Effect of microwave treatments on the quality of a smoothie

M. Arjmandia, M. Otónb, F. Artés-Hernándeza,b, P. A. Gómezb, F. Artésa,b, and E. Aguayoa,b

aPostharvest and Refrigeration Group. Universidad Politécnica de Cartagena (UPCT). Paseo Alfonso XIII, 48. 30203 Cartagena, Murcia, Spain.

bQuality and Health Unit. Instituto de Biotecnología Vegetal (UPCT).

Keywords:pasteurization, microbial quality, enzyme activity, vegetable beverages.

INTRODUCTION

Fruit and vegetables are a rich natural source of many antioxidants including carotenoids, flavonoids,phenolic compounds and vitaminsthat provide protection against harmful free radicals. Antioxidants block the oxidation processes by neutralizing free radicals and reducing the risk of certain types of cancer and other diseases (Azizah et al. 2009). At present, consumers demand the best preservation of the sensory, nutritional and health-related characteristics of plant-derived food products. Thermal processing is necessary for destroying harmful pathogenic microorganisms and inactivation of endogenous enzymes. Conventional thermal processing generally induces detrimental changes, lowering the quality attributes of products, especially nutritional value and sensory properties in terms of color and flavor (Math et al. 2014).Microwave pasteurization (MWP) heat treatments have gained attention as an alternative to conventional pasteurization (CP) of liquid foods, such as milk and fruit juices because it is a fast heating method and also it has been accepted to reduce the adverse thermal degradation in food quality while ensuring food safety (Clare et al. 2005). The objective of the present work was to compare the effects of MWP and CP on the chemical, functional, microbial and sensorial parameters of a fresh orange smoothie.

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MATERIALSANDMETHODS

Sample preparation

After several preliminary compositional and sensory tests, an orange-colored smoothie was prepared with 126 g of tomato, 61 g of carrot, 29 g of pumpkin, 4 mL lemon juice ( to reach a pH of 4.45), 50 mL mineral water and 0.3 g marine salt. All ingredients were blended for 3 min in a thermomix (Vorwerk elektrowerke, Model TM 31-1, France).

Treatments

For MWP a semi-industrial prototype of continuous MW oven (Sairem Iberica S.L. SI-MAQ0101, Barcelona, Spain) was used for the current experiments. 600 mL smoothies were treated under low power/long time (210 for 646 s) and high power/short time (3,600 W for 93 s). For CP, samples (600 mL) were heated in the thermomix. In both MWP and CP treatments, the temperature reached in the samples was 90 ± 2 °C for 35 s.

Sensory evaluation

Sensory analyses were performed according to international standards (ASTM STP 913 1986). Appearance, flavor and overall quality of smoothies were evaluated by a twelve- persons (aged 24-68) panelat room temperature (20 °C). They completed a rating sheet based on a nine-point scale where 1 = unacceptable, 3 = fair, low quality, 5 = moderate, 7 = high, 9 = very high quality.

Microbial analysis

To determine mesophilic bacteria, molds andyeasts 10 g of smoothie werehomogenized for 1 min in 90 mL of sterile peptone-buffered water (Scharlau, Barcelona, Spain) in a sterile stomacher bag (Colorworth Stomacher 400, Steward Laboratory, London, UK). For Listeria monocytogenes, 25 g of samples were put into 225mL Fraster Listeria Broth Base (Cultimed, Panreac, Barcelona, Spain) and incubated at 37 °C for 24 h. Itwas determined using the spread plate method with Listeria Oxford Selective Supplement (Scharlau Scharlab S.L. Barcelona, Spain) added to Oxford Agar Base (Scharlau Scharlab S.L. Barcelona, Spain)according to Regulation EC. Microbial counts were reported as log10 colony forming units per gram of smoothies (log CFU g-1).

Physical quality analysis

The color of the samples was monitored by a photo-colorimeter (Minolta CR-300, Ramsey, NJ, USA). Color was expressed as Hunter L*, a*, b*and hue angle (h⁰ = tan-1b*/a*).

Chemical quality attributes

Total soluble solids (TSS) of smoothies were determined using a digital refractometer (Atago, Tokyo, Japan) and expressed as °Brix. The pH of samples was measured with a pH-meter calibrated with phosphate buffers, pH 4 and 7 (Crison 2001 pH-meter, Crison Instruments S.A., Barcelona, Spain).Titratable acidity (TA) was performed by titrating 5 mL of homogenized sample with NaOH (0.1 N) to an end point of pH 8.1 (716 DMS Titrino, Metrohm, Herisau, Switzerland) (AOAC, 1984).

Total phenolic compounds (TPC) and total antioxidant activity (TAC)

TPC and TAC using Ferric Reducing Antioxidant Power (FRAP) reagent were measured following Arjmandi et al. (2016a). TPC was expressed as mg chlorogenic acid equivalents (ChAE) kg-1 fresh weight (FW). Total antioxidant capacity results were expressed as mg ascorbic acid equivalent (AAE) kg-1 (FW).

Vitamin C ontent

The ascorbic acid (AA) determination was performed as described by Arjmandi et al. (2016a).Vitamin C was quantified through a calibration curve made with AA standards and results were expressed as mg (AA) kg-1 FW.

Carotenoids

Carotenoids were measured according to the method of Nagata and Yamashita(1992)with slight modifications. Results were expressed as mg lycopene or -carotene kg-1 FW.

Enzymes analysis

Peroxidase (POD),pectin methylesterase (PME),and polygalacturonase (PG), were determined following Arjmandi et al. (2016b). For all analysis, each of the three replicates was analyzed by triplicate.

Statistical analysis

Data were subjected to one-way analysis of variance (p≤ 0.05) using Statgraphic Plus 5.1, Manugistic Inc, Rockville, MD, USA). Mean values were compared by multiple range least significant difference (LSD) test to identify significant differences among treatments.

RESULTSANDDISCUSSION

Regarding sensory evaluation, among the applied heating treatments, a similar appearance score was given to them (8.3 to 7.7). In all heated samples, overall quality scores were above the limit of acceptability for consumption.

The unheated smoothies showed a high mesophilic bacterial load (5.1 log CFU g-1) that was significantly reduced when any of the heat treatments were applied (< 1 log CFU g-1). Yeast growth was also reduced from 2.5 log CFU g-1 in control samples to < 2 log CFU g-1 in heated samples. The initial mold load was under the detection limit (< 2 log CFU g-1).Any pathogenic bacterial was detected in any samples at any time. Using MWP could reduce heating time as compared to conventional-heating methods (Robinson et al. 2009), and in this case, MWP was more efficient than CP for reducing microbial counts in smoothies. These results are in agreement with those of Picouet el al. (2009), who reported that the MW processing of apple products (652 W/35 s) reduced pathogenic microorganisms, while maintaining the nutritional and sensorial attributes, and could be used to increase the competitiveness of the fruit sector.

The effects of thermal treatment on smoothies color are shown in Table 1.Color evaluation ofMWP-treated samples showed a slightly higher increase of L* as compared to the CP and untreated smoothies (P < 0.05). The highest L* value (42.89 ± 0.17) was found in MWP samples treated with the high power/short time (3,600 W, 93 s). According to the results displayed in Table 1, the h° value of the smoothie increased after thermal treatment in comparison to untreated samples. This level was similar to the untreated samples when a combination of highest power/shortest time (3,600 W, 93 s) was used.Schiffmann (2001) reported that heat transfersrapidly in MWP, for that reason this technique can be used to better preserve the color of smoothies compared to CP, and this could be an advantage for this kind of product.

Table1. Effects of microwave (MWP) and conventional treatment (CP) onHue angle (h°) and lightness (L*) of smoothie

Treatment / Color parameters
Hue angle (h°) / Lightness (L*)
Non-treated / 50.36 ± 0.70*d / 39.26 ± 0.23*d
CP / 53.29 ± 0.70a / 40.29 ± 0.15c
MWP: 210W-646s / 52.33 ± 0.26b / 40.86 ± 0.09b
MWP: 3600W-93s / 51.58 ± 0.47c / 42.89 ± 0.17a

*Values are mean ± standard error (n=3). Different letters in the same row indicate significant differences(P0.05).

TSS in unheated smoothie was 5.10 °Brix and this value slightly increased to 5.13 in thermally treated samples (data not shown). No significant differences were found by type of thermal treatment. The initial pH value (4.24) was not significantly affected by heat treatments. TA was quite stable without clear differences among the differently treated samples and storage time, with 0.46% and 0.44% citric acid in unheated and heated samples, respectively.

The initial TPC of fresh smoothies was 41.36 ± 0.11 mg ChAE 100 mL-1. Immediately after pasteurization, the maximum TPC value was reached in MWP smoothies without differences between MW treatments(data not shown) This could be attributed to disruptions of the cell wall by thermal processing (Martínez-Hernández et al. 2013), with it being higher when using high-power MW such as 3,600 W compared to low MW such as 260 W or CP. Additionally, thermal processing is able to inactivate the polyphenol oxidase enzyme, thus preventing polyphenol degradation (Chuah et al. 2008).

The TAC value in unheated smoothie was 72.86 ± 0.38 mg AAE 100 mL-1 and it decreased after both heating techniques, although MWP-treated smoothies maintained higher levels (Table 2). Podşedek (2003) reported that antioxidant levels of conventionally-heated vegetables were lower than the corresponding fresh samples. In this study, after CP, the TAC decreased to 72% from its initial value as compared to unheated samples, whereas the MW treatment using low power/long time (210W/646 s) and high power/short time (3,600 W/93 s), retained 84 and 95%, respectively.

Vitamin C in unheated smoothie was 11.72 ± 0.02 mg 100 mL-1 (Table 2). Changes in vitamin C content of the thermal treatments were statistically different (p0.05). Smoothies treated under CP obtained 9.5 ± 0.08 mg 100 mL-1 of vitamin C (a reduction of 19%). In contrast, smoothies under high power/short time MWP had the lower reduction of vitamin C content (4%) compared to CP.Generally, the current results showed that thermal processing led to degradation of vitamin C due to oxidative processes. These results are in agreement with findings by Leoni (2002) who found that vitamin C is a heat-sensitive compound in the presence of oxygen. In this case the use of high power/short time MWP treatments could be used as a tool to help keep the qualitative factors of refrigerated smoothies.

The initial lycopene content in fresh smoothie was 10.78 ± 0.02 mg L-1 (Table 2), with this amount increasing slightly after all the different heat treatments, in particular under high power/short time MW treatment (12.93 ± 0.09 mg L-1). The heat processing could lead to a more efficient extraction of lycopene from the matrix by breaking down cell walls, making it more accessible (Azizah et al. 2009).

The β-carotene content also increased after both methods of heat treatment, but no significant differences among them were found. The initial amount of β-carotenes in unheated smoothie was 5.89 ± 0.07 mg 100 L-1. This amount was incremented by all heat treatment methods and achieved the maximum value in the combination of high power/short duration of MWP (6.88 ± 0.09 mg 100 L-1). These results confirm those found by Stahl and Sies (1992) and Azizah et al. (2009) who reported that heating treatments enhanced lycopene and β-carotene content in cooked tomato, carrot, spinach and pumpkin as compared to fresh products. This enhancement could be attributed to cell membrane and wall disruption produced by thermal processing, making β-carotene more accessible for extraction (Van het Hof et al. 2000).

Table2. Effects of microwave (MWP) and conventional pasteurization (CP) onantioxidant capacity (mg AAE 100 mL-1), vitamin C (mg100 mL-1) and lycopene (mg L-1) of smoothie.

Treatment
TAC / Vitamin C / Lycopene
Non-treated / 72.86 ± 0.38*a / 11.72 ± 0.02*a / 10.78±0.02c
CP / 52.64 ± 0.14d / 9.50± 0.08d / 11.57±0.11b
MWP: 210W-646s / 61.69 ± 0.57c / 10.92 ± 0.02c / 12.46± 0.47 a
MWP: 3600W-93s / 69.50 ± 0.40b / 11.26 ±0.05b / 12.93 ± 0.09a

*Values are mean ± standard error (n=3). Different letters in the same row indicate significant differences (P0.05).

Results indicate that both heating treatments were able to decrease the POD activity in smoothies (Table 3). After treatment, MWP treatments at high power/short time diminished POD activity in the smoothies up to 96%.In comparison, the CP and low power/long time treatment reduced the initial POD activity by 70 and 91%, respectively in the smoothie.Similar results have been reported in the MW inactivation of POD of green coconut water (Matsui et al., 2008).

After MWP and CP treatments, PME activity was significantly reduced (p < 0.05). The statistical analysis showed a significant effect (p < 0.05) of MW doses on the residual enzymatic activity; the maximum inactivation (92%) was reached using the high power and short time (3,600 W/93 s), in comparison with the 81% PME reduction using 210 W/246 s and 73% for CP (Table 3). These results confirm those reported by Tajchakavit and Ramaswamy(1997) who reported that at the same temperature (60 °C), MW (7.37 s) heating largely enhanced PME inactivation as compared to CP (154 s) in orange juice.

Similarly to the other two enzymes, significant differences (p < 0.05) among treatments were found (Table 3). MWP with high power/short time (3,600 W/93 s), provided the best enzyme activity reduction (50%). In CP, the enzyme’s residual activity was only reduced to 69%. Our results are similar to those reported by Aguiló-Aguayo et al. (2008), who found that the residual PG activity decreased after a conventional heat treatment, finally reaching 78% and 56%, in tomato juice heated at 90 °C for 30 and 60 s, respectively. Nevertheless, pectin compounds are also broken by the combined action of PG and PME, therefore a reduction of PME activity leads to a decrease of PG action (Gross, 1982; Aguiló-Aguayo et al., 2009).

Table3. Residual activity (RA%) of peroxidase (POD), pectin methylesterase (PME), and polygalacturonase (PG) in unheated (control), conventional (CP) and microwave (MWP) pasteurized smoothie.

Treatment
POD / PME / PG
Non-treated / 100.00 ± 0.00*a / 100.87± 0.00a / 100.00±0.02a
CP / 29.79± 0.05b / 26.74 ± 0.01b / 69.28±0.1b
MWP: 210W-646s / 8.51 ± 0.05c / 18.67 ± 0.02c / 57.48± 005c
MWP: 3600W-93s / 4.14± 0.06d / 7.89 ± 0.05d / 50.33 ± 0.09d

*Values are mean ± standard error (n=3). Different letters in the same column indicate significant differences among mean values of different treatments (p < 0.05). Low MWP: Microwave pasteurization at low power and long time. Medium MWP: Microwave pasteurization at medium power and medium time. High MWP: Microwave pasteurization at high power and short time.

CONCLUSIONS

Thefollowingconclusionscanbedrawnfromthestudy:

-Based on the overall sensory and microbial quality, MWP particularly with high power/short time is able to keep better the quality of smoothie compared to CP and low power/long time.

-Continuous MWP (high power/short time) could reduce food processing time, increasing the industry’s efficiency and leading to better color retention of final product.

-MWP treatments, in particular the treatments withhighest power/shortest time, provided the best levels of antioxidant capacity and vitamin C and increased total phenolic compounds (TPC) and lycopene.

-A lower residual enzyme activity from POD, PME and PG was obtained under MWP heating, specifically when using a higher power/short time.

ACKNOWLEDGEMENTS

This work was financially supported by MINECO-FEDER (AGL2013-48830-C2-1-R). Thanks are due to Instituto de Biotecnología Vegetal (IBV-UPCT) for providing some of the equipment.

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