝑝𝑒𝑒𝑑 (𝑟𝑝𝑚) 𝑚𝑎𝑥 𝑠𝑐𝑟𝑒𝑤 𝑠𝑝𝑒𝑒𝑑 (𝑟𝑝𝑚)

×

𝑡𝑜𝑟𝑞𝑢𝑒 (%) 100

𝑚𝑜𝑡𝑜𝑟 𝑝𝑜𝑤𝑒𝑟 (𝑘𝑊)

× 𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 (𝑘𝑔/ℎ)

(1)

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SME (kW h/kg) =

where max screw speed and motor power were 500 rpm and 2.2 kW, respectively. Torque

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value was presented in the unit of “%”, where 100% equals 9.5 N m. SME was presented as

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the average value calculated from the duplicated extrusion runs. Extrudates were cooled to room temperature, dried overnight at 50 oC to achieve a moisture content lower than 10% and

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stored in zipped plastic bags to prevent moisture absorption from the environment. 2.3. Extrudate density and radial expansion index Extrudate density (𝜌𝐸 ) and radial expansion index (EI) were measured using the method reported by Ryu and Ng (2001), and modified by Koksel and Masatcioglu (2018),

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respectively. Extrudate density was calculated by a canola seed displacement technique using Eq. (2): 𝜌𝐸 =

𝑚𝑒 𝑉𝑒

=

𝑚𝑒 ×𝜌𝐶

(2)

𝑚𝐶

where 𝜌𝐸 , 𝑉𝑒 , 𝑚𝑒 , 𝜌𝐶 , and 𝑚𝐶 represent density of extrudates, volume of extrudate piece, extrudates mass (weighed on a balance), density of canola seeds, and the mass of canola

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seeds with a volume same as the extrudate piece, respectively. Five measurements were conducted for each treatment. For radial expansion index, two measurements were conducted for each treatment, where each measurement involved averaging five random readings along the length of an extrudate. Radial expansion index was calculated using Eq. (3): D

EI = DE

(3)

d

where DE is the diameter of extrudate, and Dd is the diameter of extruder die (2.3 mm). Both

duplicate extrusion runs for each treatment. 2.4. True density and gas volume fraction measurements

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extrudate density and radial expansion index results were presented as the mean value of

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A gas displacement pycnometer (Ultrapyc 1200e, Quantachrome Instruments,

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Boynton Beach, FL, USA) was used to measure the true density of extrudate cell walls and to calculate the gas volume fraction of extrudates. The pycnometer consisted of two chambers,

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one to hold the extrudate sample and the other with a fixed reference volume. Extrudate samples were ground finely and then sifted through a 212 µm test sieve. Approximately 2 g

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of ground and sifted extrudate sample was placed into the sample chamber. The chamber was hermetically sealed and purged with pressurized helium gas at 131 kPa. The extrudate sample volume (VS), extrudate cell wall density (ρcw) and gas volume fraction (i.e., porosity, ∅) were calculated using Eq. (4), (5) and (6), respectively. 𝑉𝑟 𝑃1 ( )−1 𝑃2

]

(4)

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𝑉𝑆 = 𝑉𝑐 − [ 𝜌𝑐𝑤 =

𝑚𝑠

(5)

𝑉𝑠

𝜌

∅ = [1 − (𝜌 𝐸 )] × 100

(6)

𝑐𝑤

where Vc is the volume of the empty sample chamber (determined from a prior calibration step), Vr is the fixed volume of the reference chamber, P1 is the pressure in the sample 7

chamber alone, P2 is the pressure after expansion of the gas (i.e., He) into the combined volumes of the sample and reference chambers, ms is the extrudate sample mass, and ρE is the extrudate density. Pycnometer tests were performed in triplicates from which average cell wall density (ρcw) and gas volume fraction (∅) were obtained as the mean values of duplicate extrusion runs for each treatment. The gas volume fraction (i.e., porosity ∅) was used to calibrate the threshold of the X-ray microcomputed tomography images (see Section 2.5).

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2.5. X-ray microcomputed tomography Extrudate samples, representative of specific extrusion conditions, were selected and cut into ~1.5 cm length and individually mounted onto the rotating stage of an X-ray

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microcomputed tomograph (SkyScan 1275, Bruker, Zaventem, Belgium) using transparent wax as an adhesive. The rotating stage was set 6.8 mm away from the X-ray source in order

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to scan the entire sample. The X-ray source (without a filter) was set at 40 kV and 208 µA.

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These voltage and current settings were established as the optimal settings for the X-rays to penetrate through the extrudate sample and to provide images of distinguishable extrudate cell walls and air cells. During the tomography scans, a rotation angle of 0.2° over 180° was

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used with a 48 ms exposure time at each rotation angle to obtain a high-contrast 3D model of the exudate sample (Erkinbaev et al., 2019). Images were taken by averaging four frames followed by cropping to a partial width of 59% to create 1000 cross-sectional images of

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1944×1382 pixels at a resolution of 13.8 µm/pixel. Out of these 1000 cross-sectional images, 500 images were used to represent the sample and reconstructed into a 3D hypercube using NRecon software (Bruker, Zaventem, Belgium). These 500 images were selected by avoiding unusually large cells along the extrudate length, as well as the top and the bottom edges of the extrudate.

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The reconstructed 3D hypercube was analyzed using a graphics processing unit (GPU) enabled computer (Precision T7810, RAM 64 GB, 3.2 GHz, Dell Corporation, Round Rock, USA). A custom-written pre-processing algorithm was applied to extract morphometric characteristics of the extrudates using CTan software package (Bruker, Zaventem, Belgium) by: (1) converting images to grey scale, (2) selecting a region of interest (ROI) using the ‘round’ function in CTan with a diameter of 337 pixels (i.e., 4.56 mm), and (3) thresholding the ROI to objectively calibrate the gas volume fraction. The threshold values of grey scale

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ROIs were individually set for each extrudate sample so that the calculated gas volume fraction matched with the pycnometer results. 2.6. Texture

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Texture attributes of extrudates were tested using the method of Koksel and Masatcioglu (2018) using a texture analyzer (TA-XT-plus, Stable Micro System, Gudalming,

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UK) equipped with a Warner-Bratzler shear blade probe. For each treatment, three

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measurements were conducted. Each measurement involved averaging the results (hardness, crispiness, crunchiness) for five random extrudate pieces, each 4 cm long. For each measurement, an extrudate was placed on the equipment platform, with a 90o shear angle to

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the blade. Exponent software (version 6, Stable Micro System, Gudalming, UK) was used to plot a time versus force graph from which textural properties were extracted. Hardness (N) is the peak force; crispiness is the number of positive peaks in the force versus time graph, and

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crunchiness (N s) is the linear distance of the rugged lines obtained from the same graph. The higher the number of positive peaks, the higher is the number of fracture events and the higher the crispiness of the extrudate. The longer the linear distance, the longer is the drop from peak to through for each fracture event on average and crunchier the extrudate. The results were presented as the average value of duplicate extrusion runs for each treatment. 2.7. Colour

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Extrudate colour parameters L* (lightness/darkness), a* (redness/greenness) and b* (yellowness/blueness) were measured following the method reported by Koksel and Masatcioglu (2018) using a colour spectrophotometer (CM-3500, Minolta, Osaka, Japan). The overall colour difference (∆E) between an extrudate and the reference (Lref*=0, aref*=0, bref*=0, i.e., black) was calculated using Eq. (7): ∆E = √(𝐿∗ − 𝐿𝑟𝑒𝑓 ∗ )2 + (𝑎∗ − 𝑎𝑟𝑒𝑓 ∗ )2 + (𝑏 ∗ − 𝑏𝑟𝑒𝑓 ∗ )2

(7)

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For assessment of overall colour difference (∆E), three measurements were conducted for each treatment, where each measurement involved averaging ten random readings at different locations of colour spectrophotometer’s sample holder. Colour results were presented as the mean value of duplicate extrusion runs for each treatment.

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2.8. Statistical analysis

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Statistical differences of extrudate properties among different treatments were

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3. Results and discussion

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determined by ANOVA and Tukey’s test (p<0.05) using SAS software (Version 9.2).

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The experimental design, torque, die pressure and specific mechanical energy (SME) values during extrusion are presented in Table 1. For all extrusion treatments, regardless of the feed moisture content and the gas injection pressure used, an increase in screw speed from 150 rpm to 200 rpm resulted in a decrease in torque values and die pressure, typical for extrusion carried out with partially empty barrels operated at low feed flow rates (Vanhoorne et al., 2016). In agreement with our findings, an increase in SME with an increase in screw speed was reported for banana-corn flour blend extrudates (Kaur et al., 2014), and corn flour-

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brewers spent grain extrudates (Ainsworth et al., 2007). SME values ranged between 139 and 224 Wh/kg (499-807 kJ/kg), in line with SME results for pea protein and fibre fortified puffed snacks produced at slightly higher moisture

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and screw speed but lower die temperature (Beck et al., 2018). When the screw speed and the N2 injection pressure were held the same, an increase of moisture content resulted in a

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decrease in torque, die pressure and SME values due to the plasticizing effect of water,

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decreasing the viscosity of the melt (Chen et al., 2006). With a moisture content of 18%, an increase in N2 injection pressure has caused decrease in both torque and SME values, suggesting that injected N2 possibly acted as a plasticizer at low moisture content and caused

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a reduction in melt viscosity. However, the plasticizing effect was not obvious with higher moisture contents (i.e., 20% and 22%), probably due to the low solubility of nitrogen gas in water (Di Maio et al., 2005). Similar results for the effect of increasing moisture content and

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N2 injection pressure were reported for yellow pea flour and red lentil flour (Chan et al., 2019; Koksel and Masatcioglu, 2018). The change in N2 injection pressure had no clear trend in relation to the changes in torque, die pressure or SME input values during extrusion of red lentil flour extrudates at both screw speeds studied and at moisture contents of 20% and 22%. These dependent variables (i.e., torque, die pressure and SME) were previously shown to be functions of the type of blowing agents (N2, CO2, supercritical CO2), raw materials (e.g.,

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yellow pea flour, pre-gelatinized corn starch, carrot powder with cornmeal) and processing conditions, such as barrel temperature (Koksel and Masatcioglu, 2018; Mariam et al., 2008; Samard et al., 2017). 3.1. Extrudate density and radial expansion index A significant three-way interaction between feed moisture content, screw speed, and N2 injection was observed for extrudate density results (Table 2). Extrudate density values for different extrusion treatments are presented in Figures 1a and b, for screw speed of 150 and

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200 rpm, respectively. For both screw speeds, density of extrudates increased with an increase in feed moisture content, pointing to lower levels of overall expansion. Similar results were reported for whole grain wheat flour (Oliveira et al., 2017) and chickpea flour-

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based extrudates (Meng et al., 2010).

Extrudates with no N2 injection (conventional extrusion) generally had the highest

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extrudate density, regardless of feed moisture content, followed by extrudates with N2

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injection at the highest pressure (500 kPa). The lowest extrudate density was observed at the lowest N2 injection pressure (300 kPa). A decrease followed by an increase in density as a function of blowing agent pressure was also observed for starch-based extrudates produced

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using supercritical CO2 (Mariam et al., 2008). The increase in density at relatively higher N2 injection pressures might be caused by burst and collapse of extrudate cell walls when the pressure inside the cell is too high compared to the atmospheric pressure at the die exit so that

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gas cannot be held in and is lost to the atmosphere (Cho and Rizvi, 2009). For extrudates with N2 injection, an increase in screw speed from 150 to 200 rpm caused a decrease in their density, meaning that extrudates produced at the higher screw speed expanded more. A similar trend for the relationship between screw speed and extrudate density was also reported for chickpea flour-based snacks (Meng et al., 2010).

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Extrudate radial expansion index values were significantly influenced by two-way interactions of feed moisture content by N2 injection pressure and screw speed by N2 injection pressure (Table 2). The extrudate radial expansion index values for different extrusion treatments are presented in Figures 2a and b, for screw speeds of 150 and 200 rpm, respectively. Extrudate radial expansion index was feed moisture content, N2 injection pressure and screw speed dependent. With an increase in feed moisture content, a decrease in radial expansion index was observed in line with the results of increasing extrudate density

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(Figure 3a and 3b). Similar results were reported for whole grain wheat flour extrudates (Oliveira et al., 2017), wheat-based expanded snacks (Ding et al., 2006), and chickpea flourbased snacks (Meng et al., 2010). Increased moisture in feed causes a reduction of elasticity

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of the melt in barrel and drives expansion in longitudinal direction (Alvarez-Martinez et al., 1988), explaining reduced radial expansion with increased feed moisture content.

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For the two screw speeds studied, the lowest N2 pressure (300 kPa) produced

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extrudates with higher or similar radial expansion index values when compared to conventional extrudates. The radial expansion index values of extrudates decreased with a further increase in N2 injection pressures, in line with the results of extrudate density. An

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increase followed by a decrease in radial expansion index values with increasing physical blowing agent concentration has also been previously reported for yellow pea extrudates in the presence of N2 (Koksel and Masatcioglu, 2018). This trend was explained by the

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impairment of the gas holding properties of food polymers in protein-rich media. Higher protein concentration disrupts the starch matrix, as the aggregated protein molecules would disrupt the continuous starch phase surrounding the bubbles during their expansion (Cho and Rizvi, 2009; Kristiawan et al., 2018) and causes gas to be lost to the atmosphere at elevated physical blowing agent concentrations. For extrudates with N2 injection, increase in screw

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speed caused an increase in radial expansion index of red lentil extrudates, in agreement with previous findings for conventional chickpea flour-based extruded snacks (Meng et al., 2010). 3.2. Extrudate porosity and microstructure Extrudate cell wall density was independent of screw speed, feed moisture content, or N2 injection pressure, and ranged between 1.48 and 1.53 kg m-3. At the same moisture content and N2 gas injection pressure, extrudate gas volume fraction (i.e., ∅, porosity) was not substantially affected by screw speed. For example, at moisture content of 18% and 0 kPa

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of N2 gas injection, extrudate porosity at 150 and 200 rpm were 83.48% and 82.34%, respectively. Generally, porosity decreased with an increase in moisture content, in line with the results of Thymi et al. (2005) and Bhattacharya et al. (1986) who studied corn starch and

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corn-soy protein blend extrudates, respectively. Contrary to these findings, the porosity values of extrudates made from pea and wheat blends increased with increasing moisture in

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the range of 18-24% moisture (flour weight basis) (Zarzycki et al., 2015). Moisture content’s

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influence on extrudate porosity is evidently a function of the raw materials, and the range of moisture contents studied. The latter is brought about by moisture content’s substantial influence on glass transition temperature which dictates the temperature at which extrudates

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solidify. The time to reach glass transition temperature is linked to extrudate collapse, and thus overall expansion (Moraru and Kokini, 2003). For both screw speeds and all moisture contents studied, with the use of N2 gas,

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porosity initially increased (0 to 300 kPa injection), then decreased (i.e. 400 and 500 kPa), in line with the results of radial expansion index (Figure 3). For example, extrudates produced at 20% moisture content and 200 rpm, porosity at 0, 300, 400, and 500 kPa N2 injection pressure were 78.23%, 90.38%, 85.27%, and 83.03%, respectively. Extrudates produced by 300 kPa N2 injection consistently had the highest porosity compared to the other N2 injection levels, while control extrudates (i.e., without N2 injection) generally had the lowest porosity,

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in line with the results of extrudate density (Figure 2). Singkhornart et al. (2014) demonstrated that, in cereal-based extrudates, injecting CO2 (500 mL min-1) as a blowing agent resulted in higher extrudate expansion when compared to extrudates produced without blowing agents in agreement with our findings. However, the positive effect of physical blowing agents on extrudate porosity was reported to be also dependent on die temperature. In some cases, extrudates had low porosity in the presence of CO2 when compared to conventional extrudates. For example, Samard et al. (2017) reported an increase in extrudate

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expansion with CO2 injection at 80°C at the die, followed by a decrease in extrudate expansion at 120°C due to the temperature dependence of gas diffusion coefficients (i.e., as temperature increases, gas diffusivity increases). Since a constant temperature profile was

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used in our study, the interactive effects of die temperature and physical blowing agent concentration on extrudate expansion will not be discussed further.

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X-ray micro-computed tomography was utilized to demonstrate the impact of

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extrusion treatments on red lentil extrudate microstructure. Different pairs of extrudates produced at various extrusion conditions were chosen (Figure 4) to better represent individual effects of screw speed, moisture content and N2 injection pressure. In Figure 4a and 4b, the

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effect of screw speed (150 and 200 rpm) on extrudate microstructure is compared for extrudates produced at 20% moisture content and 400 kPa N2 injection. An increase in screw speed caused a slight increase in porosity, which is in agreement with the radial expansion

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index results. Qualitatively, average cell size of extrudates produced at the lower screw speed were smaller, while the number of cells were higher when compared to those produced at higher screw speed. Overall, screw speed had the least overall impact on porosity and radial expansion index in comparison to moisture content and N2 injection. The effect of moisture content (18 and 22%) on extrudate microstructure at constant screw speed and N2 injection pressure is presented in Figures 4c and 4d. Relative to the other

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parameters (i.e., screw speed and N2 injection pressure), porosity was most affected by moisture content. The low moisture content (18%) extrudates were characterized with a greater expansion and porosity compared to the higher moisture content (22%) ones, but they had fewer number of cells which were larger and unevenly distributed. At a higher moisture content, however, extrudates had numerous and more evenly distributed cells that were relatively smaller and had thicker cell walls. This could be attributed to the higher moisture levels that allow more even hydration of proteins and carbohydrates, which in turn create a

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stronger network that resists expansion at the die exit (Li et al., 2005). In Figure 4e and 4f, the effect of N2 injection pressure (0 and 500 kPa) on extrudate microstructure is presented at constant screw speed and feed moisture content. The extrudates

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without N2 had fewer cells and were irregularly shaped. The X-ray results shown that N2 injection created numerous small and more evenly distributed cells, as physical blowing

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agents, like N2, may increase the number of available nucleation sites for bubbles (Ishikawa

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et al., 2012; Sun et al., 2015). The high number of small cells have also been enhanced by N2 injection in extrudates made from red lentil and yellow pea flours at slightly different

2018).

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moisture contents when compared to this study (Chan et al., 2019; Koksel and Masatcioglu,

3.3. Texture

Extrudate textural properties (hardness, crispiness, and crunchiness) as a function of

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feed moisture content, screw speed and N2 injection pressure are presented in Table 3. Twoor three-way interactions of feed moisture content, screw speed, and N2 injection pressure did not significantly influence textural quality attributes (Table 2). On average, extrudate hardness increased with an increase in feed moisture content, in line with previous studies on extrudates made from whole wheat (Oliveira et al., 2017) and chickpea flours (Meng et al., 2010). Extrudate hardness was a function of screw speed, with hardness values increasing on

16

average with a decrease in screw speed from 200 rpm to 150 rpm. Similar results were also reported for whole grain corn flour based snacks extruded in a similar screw speed range (100 – 300 rpm) (Ainsworth et al., 2007). Crispiness of extrudates decreased with an increase in feed moisture content, regardless of the intensity of the screw speed (Table 3). Extrudates produced at 300 kPa N2 injection were the crispiest, generally followed by no N2 injection, at 400 kPa N2 injection, and at 500 kPa N2 injection, demonstrating the non-linear relationship between N2 injection

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pressure and extrudate texture. This could possibly due to a combination of higher number of cells caused by the injection of N2 and the thinner cell wall of extrudates produced at 300 kPa compared to extrudates produced at other N2 injection pressures, as the crispiness is found to

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be associated with the rupture of the cellular structure occurred when force is applied to the snacks (Mazumder et al., 2007). Extrudates produced at screw speed of 200 rpm were in

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general crispier than those at screw speed of 150 rpm. The dependence of crispiness on N2

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injection pressure and feed moisture content had the opposite trend with extrudate density (Figure 3), meaning that the higher the extrudate density, the lower the crispiness. When compared to conventional extrusion, N2 injection slightly increased the crunchiness of red

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lentil extrudates. However, this increase was not statistically significant. Extrudates produced at 150 rpm were, on average, slightly crunchier than extrudates produced at 200 rpm. Feed moisture content had no obvious effect on crunchiness.

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Porosity was strongly and negatively correlated to hardness (R=−0.958), but strongly and positively correlated to crispiness (R=0.939) (i.e., extrudates with a lower gas volume fraction were harder and extrudates with a higher gas volume fraction were crispier). The inverse relationship between hardness and porosity is attributed to an increase in extrudate density, an increased cell wall thickness and reduced cell size (Jin et al., 1995; Lazou and Krokida, 2010; Philipp et al., 2017) in harder extrudates. These findings are also in line with

17

Chanvrier et al. (2014) and Ramos Diaz et al. (2015) who studied expansion and texture of cereal-based puffed extrudates. 3.4. Colour Extrudate colour parameters L* and ∆E are presented in Table 3. Pictures of selected extrudate samples are shown in Figure 4 for visual comparison. A significant three-way interaction between feed moisture content, screw speed, and N2 injection was observed for L* and ∆E (Table 2). Compared to conventional extrusion, N2 injection caused an increase in

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extrudate L* values, meaning that N2 injection produced lighter coloured extrudates, for example in Figure 4, from sample (a) to sample (d), the extrudates are visually lighter with an increase in N2 injection pressure. Moreover, with an increase of N2 injection pressure, L* of

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extrudates increased, becoming even lighter in colour. This is opposite to what one might expect because without the information on cell size (and the effects of diffuse reflection), a

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lower level of expansion generally resulted in a darker extrudate colour. These changes in

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extrudate brightness are expected due to the higher number of cells produced as the N2 injection pressure is increased (Figure 3), and indicates an interaction between extrudate microstructure and how light is reflected from the surface of extrudates; how it is diffused

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within the extrudates and then scattered at the interfaces of cells and cell walls (Koksel and Masatcioglu, 2018). The effect of feed moisture content on L* values were not as substantial when compared to the effect of N2 injection, possibly due to the narrow range of moisture

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contents studied. An increase in screw speed from 150 to 200 rpm also had no obvious effect on L* values.

Generally, the ∆E value (i.e., overall colour difference) of extrudates increased with

an increase in feed moisture content (Table 2). Extrudates with no N2 injection had the lowest ∆E, suggesting that these extrudates were closest to the reference colour. An increase in ∆E was observed with an increase in N2 injection pressure. Same effects of increasing physical

18

blowing agent (CO2) pressure on ∆E were also reported for cornmeal extrudates (Samard et al., 2017). In this study, red lentil extrudates produced at a screw speed of 150 rpm had higher ∆E values when compared to extrudates produced at a screw speed of 200 rpm. The change in extrudate colour may have been affected by the residence time of material in the barrel, as a slower screw speed would result in a longer residence time, a longer exposure to heat and therefore, a higher level of degradation of the heat sensitive compounds in the raw materials (Kaur et al., 2014). Breakdown of such compounds [e.g., heat labile bioactive

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compounds (Arribas et al., 2019; Margier et al., 2018) or amino acids like lysine (Masatcioglu et al., 2014)] can influence the colour and possibly impair the nutritional quality of the end-product.

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4. Conclusions

The overall expansion, microstructure, texture and colour of extrudates made from red

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lentil flour extruded using different feed moisture content, screw speed, and N2 injection

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pressure were studied. An increase in feed moisture content was found to reduce the overall expansion, overall porosity, and crispiness of extrudates. To the best of our knowledge, this is the first in-depth study focusing on the effects of N2 injection on physical, mechanical and

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microstructural properties of red lentil extrudates Nitrogen gas, as a physical blowing agent during extrusion, demonstrated great potential to be used as a tool to manipulate extrudate properties, such as extrudate density and colour. Moreover, N2 injection led to a more

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uniform distribution of cells in the final product as evidenced by extrudate microstructure measured using X-ray microcomputed tomography. In the future, further studies will be performed to better understand how higher or lower screw speed and a wider range of moisture contents would affect extrudate quality in the presence of different physical blowing agents.

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Conflict of interest.

The authors declare no conflict of interest.

Acknowledgements The authors would like to acknowledge the University of Manitoba Start-up and GETS funding, and thank Ingredion Inc. for supplying the red lentil flour.

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extruded corn starch. Journal of Food Engineering, 68(4), 519–526. https://doi.org/10.1016/j.jfoodeng.2004.07.002 Trater, A. M., Alavi, S., & Rizvi, S. S. H. (2005). Use of non-invasive X-ray

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https://doi.org/10.1016/j.ijpharm.2016.03.058 Zarzycki, P., Kasprzak, M., Rzedzicki, Z., Sobota, A., Wirkijowska, A., & Sykut-Domańska, E. (2015). Effect of blend moisture and extrusion temperature on physical properties of everlasting pea-wheat extrudates. Journal of Food Science and Technology, 52(10),

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re

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6663–6670. https://doi.org/10.1007/s13197-015-1754-y

27

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re

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Figure 1. Diagram of screw configuration for the extruder.

28

Figure 2.Effects of feed moisture content and N2 injection pressure (0, 300 kPa, 400 kPa, and 500 kPa) on extrudate density at a screw speed of (a) 150 rpm and (b) 200 rpm. Error bars represent ± standard deviations. 18% moisture

20% moisture

22% moisture

0.5 0.4 0.3

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Extrudate density (kg/m3)

(a) 0.6

0.2

0 300 400 500 0 300 400 500 0 300 400 500 18 20 22 Nitrogen injection pressure (kPa)

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0.0

-p

0.1

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(b) 0.6

0.4 0.3 0.2

22% moisture

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0.5

20% moisture

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Extrudate density (kg/m3)

18% moisture

0.1 0.0

0 300 400 500 18

0 300 400 500 0 300 400 500 20 22 Nitrogen injection pressure (kPa)

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Figure 3. Effects of feed moisture content and N2 injection pressure (0, 300 kPa, 400 kPa, and 500 kPa) on extrudate radial expansion index at screw speed of (a) 150 rpm and (b) 200 rpm. Error bars represent +/- standard deviations. (a) 6

20% moisture

18% moisture

22% moisture

4 3

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Expansion index

5

2

18% moisture

2 1 0

20% moisture

22% moisture

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4

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Expansion index

5

3

0 30 40 50 0 30 40 50 20 22 0 0 0 0 0 0 Nitrogen injection pressure (kPa)

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(b) 6

0 30 40 50 18 0 0 0

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0

-p

1

0 300 400 500 18

0 300 400 500 0 300 400 500 20 22 Nitrogen injection pressure (kPa)

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Figure 4. 3D cross-sectional microstructure of red lentil extrudates at (a) 150 rpm, and and (b) 200 rpm, at constant feed moisture content (20%) and constant N2 injection pressure (400 kPa); at (c) 18% moisture content, and (d) 22% moisture content, at constant screw speed (200 rpm) and constant N2 injection pressure (400 kPa); and at (e) 0 kPa and (f) 500 kPa N2 injection pressure at constant screw speed (150 rpm) and constant moisture content (18%).

(b)

150 rpm

200 rpm

MC 20%

MC 20%

400 kPa

400 kPa

(c)

(d)

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400 kPa

MC 22%

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MC 18% 200 rpm

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(a)

200 rpm 400 kPa

(f)

0 kPa

500 kPa

150 rpm

150 rpm

18% MC

18% MC

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(e)

31

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The scale bar in (a) is 2.5 mm and is applicable to all images.

Figure 5. Pictures of red lentil extrudates at different N2 injection pressure (a) 0 kPa, (b) 300 kPa, (c) 400 kPa, (d) 500 kPa, at constant moisture content (20%) and constant screw speed

(b)

(c)

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re

(a)

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(150 rpm).

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(d)

Table 1. Effects of screw speed, feed moisture content and N2 injection pressure on torque, die pressure and specific mechanical energy (SME) values during extrusion (n=2 for extrusion runs). Torque (%)

Die pressure (kPa)

SME (Wh/kg)

0 300 400 500 0 300 400 500 0 300 400 500 0 300 400 500 0 300 400 500 0 300 400 500

87.3 84.0 83.5 82.0 75.0 76.5 76.5 75.5 63.0 67.5 67.0 66.5 76.5 73.5 72.0 72.0 67.5 66.5 65.5 63.0 61.3 60.8 58.5 60.0

6230 6350 6550 6550 5500 5650 5800 5600 4700 4600 4700 4700 5180 5450 5250 5400 4450 4850 4900 4800 4400 4100 4200 4250

192.0 184.8 183.7 180.4 165.0 168.3 168.3 166.1 138.6 148.5 147.4 146.3 224.4 215.6 211.2 211.2 198.0 190.1 192.1 184.8 179.7 178.2 171.6 176.0

18

150

20

22

20

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200

lP

18

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22

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Gas (N2) injection pressure (kPa)

-p

Moisture content (%)

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Screw speed (rpm)

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Table 2. Three-way analysis of variance on the effects of feed moisture content, screw speed, and N2 injection pressure on extrudate physical quality parameters [i.e., extrudate density, radial expansion index, hardness, crispiness, crunchiness, lightness (L*) and colour difference (∆E)]. The effects are considered as significant if P<0.05. P value Effect

DF

Extrudate

Radial expansion

density

index

Crispines

Crunchi

s

ness

Hardness

3

MC×NP

SS×NP

MC×SS

6

3

2

MC×SS 6

0.0017

<0.0001

<0.0001

<0.0001

<0.0001

<0.0001

0.0026

<0.0001

<0.0001

<0.0001

<0.0

<0.0

001

01

<0.0

<0.0

001

01

<0.0

<0.0

001

001

<0.0

0.008

001

2

0.6809

0.0348

<0.0001

0.0184

0.0101

<0.0001

0.2095

0.027 0.003 0.0053

0.0026

0.4354 0

5

0.001 0.051

0.8116

0.4541

0.2167

0.7490

Jo

×NP

<0.0001

<0.0001

<0.0001

∆E

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NP

<0.0001

<0.0001

-p

1

<0.0001

re

SS

<0.0001

lP

2

ur na

MC

L*

0.2981

0.0451

0.0537 1

7

<0.0

<0.0

001

001

0.9281

Note: MC = feed moisture content, SS = screw speed, NP = N2 injection pressure, DF

= degrees of freedom

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Table 3. Texture (hardness, crispiness and crunchiness), and colour [lightness(L*), colour difference (∆E)] quality parameters of red lentil extrudates under different feed moisture content, screw speed, and gas injection pressure treatments (n=2 for extrusion runs, n=3 for texture measurements, n=3 for colour measurements). Values were presented by average ± standard deviations. Screw

Moisture

N2

speed

content

pressure

(rpm)

(%)

(kPa)

Hardness

Crispiness

(N)

Crunchiness

L*

∆E

(N s)

0

5.67±1.42

11.40±2.31

31.26±5.20

52.71±1.56

59.13±0.85

300

7.14±1.28

12.60±2.28

38.63±4.44

55.77±0.64

63.30±0.34

400

8.42±1.05

10.20±1.91

38.60±3.82

63.09±0.93

71.93±0.36

500

9.06±0.78

8.53±0.73

38.40±2.45

66.29±1.09

74.64±0.11

0

6.27±2.88

10.60±1.98

32.97±10.22

50.78±0.82

59.79±0.49

300

8.11±1.26

11.07±2.21

37.55±6.36

59.13±1.70

67.82±0.10

400

10.84±1.86

8.77±1.22

45.49±4.76

62.78±0.28

71.80±0.18

500

10.29±1.55

7.77±1.54

37.86±4.78

68.13±0.93

76.72±0.44

0

4.99±2.30

7.03±1.27

300

9.71±1.75

7.90±1.56

400

11.89±0.52

7.50±0.69

500

11.32±1.77

0

6.63±2.71

300

5.30±0.77

400 500

25.92±6.59

50.81±0.60

60.30±0.09

37.78±5.75

56.26±1.07

64.99±0.39

43.87±2.39

62.22±0.41

71.79±0.24

6.77±0.94

38.73±6.64

64.66±0.56

73.43±0.18

10.87±2.74

32.09±9.12

50.38±1.28

58.30±0.39

15.57±1.71

35.95±7.33

55.39±0.79

62.03±0.49

6.93±1.09

11.27±1.60

34.76±4.46

61.44±0.52

69.68±0.27

8.34±1.34

9.50±1.03

37.01±4.78

63.83±0.7

71.95±0.20

lP

18

re

22

0

5.29±1.95

8.93±2.12

25.82±7.41

50.47±0.38

59.23±0.19

300

6.37±1.14

12.33±2.49

32.63±3.56

55.21±1.43

62.60±0.53

400

8.94±1.61

9.00±1.62

36.37±7.12

64.55±0.48

73.49±0.28

500

8.74±1.00

8.63±1.67

34.30±5.85

67.07±1.07

75.41±0.49

0

6.27±1.20

8.73±2.41

30.21±7.50

49.66±2.21

58.45±0.19

300

6.97±1.42

11. 40±2.91

36.19±8.79

56.41±1.65

64.21±0.48

400

10.34±1.43

8.13±1.58

40.90±8.01

62.05±0.39

71.04±0.18

500

10.52±0.87

6.93±1.64

38.27±3.94

65.45±0.33

74.39±0.19

20

Jo

200

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20

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150

ro of

18

22

35