Color, Mechanical, and Microstructural Properties of Vacuum Assisted Microwave Dried Saskatoon Berries

Abstract

Saskatoon berries were dried using a vacuum assisted microwave technique. Central composite rotate design and response surface methodology were used to develop the regression models and to study the influence of drying processing variables (microwave power, drying time, and fruit load) on color, mechanical properties, and microstructure of dried berries. All the three variables had significant effects on the above mentioned properties (p< 0.05). Drying affected the color at variable levels and increased the lightness of dried berries compared to frozen berries. Hardness, gumminess, and chewiness of dried berries increased and springiness and cohesiveness of dried berries decreased with microwave power and drying time. The porosity and the destruction level in the microstructure of dried berries increased with the microwave power. The findings of this study will be useful to identify desirable microwave-vacuum drying operating conditions for targeted dried Saskatoon berry products.

Keywords:

• Vacuum assisted microwave drying
• Saskatoon berries
• Color
• Mechanical properties
• Microstructure

INTRODUCTION

Saskatoon berries (Amelanchier alnifolia Nutt.), commonly known as Saskatoons, are purplish-blue berries that possess high nutritional value and nutraceutical potential. The Saskatoon berries had a moisture content of 82–84 g/100 g berries, carbohydrate, 15–20 g/100 g berry sugar, significant amounts of protein, fat, and fiber and relatively large amounts of potassium, iron, magnesium, and phosphorous.[¹] The vitamins found in Saskatoon berries include vitamin A, vitamin E, vitamin C, thiamin, riboflavin, pantothenic acid, and vitamin B6. The Saskatoon berries are a rich source of antioxidants as they contained anthocyanins and other phenolic compounds.[²] The Canadian aboriginal people used to use the Saskatoon berry juice for the treatment of stomach ailments and also to prepare eye and eardrops from the matured Saskatoon berries.[³⁴] The commercial cultivation of Saskatoon berries in Canada started several decades ago and now the berries are being increased a strong commercial importance.[⁵] In 2011, Canada produced around a total 309 tons of Saskatoon berries, which had market value of total $1,212,000. About 134 tons (market value of $445,000) of Saskatoon berries were used for the processing industry and about 175 tons (market value of $767,000) of Saskatoon berries were used to meet the fresh fruit demand.[⁶] About a half of the total Saskatoon berries are produced in Prairie Provinces (Alberta, Manitoba, and Saskatchewan) of Canada. Approximately 500 to 800 hectares in prairies are being cultivated to produce Saskatoon berries. However, the demand for Saskatoon berries exceeds the supply. It has been projected that over 4000 hectares will eventually be used in the prairies for producing Saskatoon berries to meet the fruit demand.[⁷] The Saskatoon fruit industry is growing rapidly, and the Saskatoon berries are being exported from Canada to other countries such as Japan and Europe in the recent years.[³] The berries are being consumed as fresh or baked in the form of pies or processed into value added products such as jams, wines, cider, beers, and spreads.[⁴] The Canadian aboriginal people consumed the berries in soups, meat dishes, and dried cakes. Raisins or pemmican can be made from dried Saskatoon berries. The Saskatoon fruits can be used as natural food colorants because of the abundant content of anthocyanins in them.[²] There is a high demand for fresh Saskatoon fruits and processed products of Saskatoon berries. The bloom of fresh produce industry and the Saskatoon berries processing industry have faced draw backs due to the lack of sustainable and cost-effective preservation systems. The fresh berries possess a very short shelf life due to their high moisture content (80–84%). Drying the berries is needed in order to increase their shelf life and to preserve them for transportation and utilization throughout the year.

Drying is the most widespread method in the world to preserve the seasonal commodities among the various postharvest operations.[⁸] One of the concerning matters of any drying methods is to maintain the quality of the dried products by using an economical drying method. The most advanced dehydration method is freeze drying which retains the quality of the dried products but it is not feasible to introduce this method commercially.[⁸⁹] One of the most commonly used drying systems is hot air drying, which involves high temperature and prolonged drying time, and leads to the poor quality of dried products.[¹⁰] Vacuum drying is applicable for foods which are heat sensitive. The vacuum dried products have better quality in terms of nutrition and volatile aroma retention. The vacuum drying method is being limited due to the higher costs and longer drying duration. The microwave technique combined with vacuum assisted drying is rapid and uniform and this drying method has potential to be used on a commercial scale to dry food products with proper drying conditions.[¹¹]

The microwave oven has been shown to have low energy consumption.[⁹] The two prime factors of volumetric heating and reduced processing time make the vacuum assisted microwave drying technique an attractive source of thermal energy.[¹⁰] The vacuum pressure reduces the boiling temperature of water and provides phase change at a low temperature which is expected to preserve the quality of the products. The vacuum assisted microwave drying can dry the materials at a low temperature with a shorter drying period and absence of air during drying diminishes oxidation. The above stated advantages improve the quality attributes (i.e., color, texture, flavor, and nutrients) of the dried products.[¹⁰¹²] Microwave-vacuum drying was proved to be a potential drying method for the high quality dried products and it improves the energy efficiency.[^9101213]

The microwave drying processing parameters (i.e., microwave power level, drying time, and initial weight of fruit load) need to be applied properly so as to retain the quality of the dried products. The color of the products plays a vital role in acceptability of the products by the consumers. Mitra et al.[¹⁴] conducted a sensory evaluation on the color of the green banana chips and it had significant effects (p < 0.01) in color difference among the banana chip samples prepared. The similar sensory results were found on the color and the texture of the different fruit enriched yogurts.[¹⁵] The quality control information and the quality changes of products due to processing, storage, and other factors can be attributed with the color of the raw and processed products.[¹⁶] The texture is a major criterion while assessing the sensory quality in food products. The textural profile analyses are primarily used to evaluate the mechanical properties (hardness, cohesiveness, springiness, gumminess, and chewiness). The structural property is also an important quality factor for the acceptance of the products by the consumers.[¹⁷] The drying processing parameters of the vacuum assisted microwave drying technique have a great influence on the above mentioned quality factors of the dried Saskatoon berries. The objectives of this study were to investigate the influence of the drying processing parameters (microwave power, drying time, and fruit load) of the vacuum assisted microwave drying technique on the color, mechanical properties, and microstructure of the dried Saskatoon berries, and to develop the regression model using central composite rotate design (CCRD) and response surface methodology (RSM).

MATERIALS AND METHODS

Materials

Riverbend Plantation (Saskatoon, Saskatchewan, Canada) supplied the frozen Saskatoon berries in bulk. The frozen berries were kept in cold storage (–25°C) in the Riverbend Plantation. Prior to microwave-vacuum drying, the frozen berries were thawed using a pre-drier to equilibrate with ambient conditions and to remove the frozen surface water of the berries. The berries were spread on the drier surface in a single layer and thawed at approximately 40°C for 30 min. The berries were then placed in the perforated ultra high density polyethylene (UHDP) made basket to drain out the surface water of the berries.

Experimental Design of Vacuum Assisted Microwave Drying of Saskatoon Berries

A CCRD was used to study the effect of microwave-vacuum drying input variables on the color, mechanical, and microstructural properties of dried Saskatoon berries. The three input variables were microwave power (5.07–6.93 kW), drying time (46.5–63.5 min), and fruit load (9.60–10.40 kg). The input variables and their ranges were selected as per the work done by Mitra and Meda.[⁴] The experimental design consisted of a total of 20 experimental runs (n = 2^k + 2k + m, where, n = total experimental points, input variables, k = 3 and central point, m = 6), which included eight factorial points, six axial points, and six central points as shown in Table 1.

Table 1  Experimental design of CCRD for vacuum assisted microwave drying of Saskatoon berries (coded values are in parenthesis)

Run	Microwave power, P(kW)	Drying time, t (min)	Fruit weight, W (kg)
	X1	X2	X3
1	5.5 (–1)	50 (–1)	9.75 (–1)
2	5.5 (–1)	50 (–1)	10.25 (1)
3	5.5 (–1)	60 (1)	9.75 (–1)
4	5.5 (–1)	60 (1)	10.25 (1)
5	6.5 (1)	50 (–1)	9.75 (–1)
6	6.5 (1)	50 (–1)	10.25 (1)
7	6.5 (1)	60 (1)	9.75 (–1)
8	6.5 (1)	60 (1)	10.25 (1)
9	6.0 (0)	55 (0)	10 (0)
10	6.0 (0)	55 (0)	10 (0)
11	6.0 (0)	55 (0)	10 (0)
12	6.0 (0)	55 (0)	10 (0)
13	6.0 (0)	55 (0)	10 (0)
14	6.0 (0)	55 (0)	10 (0)
15	5.07 (–1.68)	55 (0)	10 (0)
16	6.93 (1.68)	55 (0)	10 (0)
17	6 (0)	46.5 (–1.68)	10 (0)
18	6 (0)	63.5 (1.68)	10 (0)
19	6 (0)	55 (0)	9.60 (–1.68)
20	6 (0)	55 (0)	10.40 (1.68)

X₁ = (P – 6)/0.5;

X₂ = (t – 55) /5;

X₃ = (W – 10)/0.25.

Vacuum Assisted Microwave Drying of Saskatoon Berries

Saskatoon berries were dried using a pilot scale vacuum assisted microwave drier as shown in Fig. 1 (Model MG8KW, Enwave Corporation, Vancouver, Canada) according to microwave-vacuum drying process followed by Mitra and Meda.[⁴] The vacuum pressure during microwave drying was maintained near to 100 kPa for all the drying conditions. The desired power level of microwave was set and supplied energy to the berries in the vacuum chamber to generate the drying process. The drying was continued to the desired treatment period. After completion of microwave-vacuum drying, the dried berries were taken out and cooled to room temperature (around 20°C). The dried samples were then kept in a refrigerator until analysis. In order to study the effect of different drying conditions, drying was performed as shown in Table 1.

Figure 1  A pilot scale Enwave vacuum assisted microwave (MW) drier used in this study.

Determination of Moisture Content of Dried Saskatoon Berries

Moisture content of dried Saskatoon berries was determined according to AOAC oven drying method.[¹⁸] The microwave-vacuum dried Saskatoon berry samples (10 g) were dried at 105°C for 24 h using a Fisher convection oven drier (Fisher Scientific Company, Ottawa, Canada). Three replications were performed for each sample. The moisture content of dried Saskatoon berries was expressed as g/100 g berry.

Determination of Color of Saskatoon Berries

The color of the frozen and dried Saskatoon berries was determined using a Hunterlab Color Analyzer (Hunter Associates Laboratory Inc., Reston, Virginia, USA). The Hunterlab Color Analyzer was calibrated against standard white and black ceramic surfaced tiles. Three replication measurements were taken for each sample. The color intensity of frozen and vacuum-assisted microwave dried Saskatoon berries was determined by measuring their respective L (lightness/darkness), a (redness/greenness), and b (yellowness/blueness) values.[¹⁶¹⁹] The color difference between frozen and microwave vacuum dried Saskatoon berries for each treatment was calculated as ΔL = L– L₀, Δa = a– a₀, Δb = b–b₀. L₀, a₀, and b₀ are denoted for the color of frozen Saskatoon berries (before drying) and L, a, and b are denoted for the color of dried Saskatoon berries (after drying). The total color difference (ΔE) was evaluated as ΔE = [(ΔL) ² + (Δa) ² + (Δb) ²]^1/2.

Determination of Mechanical Properties of Dried Saskatoon Berries

A Universal Texture Analyzer TA.XT2 (Texture Technologies Corp., NY, USA) with a load cell capacity of 25 kg was used to determine the mechanical properties of dried Saskatoon berries. A two-cycle texture analysis (TPA) compression test was used to determine hardness, springiness, cohesiveness, gumminess, and chewiness of the dried Saskatoon berries. Force and displacement data were acquired and analyzed using texture expert software (Stable Micro Systems Ltd., UK). The set compression test parameters for the Saskatoon berries were pretest speed (2 mm/s), test speed (1 mm/s), post-test speed (2 mm/s), and trigger force (0.05 N). An aluminum probe (cylindrical) with a diameter of 38.1 mm (1.5 in) was used to compress the samples on a table parallel to the probe’s compression surface. The samples were oriented on the table such that the stem-calyx plane was parallel to the compression probe surface. The experiment was conducted within the temperature range of 22–24°C and ten replications (ten berries) were conducted for each sample. The height of the force peak on the first compression cycle (maximum force, N) is considered as hardness. Springiness is the measure of the ability of the product to retain its original dimension after the removal of the applied load and is determined by the distance of the detected height (first compression cycle) of the product on the second compression divided by the original compression distance. The ratio of the positive force area under the second peak to the first peak was defined as cohesiveness. Gumminess is the product of hardness and cohesiveness (hardness × cohesiveness) and chewiness is calculated by multiplying gumminess by springiness (gumminess × springiness). Gumminess is the energy required to disintegrate a semisolid food to a state of readiness for swallowing and chewiness is the energy required to masticate a solid food product.[^20–22]

Microstructural Analysis of Frozen and Dried Saskatoon Berries

A scanning electron microscope (JEOL 840A, JEOL Ltd., Japan) was used to determine the effect of vacuum assisted microwave drying on the microstructural property of Saskatoon berries. The scanning electron microscopy (SEM) was performed at 20kV with a working distance of 48 mm for low magnification images (< 200×) and 15 to 25 mm for higher magnification images. Samples were coated with a conductive material (gold) to ensure sufficient electron refraction and the images were recorded digitally.

Statistical Analysis with Response Surface Modeling

A second order polynomial equation (1) was used to fit the coded variables:

(1)

where, Y represented the experimental response, B₀, B₁, B₂, B₃, B₁₁, B₂₂, B₃₃, B₁₂, B₂₃, and B₃₁ were constants and regression coefficients of the model, and X₁, X₂, and X₃ were the independent variables. The model included linear, quadratic, and interaction terms to determine the effect of process variables on the response. The RSM was used to solve the regression equation and to investigate the effects of three independent input variables (microwave power, drying time, and fruit load) on the response variables (color and mechanical properties of dried berries). Statistical analyses were conducted using a computer program of response surface regression (PROC RSREG) in SAS (version 9.3). The analysis of variance (ANOVA) was conducted and the model was justified with F value at (Pr > F) ≤ 0.05. The linear terms were considered for the model for all responses, but only the significant terms (coded value) of quadratic and interaction were included in the predictive model. The significant terms were justified with t value at (Pr > |t|) ≤ 0.05. The response surface model was generated by presenting the response as function of two factors and keeping the third constant.

Table 2  Experimental results of moisture content, color, and mechanical properties of vacuum assisted microwave dried Saskatoon berries

		Color of dried Saskatoon berries				Mechanical properties of dried Saskatoon berries
Run	Final moisture content (g/100 g dry fruit) of dried Saskatoon berries	L	a	b	ΔE	Hardness (N)	Springiness	Cohesiveness	Gumminess (N)	Chewiness (N-mm)
1	45.15 ± 1.50	12.93 ± 0.95	0.90 ± 0.03	−0.23 ± 0.01	3.61 ± 0.03	2.60 ± 0.71	0.71 ± 0.07	0.59 ± 0.05	1.52 ± 0.67	1.09 ± 0.50
2	50.44 ± 1.50	14.39 ± 0.23	3.37 ± 0.80	0.54 ± 0.15	4.13 ± 0.40	1.86 ± 0.70	0.66 ± 0.08	0.51 ± 0.06	0.94 ± 0.36	0.62 ± 0.20
3	33.09 ± 0.96	13.09 ± 1.36	0.85 ± 0.11	−0.23 ± 0.01	3.96 ± 0.01	7.8 ± 1.22	0.62 ± 0.06	0.45 ± 0.04	3.34 ± 0.80	2.17 ± 0.30
4	43.81 ± 1.29	12.88 ± 0.16	0.94 ± 0.20	−0.27 ± 0.02	3.52 ± 0.08	8.83 ± 1.25	0.60 ± 0.11	0.45 ± 0.08	3.90 ± 0.71	2.40 ± 0.25
5	33.81 ± 1.06	12.90 ± 0.31	1.05 ± 0.12	−0.15 ± 0.01	3.44 ± 0.55	6.53 ± 0.75	0.60 ± 0.07	0.45 ± 0.06	2.81 ± 0.40	1.76 ± 0.11
6	44.84 ± 1.22	13.98 ± 1.20	1.89 ± 0.21	0.09 ± 0.01	3.91 ± 0.30	2.33 ± 0.55	0.67 ± 0.08	0.60 ± 0.06	1.35 ± 0.56	0.93 ± 0.15
7	8.05 ± 0.64	11.67 ± 0.42	1.39 ± 0.25	0.09 ± 0.01	2.41 ± 0.41	58.57 ± 5.25	0.54 ± 0.05	0.33 ± 0.04	19.60 ± 1.99	10.47 ± 1.5
8	15.27 ± 1.10	14.35 ± 0.60	1.70 ± 0.35	0.11 ± 0.05	4.18 ± 0.56	55.17 ± 8.15	0.46 ± 0.02	0.15 ± 0.01	9.60 ± 1.50	3.75 ± 0.80
9	34.92 ± 0.76	12.02 ± 0.78	1.42 ± 0.16	0.11 ± 0.03	3.17 ± 0.15	8.51 ± 1.60	0.58 ± 0.08	0.45 ± 0.07	3.83 ± 0.84	2.23 ± 0.60
10	36.70 ± 1.10	14.00 ± 0.88	1.39 ± 0.26	0.14 ± 0.03	4.01 ± 0.80	6.81 ± 0.75	0.60 ± 0.08	0.48 ± 0.08	3.24 ± 0.95	1.94 ± 0.55
11	36.48 ± 1.23	13.37 ± 1.10	1.53 ± 0.33	0.14 ± 0.01	3.39 ± 0.88	7.27 ± 0.70	0.63 ± 0.05	0.47 ± 0.03	3.40 ± 0.70	2.13 ± 0.70
12	36.68 ± 1.37	13.28 ± 0.53	1.83 ± 0.40	0.21 ± 0.02	3.15 ± 0.25	8.79 ± 1.25	0.55 ± 0.04	0.42 ± 0.08	3.71 ± 0.80	2.04 ± 0.56
13	37.50 ± 1.95	12.46 ± 0.44	1.34 ± 0.21	0.22 ± 0.04	2.83 ± 0.03	8.94 ± 1.35	0.56 ± 0.08	0.44 ± 0.03	3.95 ± 0.84	2.27 ± 0.40
14	36.52 ± 0.80	11.66 ± 1.15	1.16 ± 0.15	−0.04 ± 0.02	2.63 ± 0.01	7.59 ± 1.25	0.60 ± 0.06	0.45 ± 0.03	3.39 ± 0.75	2.03 ± 0.25
15	53.17 ± 1.85	12.41 ± 0.98	1.79 ± 0.14	0.18 ± 0.05	2.53 ± 0.02	5.38 ± 1.22	0.55 ± 0.11	0.42 ± 0.08	2.28 ± 0.60	1.24 ± 0.30
16	22.92 ± 1.31	12.53 ± 1.07	0.96 ± 0.13	−0.11 ± 0.02	3.42 ± 0.42	17.17 ± 3.20	0.53 ± 0.05	0.40 ± 0.04	6.72 ± 1.50	3.67 ± 0.75
17	49.57 ± 0.73	13.49 ± 1.33	1.92 ± 0.55	0.19 ± 0.03	3.59 ± 0.45	5.07 ± 0.56	0.62 ± 0.08	0.48 ± 0.08	2.36 ± 0.50	1.51 ± 0.25
18	17.89 ± 1.56	11.25 ± 0.53	1.84 ± 0.60	0.25 ± 0.45	1.80 ± 0.16	29.12 ± 4.41	0.53 ± 0.07	0.36 ± 0.04	10.45 ± 1.75	5.65 ± 0.95
19	32.02 ± 0.45	12.20 ± 0.09	1.78 ± 0.22	0.23 ± 0.04	2.35 ± 0.30	17.81 ± 2.55	0.54 ± 0.05	0.41 ± 0.04	7.32 ± 0.95	3.91 ± 0.67
20	42.12 ± 0.78	11.77 ± 0.55	1.57 ± 0.44	0.14 ± 0.05	2.33 ± 0.12	8.23 ± 1.50	0.51 ± 0.11	0.40 ± 0.06	3.12 ± 1.40	1.70 ± 0.60

RESULTS AND DISCUSSION

Color of the Vacuum Assisted Microwave Dried Saskatoon Berries

The experimental results of L value, a value, b value and total color difference (ΔE) of dried samples are shown in Table 2. The regression models (coded value) of L value, a value, b value and total color difference of dried Saskatoon berries are shown below.

The ANOVA analysis showed that microwave power, drying time and fruit load affected the color of the dried Saskatoon berries significantly (p < 0.05). The response surface effect of drying time and fruit load keeping microwave power constant at the center point on the total color difference of microwave-vacuum dried Saskatoon berries is shown in Fig. 2. The color difference increased with the increased fruit load and decreased with drying time. The L value, a value, and b value of the frozen (control) Saskatoon berries were 10.64, 3.43, and 0.80, respectively. The overall observations from Table 2 showed that the vacuum-assisted microwave drying increased the L value for all the drying conditions. This phenomenon of increased lightness of the dried berries for all the drying conditions may occur due to leaching of pigments such as anthocyanins and phenolic compounds[²] from the berries during the drying process. Higher drying time (63.5 min) gives lower L value and lower drying time (50 min) gives higher L value. The longer period of thermal treatment during drying causes the color change of dried products.[¹⁴²³] The a value and b value decreased compared to control (frozen berries) for almost all drying conditions. There is a significant decrease (p < 0.05) of a value when microwave power increased from 5.07 to 6.93 kW and moisture content of the dried samples decreased from 53.17 to 22.92 g/100 g berry (Table 2 for moisture content) at the same operating conditions for 50 min drying time and 10 kg fruit load. It might happen due to higher power levels resulting in product charring.[²⁴] Similarly, the increase in drying time from 46.5 to 63.5 min and fruit load from 9.60 to 10.40 kg decreased a value of dried Saskatoon berries. This phenomenon elucidates that increase in microwave power, drying time, and fruit load decreases the redness of dried Saskatoon berries compared to control (frozen berries). The b value indicates yellowness (positive b value) or blueness (negative b value) of the products. A significant difference of b value (p < 0.05) between the frozen and dried samples was observed. The b value decreased for all the drying conditions of Saskatoon berries compared to the frozen sample. This indicates that the Saskatoon berries tend to be blue in color after drying. The browning reactions (enzymatic and non-enzymatic) that take place during drying of fruits and vegetables are related to the color changes of the products. The higher color difference of the dried products than the fresh products possesses higher browning.[¹⁰] The color difference of Saskatoon berries denotes how the drying treatments affected the apparent visual color of the dried Saskatoon berries. The lowest color difference (ΔE = 1.80) was achieved at a moisture content of 17.89 g/100 g berry while drying performed at a microwave power of 6.0 kW for 63.5 min and 10 kg fruit load . This indicated that the longer drying time generated smaller color difference.

Figure 2  Effect of drying processing variables (microwave power, drying time, and fruit load) on the response surface of total color difference, hardness, springiness, cohesiveness, gumminess, and chewiness of microwave-vaccum dried Saskatoon berries.

Mechanical Properties of Dried Saskatoon Berries

Hardness

The hardness values of dried Saskatoon berries for the different drying conditions are shown in Table 2. The regression model (coded value) of the hardness of dried berries is shown

The model showed that linear terms and interaction between microwave power and drying time affected the hardness significantly. The ANOVA analysis showed that the three input variables (microwave power, drying time, and fruit load) had a significant effect on the hardness of the dried berries at p < 0.001 with an R² of 0.89. The response surface effect of microwave power and drying time keeping fruit load constant at the center point on the hardness of dried Saskatoon berries is shown in Fig. 2. The response surface model showed that the microwave power and drying time had linear effect on the hardness. The increase of these two variables increased the hardness value of the dried berries while keeping the third variable (fruit load) constant. The relationship between drying time and fruit load while keeping microwave power constant showed a negative quadratic effect on the hardness of the dried berries. The increased drying time increased hardness and increased fruit load decreased hardness while keeping microwave power constant. The observations from Table 2 showed that the moisture content of dried Saskatoon berries varied from 50.44 to 8.05 g/100 g berry and the corresponding hardness of dried Saskatoon berries varied from 1.86 to 58.57 N. The highest hardness (58.57 N) with the lowest moisture content (8.05 g/100 g berry) was found when drying was operated at 6.5 kW microwave power for 60 min with a fruit load of 9.75 kg. The higher microwave power (6.5 kW) causes a higher product temperature.[²⁵] The higher product temperature for a longer drying time (63.5 min) increased the water removal from the berries by increasing drying rate. The higher amounts of water removal from Saskatoon berries during drying led to increase the compaction force between cells of the dried berries causing the higher hardness of the dried berries. The lower drying time (46.5 min) showed a direct impact to reduce the hardness of dried Saskatoon berries achieving a high moisture content (49.57 g/100 g berry) of dried Saskatoon berries. The increment of fruit load from 9.75 to 10.25 kg while keeping constant other two processing variables decreased the hardness of dried Saskatoon berries. The results indicated that the higher drying time and the higher microwave power provided higher hardness of dried Saskatoon berries and the lower fruit load increased hardness of dried Saskatoon berries.

Springiness

The regression model (coded value) of the springiness of the dried Saskatoon berries is shown below.

The model showed that only linear terms of the second order polynomial equation affected the springiness significantly. The ANOVA analysis indicated that the value of springiness of dried berries was affected by all the three drying processing variables (microwave power, drying time, and fruit load) significantly (p < 0.05). The response surface effect of drying time and fruit load keeping microwave power constant at the center point on the springiness of dried Saskatoon berries is shown in Fig. 2. The drying time had negative linear effect and fruit load had positive linear effect on the springiness of dried Saskatoon berries while keeping the microwave power constant. The response surface models also indicated that the increased microwave power and drying time decreased springiness and the increased fruit load increased springiness of dried Saskatoon berries. This trend was opposite of the effect of microwave-vacuum drying parameters on the hardness of dried Saskatoon berries. The experimental results of springiness of the dried samples for the different drying conditions are shown in Table 2. The experimental results showed that the berries dried (45.15 g/100 g berry of moisture content) at lower microwave power (5.5 kW) and lower drying time (50 min) with a fruit load of 9.75 kg possessed the highest springiness (0.71). The berries dried (15.27 g/100 g berry of moisture content) at higher microwave power (6.5 kW) and higher drying time (60 min) with a fruit load of 10.25 kg had the lowest springiness (0.46). The results indicated that the springiness of dried Saskatoon berries decreased when hardness of dried Saskatoon berries increased. Springiness is the rate at which deformed food materials return to their original state after the deforming force is withdrawn and the springiness indirectly represents elasticity.[²⁰] Therefore, the increase in hardness of dried Saskatoon berries leads to decrease in springiness or elasticity of dried Saskatoon berries.[²²]

Cohesiveness

The regression model (coded value) of the cohesiveness of dried berries is given below.

The regression model showed that only the linear terms represented the model of the cohesiveness of dried Saskatoon berries. The ANOVA analysis indicated that the input variables (microwave power, drying time, and fruit load) affected the cohesiveness of dried Saskatoon berries significantly at p < 0.05. The response surface effect of microwave power and drying time keeping fruit load constant at the center point on the cohesiveness of dried Saskatoon berries is shown in Fig. 2. The response surface model showed that microwave power and drying time had linear negative effect on the cohesiveness of dried Saskatoon berries while fruit load was kept constant. The response surfaces also showed that the fruit load had quadratic effect and microwave power had almost negative linear effect on the cohesiveness of dried Saskatoon berries while drying time was kept constant at the center point. The experimental results of cohesiveness of dried Saskatoon berries are presented in Table 2. The results showed that the cohesiveness of dried Saskatoon berries varied from 0.15 to 0.60 and the corresponding moisture content of dried Saskatoon berries varied from 15.27 to 44.84 g/100 g berry. The results showed that the moisture content of dried Saskatoon berries is inversely related to the cohesiveness of dried Saskatoon berries. The interactions between the drying processing variables affected the cohesiveness of dried Saskatoon berries. The higher microwave power (6.5 kW), higher drying time (63.5 min), and lower fruit load decreased cohesiveness and moisture content of dried Saskatoon berries. The cohesiveness is a measure of the strength of the internal bonds of the food materials. The higher microwave power and higher drying time with lower fruit load produce higher energy and the higher energy creates the de-bonding of the dried fruit materials resulting in lower cohesiveness. This may be also due to the alteration of cell membrane in the structure of dried Saskatoon berries during heating, causing the pectin substances to redistribute.[²⁶]

Gumminess

The regression model (coded value) of gumminess of the dried berries is given below.

The model of gumminess of dried Saskatoon berries showed that the linear terms, quadratic effect of drying time, and the interactions (between microwave power and drying time and between microwave power and fruit load) had significant effect on the gumminess of the dried Saskatoon berries. The ANOVA analysis showed that the three input variables of microwave power, drying time and fruit load had significant effect on the gumminess of the dried berries at p < 0.001 with an R² of 0.92. The response surface effect of drying time and fruit load keeping microwave power constant at the center point on the gumminess of dried Saskatoon berries is shown in Fig. 2. The response surface model showed that the drying time had positive linear effect and fruit load had negative linear effect on the gumminess of dried Saskatoon berries while keeping microwave power constant. Similarly, microwave power had positive effect on the gumminess of dried Saskatoon berries while keeping drying time constant. The experimental results of gumminess of different dried berry samples are shown in Table 2. The experimental results showed that the gumminess of dried Saskatoon berries ranged from 0.94 to 19.60 N and the corresponding moisture content of dried Saskatoon berries varied from 50.44 to 8.05 g/100 g berry. The Saskatoon berries dried (8.05 g/100 g berry moisture content) at 6.5 kW microwave power for 60 min with 9.75 kg fruit load possessed the highest gumminess (19.60 N) among the dried Saskatoon berry samples. The increment of fruit load from 9.75 to 10.25 kg reduced the gumminess of dried Saskatoon berries from 19.60 to 9.60 N while keeping microwave power (6.5 kW) and drying time (60 min) constant. Overall observations showed that the higher microwave power and higher drying time contributed higher gumminess of dried Saskatoon berries. The higher fruit load contributed lower gumminess of the dried Saskatoon berries. The results also indicated that the trend of the effects of microwave-vacuum drying parameters (microwave power, drying time, and fruit load) on the gumminess of dried Saskatoon berries was similar to the effects of microwave-vacuum drying parameters on the hardness of dried Saskatoon berries. The gumminess (hardness × cohesiveness) is a function of hardness and this property of dried Saskatoon berries is correlated to hardness of dried Saskatoon berries. Bourne[²⁰] found a close correlation between gumminess and hardness of the food products, which was in agreement with the present study.

Chewiness

The chewiness is the energy required to masticate a solid food and affects the oral cavity mouthfeel.[²⁶] Hence, the chewiness of dried Saskatoon berries is one of the major considerations to consumers. The regression model (coded value) of the chewiness of the dried berries is shown below.

The model of chewiness of dried Saskatoon berries showed that the linear terms and the interactions (between microwave power and drying time and between microwave power and fruit load) had significant effect on the chewiness of the dried Saskatoon berries. The ANOVA analysis showed that the three input variables (microwave power, drying time, and fruit load) had significant effect on the chewiness of the dried berries at p < 0.001 with an R² of 0.91. The response surface effect of microwave power and drying time keeping fruit load constant at the center point on the chewiness of dried Saskatoon berries is shown in Fig. 2. The microwave power and drying time had positive linear effect on the chewiness of dried Saskatoon berries while keeping third variable fruit load constant at the center point. Similarly, the fruit load had negative linear effect on the chewiness of dried Saskatoon berries while keeping drying time constant at the center point. The experimental results of the chewiness of dried Saskatoon berries are presented in Table 2. The chewiness of the dried berry samples ranged from 0.62 to 10.47 N-mm and the corresponding moisture content of dried Saskatoon berries varied from 50.44 to 8.05 g/100 g berry. The maximum chewiness was observed while samples dried at 6.5 kW microwave power for 60 min with a fruit load of 9.75 kg. But the chewiness decreased significantly when fruit load was increased from 9.75 to 10.25 kg keeping other two variables (microwave power and drying time) constant. It has been observed that the chewiness of dried Saskatoon berries increased while microwave power increased from 5.07 to 6.93 kW and drying time increased from 45.5 to 63.5 min keeping the fruit load constant. The overall observations of the results of chewiness of dried Saskatoon berries showed that the chewiness of the dried berries increased with microwave power and drying time and, was inversely related to fruit load. The results also showed that the drying effects on chewiness of dried Saskatoon berries was similar to drying effects on hardness of dried Saskatoon berries showing a relationship between chewiness and hardness of dried Saskatoon berries. This result is in agreement with that reported by Bourne[²⁰] who observed that a correlation between chewiness and hardness of the food products.

Microstructural Analysis of Saskatoon Berries

Initial moisture content of frozen Saskatoon berries was around 84%. The most microwave-vacuum dried (8.05 g/100 g berry moisture content of dried berries) sample while drying at microwave power of 6.5 kW for 60 min with a fruit load of 9.75 kg (drying condition 7, Table 1) and the least microwave-vacuum dried (53.17 g/100 g berry moisture content of dried berries) sample while drying at microwave power of 5.07 kW for 55 min with a fruit load of 10 kg (drying condition 15, Table 1) and their respective frozen berry sample were considered to analyze the microstructural features using SEM compared to the structural changes of their respective frozen Saskatoon berry samples. The SEM images of frozen and dried berry samples are shown in Fig. 3. It has been observed that the interior pore spaces of the frozen samples (control) shown in Fig. 3a and 3c are smaller as well as tighter packing and stronger connections between cells compared to dried samples. The microwave-vacuum drying of Saskatoon berries changes the internal microstructure of dried Saskatoon berries considerably by increasing the porosity causes a higher level of destruction in its microstructure. In terms of level of destruction in microstructure of dried Saskatoon berries, there is a significant difference between drying condition 7 and drying condition 15 as shown in Fig. 3b and 3d. It also showed that the higher microwave power (6.5 kW) resulted in higher level of microstructure destruction than the lower microwave power (5.07 kW) applied for drying of Saskatoon berries. The higher microwave power generates massive and fast vaporization during microwave-vacuum drying and vapor bubbles can increase total pressure gradient inside Saskatoon berries resulting in increased porosity of dried Saskatoon berries.[²⁷] The porosity of the mass of the berries indicates the resistance to airflow at the time of aeration and drying process. The massive and fast evaporation of water in cells collapse the internal cell structures and weaken the connections between cells causing destruction in the microstructure of dried Saskatoon berries.[¹³]

Figure 3  SEM images of internal micro-structures of control (frozen) and vaccum-assisted microwave dried Saskatoon berries: (a) drying condition 7 (control); (b) drying condition 7 (most dried); (c) drying condition 15 (control); (d) drying condition 15 (least dried).

CONCLUSIONS

RSM combined with CCRD analysis showed that all three drying processing variables (microwave power, drying time, and fruit load) had significant (p < 0.05) effects on color (L value, a value, b value and total color difference), mechanical properties (hardness, springiness, cohesiveness, gumminess, and chewiness), and microstructural changes of dried Saskatoon berries. The regression and response surface analysis within the experimental range generated by RSM combined with CCRD modeled response variables (color and mechanical properties) effectively as a function of the independent variables (microwave power, drying time, and fruit load). The moisture content of dried Saskatoon berry samples varied from 53.17 to 8.05 g/100 g berry, whereas, the frozen Saskatoon berries contained 84 g /100 g berry moisture content. The vacuum assisted microwave drying increased the lightness of dried Saskatoon berries increasing L value for all drying conditions compared to control (frozen berries). However, higher drying time decreased the L value of dried Saskatoon berries. In comparison with control (frozen berries), the redness and yellowness of all dried Saskatoon berry samples decreased because of decreasing a value and b value of all dried Saskatoon berry samples. Higher microwave power (i.e., 6.5 kW), higher drying time (i.e., 63.5 min and lower fruit load (i.e., 9.75 kg) increased hardness, gumminess, and chewiness of dried Saskatoon berries. Springiness and cohesiveness of dried Saskatoon berries decreased with the lower microwave power, lower drying time, and higher fruit load. The higher microwave power increased the porosity of dried Saskatoon berries resulting in the higher level of microstructure destruction than the lower microwave power applied for drying of Saskatoon berries. In order to understand the effects of the microwave-vacuum drying processing variables (microwave power, drying time, and fruit load) on the color, mechanical properties, and microstructural property are deemed valuable for the commercial dried Saskatoon berry products development using microwave-vacuum drying technology.

ACKNOWLEDGMENTS

The authors would like to thank Mrs. Grace Whittington, owner of the Riverbend Plantation for providing Saskatoon berries and drying facilities.

FUNDING

The authors gratefully acknowledge the Agricultural Development Fund (ADF), SK, Canada for financial support.