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Keywords:

  • dose uniformity ratio;
  • dosimetry;
  • quality;
  • spoilage

Abstract

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Combination Treatment: VI and Irradiation at 1 kGy
  7. Effect on Microbial Quality
  8. Conclusions
  9. References

This study assessed the application of an antibrowning solution using vacuum impregnation (VI) and then electron-beam irradiation as a means to extend the shelf life of sliced white button mushrooms (Agaricus bisporus). A preliminary study helped to determine the best antibrowning solution and VI process parameters. Mushroom slices were impregnated with 2 g/100 g ascorbic acid + 1 g/100 g calcium lactate; 2 g/100 g citric acid + 1 g/100 g calcium lactate; 1 g/100 g chitosan + 1 g/100 g calcium lactate; and 1 g/100 g calcium lactate at different vacuum pressures and times and atmospheric restoration times. Selection of the antibrowning solution and VI parameters was based on texture and color of the mushroom slices. Next, the slices were irradiated at 1 kGy using a 1.35-MeV e-beam accelerator. Physicochemical, sensory, and microbial quality of mushrooms was monitored for 15 d at 4 °C. The best impregnation process in this study was 2 g/100 g ascorbic acid and 1 g/100 g calcium lactate at 50 mm Hg for 5 min and an atmospheric restoration time of 5 min. The control (untreated) samples suffered structural losses throughout storage. Only the vacuum impregnated-irradiated samples had acceptable color by the end of storage. Sensory panelists consistently preferred the samples produced with VI and irradiation because exposure to ionizing radiation inhibited growth of spoilage microorganisms.

Practical Application

Extending the shelf life of mushrooms is important for their marketing and distribution and reliable preservation methods are still needed. Treating sliced mushrooms with vacuum impregnation and electron-beam irradiation introduces physiologically active components such as beneficial antioxidants, vitamins, and cations, while assuring safety and maintaining quality.


Introduction

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Combination Treatment: VI and Irradiation at 1 kGy
  7. Effect on Microbial Quality
  8. Conclusions
  9. References

White button mushrooms have an impact on treatment and prevention of breast cancer, mainly in postmenopausal women; they seem to inhibit the activity of aromatase, an enzyme involved in estrogen production (Teichmann and others 2007; Wani and others 2009).

The mushroom industry presents about 5% to 25% of its fresh produce as slices and consumer demand for ready-to-use vegetables has increased the market for sliced mushrooms (Brennan and Gormley 1998). However, it is challenging to maintain their quality because of the lack of cuticle to protect them. Their larger surface area broadens the spoilage problems. Some critical changes include browning and softening (Brennan and Gormley 1998; Sommer and others 2010).

Preservation of mushrooms has been attempted using packaging (Lopez-Briones and others 1992), chemicals (Sapers and others 2001), washing (Cliffe-Byrnes and O'Beirne 2008), tyrosine inhibitors (Singh and others 2010), ozone (Yuk and others 2007), and irradiation (Roy and Bahl 1984; Beaulieu and others 2002). However, these techniques have associated drawbacks including safety, discoloration, and off-flavors production, and may be unsuitable for use on an industrial scale (Duan and others 2010).

In the last decades, vacuum impregnation (VI) has been used as a means to introduce liquids into porous foods. This technique alters the product's composition, physical, and chemical properties to improve its nutritional value (Fito and others 2001; Hironaka and others 2011). Furthermore, VI of a coating solution may be an alternative to improve the dispersion retention and to form a thicker, more effective coating. Vargas and others (2009) observed that chitosan-based edible coatings applied to fresh-cut carrots by VI enhanced all the positive effects of the coating.

Sometimes, effective preservation of fresh-cut products may be achieved using a combination of several treatments (Garcia and Barrett 2002). Irradiation using gamma rays and electron beams maintains the produce quality and enhances shelf life in 2 ways: it kills spoilage organisms and retards plant ripening (Moreira and Castell-Perez 2012; Mami and others 2013). The recommended dose for enhancing the shelf life of mushrooms in different countries ranges from 1 to 3 kGy (Akram and Kwon 2010). However, irradiation at doses greater than 1 kGy can induce negative quality effects such as texture loss due to tissue softening (Koorapati and others 2004; Niemira and Fan 2009; Moreira and Castell-Perez 2012). Thus, the need to explore the synergistic effect of VI and irradiation in extending the shelf life of fresh-cut mushrooms.

The objectives of this study were to (1) determine the best VI setup in terms of mushroom color and texture, and (2) evaluate the effect of VI and e-beam irradiation on the physicochemical, microbiological, and sensory quality of fresh-sliced mushrooms.

Materials and Methods

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Combination Treatment: VI and Irradiation at 1 kGy
  7. Effect on Microbial Quality
  8. Conclusions
  9. References

Experimental design

In a preliminary study, mushroom slices were vacuum impregnated (VI) with several solutions. During storage at 4 °C, color and texture were monitored and the best conditions (impregnation solution, vacuum pressure and duration, and atmospheric restoration time) determined. The best conditions were then used to design the next set of experiments, which consisted of 1 set with 3 experiments: (1) irradiation, (2) VI, and (3) VI plus irradiation (see Irradiation experiment section).

Four groups of samples were used for product quality and shelf life analyses: (1) control (fresh samples without any treatment), (2) impregnated, (3) irradiated, and (4) impregnated-irradiated slices. The experimental design was a 4 × 4 × 2 experiment consisting of 3 factors: impregnating solution (4 different solutions), vacuum pressure (50, 75, 100, and 125 mm Hg), pressure duration (5 and 10 min), and atmospheric pressure restoration (5 and 10 min). The experiment was conducted in duplicate.

Sample preparation

Locally grown button mushrooms (Agaricus bisporus) were stored at 10 °C and 95% relative humidity in plastic bags, washed under tap water, dried with absorbent paper, and sliced (Farberware, Hillsboro, Tex., U.S.A.). The head sides and long stems where discarded. The equipment (slicer, knife, beakers, and strainers) was sanitized using a 300 μL/L chlorine solution. Slices thickness was 6.5 ± 0.3 mm with a 3 to 5 mm cap length.

Preparation of impregnation (antibrowning) solutions

Antibrowning solutions

To prepare the chitosan solution, 0.5 g/100 g acetic acid (Glacial, Mallinckrotd Baker Inc., Paris, Ky., U.S.A.) and 1 g/100 g calcium lactate pentahydrate (Sigma-Aldrich, St. Louis, Mo., U.S.A.) was dissolved in distilled water at room temperature. Next, chitosan (medium molecular weight, Sigma-Aldrich) was added at 1 g/100 g concentration (Hernandez-Munoz and others 2006). Ascorbic acid (L-Ascorbic acid, Sigma-Aldrich) and citric acid (Citric acid Anhydrous, Fisher Scientific, Fair Lawn, N.J., U.S.A.) solutions were prepared by dissolving 2 g/100 g of each acidulant and 1 g/100g calcium lactate pentahydrate (Sigma-Aldrich) in distilled water at room temperature. Similarly, 1 g/100g of calcium lactate pentahydrate was dissolved in distilled water at room temperature.

Preliminary study

The best impregnation solution, time, and pressure combinations were determined by monitoring color and texture during 15 d at 4 °C.

Impregnation procedure

A VI system composed of a vacuum pump (Emerson Motor Div., St. Louis) and a vacuum glass desiccator (Pyrex®, Brazil) was used. Sliced mushrooms were immersed in beakers (approximately 13 slices per beaker, a strainer used to keep them immersed) containing the different impregnation solutions (ascorbic acid, citric acid, calcium lactate, and chitosan), 1 solution at a time. During the vacuum step, different pressures (50, 75, 100, and 125 mm Hg) were applied for 5 and 10 min and afterward, the atmospheric pressure was restored for 5 or 10 min. The impregnated samples were then drained and excess liquid was removed from the surface with a paper towel. The groups without any treatment served as controls. The impregnated and control samples were placed into plastic containers (Ziploc®, 591 mL, with plastic lids) and stored at 4 °C for analysis at days 0, 4, 9, 12, and 15 of storage.

Impregnated liquid fraction (X)

Sample weight before (W1) and after (W2) the impregnation treatment was measured and impregnated liquid fraction (X) values were calculated. The impregnated liquid fraction represents the total external liquid that penetrates into the tissue. It was calculated by the weight difference of the samples before and after the treatment (Ortiz and others 2003) using the following formula:

  • display math(1)

Ten slices were used for each treatment with 8 replications. Due to the sheer amount of samples required, the change in mushroom composition was calculated after the impregnation process only after determining the most effective VI process. USDA food composition values were used as reference (USDA 2011). The change in water content after impregnation was confirmed by measuring the moisture content of the control and the selected impregnated sample.

Moisture content

Moisture content of impregnated (ascorbic acid + calcium lactate) and control slices were determined by weight loss after drying in a vacuum oven at 70 °C until constant weight (AOAC 1990). The samples were randomly chosen. Each sample's weight was recorded before and after drying. Moreover, the weight of canisters was recorded for higher accuracy. The samples (cap and stem) were first chopped into small pieces (approximately 10 g) and then placed in aluminum canisters prior the drying process. After removal from the vacuum oven, the samples were placed in a desiccator before recording the final weight. Measurements were carried out at room temperature in triplicate for the control and the selected treatment.

Electron-beam irradiation

Uniform dose distribution within a sample is important when designing irradiation treatments for food. Uniformity is described by a low dose uniformity ratio (DUR), the ratio between the maximum and minimum dose absorbed by the sample. Although the ideal DUR should be close to 1, accepted DUR values for practical applications are between 1.5 and 2 (IAEA 2002). A dose mapping study was conducted using an ion farm chamber to determine dose uniformity of the irradiation setup. Irradiation dosage was measured by placing Radiochromic film dosimeters (Far West Technology Inc., Goleta, Calif., U.S.A.) at the front, center, and backside of the mushroom slices, for a total of 3 dosimeters. The radiochromic films were read after stabilization using a Radiochromic reader model 92 (Far West Technology Inc.). The DUR in this study was 1.44, ensuring uniform dose distribution within the mushroom slice. The entrance dose was approximately 0.45 kGy, maximum dose was approximately 0.65 kGy, and backdose was 0.6 kGy for an applied dose of 0.5 kGy. For a target of 1 kGy (this study), the same relationship was applied, with entrance, maximum and backdoses of 0.90, 1.30, and 1.2 kGy, respectively (DUR = 1.44.) It should be noted that sample thickness was reduced to 3.91 ± 0.16 mm because the DUR was too high with samples 6.5 mm thick. This sample thickness was used for the shelf life study.

Irradiation experiment

A group of 4 different samples was evaluated to determine the best treatment: (1) control (no

treatment), (2) impregnated (2 g/100 g ascorbic acid + 1 g/100 g calcium lactate), (3) irradiated, and (4) impregnated-irradiated slices. Samples were prepared using the same procedure described above. Prior to irradiation, a single slice was placed in a plastic bag (Mylar) (Zip Seal™, 8.64 × 10.16 cm,

48GaPET/PE/0.00035Foil/LLDPE, Transilwrap Co., Franklin Park, Ill., U.S.A) and sealed. The slices were irradiated at 1 kGy, the maximum allowed by the FDA for fresh produce. We did not irradiate at lower or higher doses because it has been proven ineffective (Sapers and others 1994; Koorapati and others 2004). Irradiation was carried out at room temperature using a 1.35 MeV Van de Graaff e-beam accelerator. After irradiation, samples were stored at 4 °C up to 15 d for shelf life analysis.

Quality parameters

Soluble solids

Soluble solids concentrations in the (a) impregnated, (b) irradiated, (c) impregnated-irradiated, and (d) control samples were determined at room temperature using a handheld refractometer (Brix 35HP, Reichert Analytical Instrument, Inc., Buffalo, N.Y., U.S.A) and expressed in °Brix scale. About 20 to 25 g of mushrooms were placed in stomacher bags and squeezed to yield around 10 g of juice. Measurements were carried out at room temperature and in triplicate for each treatment and control group.

PH

The pH was measured using a digital pH meter (Cole Parmer, pH 500 series, #59003-20, Singapore) calibrated with standard solutions, pH 4, 7, and 10 before the experiment. About 20 to 25 g of mushrooms were placed in stomacher bags and squeezed to yield around 10 g of juice. Measurements were carried out at room temperature and in triplicate for each treatment and control group.

Color

The color properties of the sliced mushrooms were determined using 20 pieces from the treated and control samples at each sampling interval at room temperature using a Lab Scan XE colorimeter (Hunter Lab, Inc, Va., U.S.A.) calibrated with a standard plate (Y = 94.00, x = 0.3578, y = 0.4567). Since our objective was to test whether the treatments maintained the whiteness of the mushroom slices, only L* values (lightness) were reported. The cap of the mushroom slices was placed in the aperture of the colorimeter. New samples were used for each sampling interval (days 0, 4, 9, 12, and 15) to avoid microbial cross-contamination of the slices.

Texture

A shear test was applied with the Warner–Bratzler probe at 1.0 mm/s using a Texture Analyzer (TA.XT2i, Texture Technologies Corp., Scardale, N.Y.). Texture was defined as the maximum force required to shear (cut) the samples. A preliminary study showed that better results were obtained when the cap length was reduced to 3 ± 0.2 cm by cutting the sides of the mushroom slices and then combining 3 slices together. Thus, 3 slices (slice thickness approximately 6.5 ± 0.3 mm, total thickness approximately 19.5 mm) were used for the impregnation experiments. As stated earlier, we had to adjust the thickness of the slices when conducting the irradiation tests to ensure proper dose uniformity within the slices. Therefore, samples consisted of 5 (instead of 3) slices with total thickness of 19.5 mm. Ten replications were done for each shear test at room temperature.

Microbiological analysis

Total aerobic plates, psychrotrophic, and yeast and mold counts were determined on days 0, 4, 9, 12, and 15 of storage. Under sterile conditions, 10 g of mushroom slices (cap and stem) from each treatment were stomached inside a sterile stomacher bag, mixed with 90 mL of 0.1 g/100 g buffered peptone water, and homogenized for 1 min; afterward, 10-fold dilutions were made in this diluent. All counts were performed using petrifilms (3M yeast and mold count plates, 3M aerobic plate count (APC), 3M microbiology, St. Paul, Minn., U.S.A.). APCs were incubated at 37 °C for 48 h; psychrotrophic count plates at 4 °C for 7 d; and yeast and mold count plates were incubated at 20 °C for 7 d. After incubation, colonies were enumerated and results reported as log colony forming units (CFU)/g of sample. The experiments were carried out in triplicate.

Sensory evaluation

Thirty-five students, faculty, and staff members at Texas A&M Univ. formed the consumer panel. Evaluation of control, impregnated, irradiated, and impregnated-irradiated slices was carried out under the same conditions on days 1, 9, and 15 of storage. The samples were placed into white plastic plates labeled with 3 random digits and presented to the panelists (Meilgaard and others 1998) who were asked to score the samples based on odor, color, firmness, and overall quality using a 9-point hedonic scale (a score of 1 represents “dislike extremely” and a score of 9 represents “like extremely”). Scores higher than 5 were considered acceptable.

Statistical analysis

Data analysis was performed using SPSS software (version 20.0 for Windows 2011). Statistical differences between variables were analyzed for significance by one-way analysis of variance using Tukey's multiple range tests. Statistical significance was expressed at the P < 0.05 level.

Results and Discussion

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Combination Treatment: VI and Irradiation at 1 kGy
  7. Effect on Microbial Quality
  8. Conclusions
  9. References

Preliminary study on VI

Color

By the end of the study (day 15), impregnation with citric acid, calcium lactate alone, and chitosan solutions by VI did not help maintain the lightness of mushroom slices, with L* values significantly (P < 0.05) lower than the control group (data not shown). This effect was true for all combinations of vacuum pressure and restoration time tested, demonstrating that these antibrowning agents are ineffective and higher concentrations of these agents may be required to achieve the desired browning inhibition effect, which may induce flavor changes in the produce (Harris and others 2007).

The 2 g/100 g ascorbic acid solution containing 1 g/100 g calcium lactate applied at a 50 mm Hg for 5 min and 5 min restoration time was the most effective VI treatment in terms of color. L* values ranged from 60.56 on day 0 to 52.61 on day 15 compared to the control (72.91 and 48.6 on days 0 and 15, respectively). This finding is not surprising because ascorbic acid is the most effective antibrowning solution (Wang and others 2013) because of the interaction of the antibrowning agent with calcium and the mushroom's tissue (Wang and others 2013). Perez-Cabrera and others (2011) found similar results on a study with minimally processed pears.

Texture

The maximum force to shear the control samples on day 0 was around 30 N. On day 15, the force value was significantly (P < 0.05) higher since the samples were very rubbery and hard to shear. By day 15, the slices impregnated with ascorbic acid had the most similar texture characteristics to the control group on day 0, with differences (P < 0.05) due to the different conditions applied during VI (applied vacuum pressures and atmospheric restoration times) (Table 1).

Table 1. Effect of impregnation treatment (ascorbic acid and citric acid + calcium lactate) on texture (firmness) of sliced mushrooms impregnated at different vacuum pressures and different atmospheric restoration times during storage at 4 °C
Time 50 mm Hg50 mm Hg75 mm Hg75 mm Hg100 mm Hg100 mm Hg125 mm Hg125 mm Hg
(days)Control–5 min–10 min–5 min–10 min–5 min–10 min–5 min–10 min
2 g/100 g ascorbic acid + 1 g/100 g calcium lactate solution (maximum force in newton)
  1. 1Standard deviation.

  2. a,b,cMeans within a row, which are not followed by a common superscript letter, are significantly different (P < 0.05).

  3. w,x,y,zMeans within a column, which are not followed by a common subscript letter, are significantly different (P < 0.05).

0w30.297aw20.05b,c,dw18.846b,cw25.19a,dx26.74aw15.562cw18.76b,cw,x19.244bw,x28.127a
 1(2.092)(1.663)(1.562)(0.756)(0.559)(0.686)(1.473)(0.864)(0.394)
4w28.257bw20.346ay30.552bw,x28.274by35.33cx22.342aw,x21.585ax23.162aw,x32.093b,c
 (1.010)(1.904)(2.270)(1.568)(1.938)(1.271)(2.423)(1.809)(1.920)
9y47.79hx25.787a,bx24.748ax29.39b,cy35.59c,gy35.027c,fy32.287a,c,ey30.505a,c,dx35.86b,d,e,f,g
 (1.237)(1.020)(1.337)(0.871)(1.548)(2.822)(2.770)(2.950)(3.226)
12x42.753fy39.704e,fy32.816b,c,dw,x29.05a,by35.616d,ey29.353a,b,cx25.236ay35.1c,d,ew,,x32.53b,c,d
 (2.409)(1.924)(1.331)(1.836)(1.998)(0.750)(2.594)(3.910)(1.904)
15x43.185ew20.122ay32.97by34.246b,c,ex25.027cx20.805aw18.457aw18.032aw25.727a,b,c
 (2.315)(0.751)(1.008)(2.413)(1.510)(1.430)(1.858)(0.978)(2.908)
2 g/100 g citric acid + 1 g/100 g calcium lactate solution (maximum force in newton)
0w30.297ax23.881ax31.328c,dx,y35.573dx29.297b,cx26.127a,bx31.145b,c,dx28.87a,b,cx30.116b,c
 1(2.092)(2.177)(2.655)(2.039)(2.495)(2.036)(1.906)(2.323)(2.243)
4w28.257by35.040by49.355cx32.613by48.436cy61.34ey53.315dy46.656cz67.543f
 (1.010)(1.897)(1.351)(1.952)(1.559)(0.789)(1.858)(0.800)(1.174)
9y47.79hy38.500a,bz43.705b,cy46.236cy46.32cz43.385b,cz37.55a,bx34.623ax,y34.83a
 (1.237)(2.116)(0.106)(3.136)(0.494)(0.007)(0.961)(1.773)(2.851)
12x42.753fx,y47.840a,bx,z24.820bx,y50.545a,bx,y45.77a,bx,y,z39.86a,bx,y,z58.775a,bz55.805a,by39.31a,b
 (2.409)(10.534)(1.796)(3.500)(4.341)(3.464)(7.177)(3.655)(3.705)
15x43.185ey45.723ax,y,z33.96a,bx,y28.415a,bx,y52.46a,bx,y,z26.36a,bx,y,z18.415a,bz56.01aw19.605b
 (2.315)(2.956)(4.723)(8.478)(3.775)(3.535)(5.607)(3.054)(1.859)

Impregnation with citric acid (Table 1) yielded samples with different texture characteristics. Similarly, calcium lactate alone was not effective when applied by VI due to loss of quality in terms of structural losses (Table 2). The structural deformation was caused by the pressure changes during the VI process as observed by other authors (Perez-Cabrera and others 2011). Increasing vacuum pressure and vacuum time resulted in more structure deformation due to the applied pressures. Impregnation with chitosan (Table 2) produced sticky and rubbery samples by day 9 with force values lower (P < 0.05) than the controls by day 15. In brief, ascorbic acid + calcium lactate impregnation at 50 mm Hg for 5 min and 5 min restoration time effectively maintained the firmness of the sliced mushrooms stored at 4 °C for 15 d. This VI treatment was also effective in increasing the amount of physiologically active components in the mushroom composition with calcium increasing by 10 times and ascorbic acid by 150 times, compared to the USDA database values of 0.003 g/100 g and 0.0021 g/100 g, respectively (USDA 2011). After 5 min under 50 mm Hg and 5 min restoration time, W1 = 22.96 ± 2.29 g, W2 = 27.19 ± 3.17, and X = 18.42 ± 3.04%. Control and ascorbic acid–calcium lactate–impregnated samples had moisture contents of 92.2% and 92.9% wet basis, respectively.

Table 2. –Effect of impregnation treatment (calcium lactate alone and chitosan + calcium lactate) on texture (firmness) of sliced mushrooms impregnated at different vacuum pressures and different atmospheric restoration times during storage at 4 °C
Time 50 mm Hg50 mm Hg75 mm Hg75 mm Hg100 mm Hg100 mm Hg125 mm Hg125 mm Hg
(days)Control–5 min–10 min–5 min–10 min–5 min–10 min–5 min–10 min
1 g/100 g calcium lactate solution (maximum force in newton)
0w30.297a,by34.61by25.125ay29.106ay30.443a,by32.313a,bz27.525ay24.84az36.147b
 1(2.092)(0.573)(0.946)(0.965)(1.336)(2.370)(1.932)(0.47)(1.556)
4w28.257a,b,cx29.74a,b,c,dz31.87b,c,dz40.717ey33.65dy33.408c,dy33.57dy26.226ax27.773a,b
 (1.010)(3.109)(3.387)(2.368)(1.828)(1.374)(1.584)(1.740)(0.575)
9y47.79ez40.503dy,z26.97by31.71b,cx19.666ay33.066cx14.943ax15.03ay31.478b,c
 (1.237)(1.752)(2.036)(1.528)(1.605)(1.720)(3.334)(1.58)(0.712)
12x42.753bw15.35ax15.94ax11.675ax15.87ax19.675a   
 (2.409)(0.883)(0.806)(0.926)(1.256)(0.968)*********
15x43.185        
 (2.315)************************
1 g/100 g chitosan + 1 g/100 g calcium lactate solution (maximum force in newton)
  1. 1Standard deviation.

  2. a,b,cMeans within a row, which are not followed by a common superscript letter, are significantly different (P < 0.05).

  3. w,x,y,zMeans within a column, which are not followed by a common subscript letter, are significantly different (P < 0.05).

0w30.297cy31.972c,d,ey25.52a,bx,y36.182ex29.9b,cy22.47ay,z32.426c,d,ey,z30.787c,dy35.17d,e
 1(2.092)(2.012)(1.858)(2.173)(2.326)(0.980)(1.578)(1.622)(1.341)
4w28.257a,bx21.02a,b,cy28.615b,cy38.713dx31.583cz44.81cy30.56cz37.846ay30.8067a,b
 (1.010)(1.230)(2.525)(2.896)(2.323)(1.258)(1.654)(2.838)(2.731)
9y47.79a,cz49.88cy29.303bx30.525b,c,d,ey45.41a,cz41.975a,dz43.77a,b,cy26.53a,b,cz60.595a,b,c
 (1.237)(0.494)(0.241)(1.421)(0.895)(0.601)(7.254)(4.652)(3.486)
12x42.753dy31.536b,cz35.996c,dx,y34.85c,dx28.35b,cx15.946ay,z31.24b,cx,y23.853a,by34.23c,d
 (2.409)(3.195)(2.296)(3.255)(1.456)(3.506)(1.103)(2.896)(5.624)
15x43.185a x13.07bx,y33.68a,bx25.9a,b z14.23a,bx16.785a,bx15.695a,b
 (2.315)***(2.701)(3.422)(3.733)***(4.101)(3.288)(5.395)

Next, we explored whether the above VI treatment combined with irradiation at 1 kGy would help preserve the physicochemical, sensory, and microbiological quality of the mushroom slices.

Combination Treatment: VI and Irradiation at 1 kGy

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Combination Treatment: VI and Irradiation at 1 kGy
  7. Effect on Microbial Quality
  8. Conclusions
  9. References

Effect on quality attributes

PH

The pH of fresh mushrooms was approximately 6.5 (Table 3). The pH values of all the impregnated samples were significantly (P < 0.05) lower than the nonimpregnated (control and irradiated alone) samples, because of the acidic nature of ascorbic acid. On day 15, all samples had a pH of 6.2, still acceptable for consumption (US FDA/CFSAN 2012).

Table 3. pH, color L*, and maximum force values for the control, impregnated, irradiated, and impregnated-irradiated samples stored at 4 °C for 15 d
Time   Impregnated-
(days)ControlImpregnatedIrradiatedIrradiated
pH
  1. 1Standard deviation.

  2. a,b,cMeans within a row, which are not followed by a common superscript letter, are significantly different (P < 0.05).

  3. w,x,y,zMeans within a column, which are not followed by a common subscript letter, are significantly different (P < 0.05).

0y6.51cx6.17a,by6.38b,cx6.05a
 1(0.08)(0.12)(0.08)(0.01)
4y,x6.30ax6.18a,by,x 6.30ax6.15a
 (0.08)(0.08)(0.11)(9.862)
9x6.18ax6.07ax6.17ay6.23a
 (0.12)(0.07)(0.10)(0.10)
 x6.18bx6.11ax6.20ax,y6.19a
12(0.12)(0.06)(0.09)(7.796)
15x6.20ax6.11ax6.15ax6.17a
 (0.03)(0.07)(0.03)(0.02)
Color L*
0z70.701bz70.142a,bz73.333cw68.503a
 1(2.501)(1.633)(2.185)(2.650)
4y67.902cy61.037ay65.051b,cx,y,z61.821a,b
 (1.900)(4.298)(4.262)(6.275)
9x59.994bw53.637ay65.627cy56.516a,b
 (4.210)(2.599)(3.095)(5.109)
12w,x55.743a,bx57.230bx53.226ay58.077b
 (6.598)(2.454)(3.243)(3.384)
15w52.631aw,x,y57.216bx55.186a,bz63.525c
 (3.293)(4.835)(2.823)(2.388)
Maximum force [N]
0x28.849bw23.040aw30.348bw30.235b
 1(5.571)(3.252)(4.178)(5.604)
4x,y34.095ay37.328aw,x35.985aw33.563a
 (4.958)(6.322)(7.757)(9.862)
9y37.150a,bw,x,y32.185ax,y42.789bw35.313a,b
 (5.707)(8.353)(6.155)(10.574)
12y36.640bw,x28.199ay,z49.718cw30.605a,b
 (5.622)(6.168)(8.239)(7.796)
15y44.210b,cx,y31.781az51.575cw39.670a,b
 (8.742)(6.538)(6.696)(7.378)
Soluble solids (oBrix)

All samples had °Brix values between 5.0 and 6.0 (data not shown) but no significant (P > 0.05) differences were found among the samples.

Color

By day 15, lightness values between the control and the treated samples were different (P < 0.05) (Table 3). On day 9, the impregnated samples had higher L* values than the control and irradiated samples, probably due to the precipitation of calcium in the cell walls, which may have increased the sample's opacity. Furthermore, dark brown spots and dark staining occurred on the surface of the controls. Figure 1 and 2 show the appearance of the slices on days 0 and 15 of storage, respectively. Figure 2 shows the beneficial effect of the combined vacuum impregnation and irradiation treatment in the color of the samples. Although ascorbic acid is effective in preventing enzymatic browning once it oxidized completely, darkening of samples occurred in impregnated samples due to melanin formation. Irradiation seems to reduce browning by slowing down enzyme oxidation (Niemira and Fan 2009).

image

Figure 1. Mushroom slices treated under different conditions (day 0 at 4 °C.)

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image

Figure 2. Mushroom slices treated under different conditions (day 15 at 4 °C.)

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Texture

Irradiation induced softening of the sliced mushrooms (Table 3). This is a problem when exposing fruits and vegetables to ionizing radiation that causes depolymerization of cellulose, hemicelluloses, starch, and pectin, which results in tissue softening (Niemira and Fan 2009; Moreira and Castell-Perez 2012). VI also induced softening due to loss of structure. However, application of the antibrowning solution (2 g/100 g ascorbic acid) containing 1 g/100 g calcium lactate maintained the firmness of the sliced mushrooms (approximately 35 N). Impregnated-irradiated mushroom slices had more fresh-like texture than the irradiated slices, demonstrating the beneficial effect of VI with a calcium-containing solution when samples are going to be irradiated. The controls and nonimpregnated samples suffered considerable structural loss (very soggy); hence, the blade could not shear the samples.

Effect on Microbial Quality

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Combination Treatment: VI and Irradiation at 1 kGy
  7. Effect on Microbial Quality
  8. Conclusions
  9. References

Three sample groups were evaluated: (1) untreated control, (2) impregnated, and (3) impregnated-irradiated.

Aerobics

Impregnation with ascorbic acid–calcium lactate solution was effective in reducing the aerobics counts (P < 0.05) (Table 4). As expected, the combined VI and irradiation treatment was effective (P < 0.05) in reducing microbial growth (Kooropati and others 2004). The trend of microbial growth for the control and impregnated samples was similar, with an increase in counts with storage time. Impregnation treatment alone reduced counts only by 1-log compared to the combined treatment (under the detection limit).

Table 4. Aerobic, psychrotrophics, and yeast and mold plate counts for the control, impregnated, and impregnated-irradiated samples stored at 4 °C for 15 d
Aerobic plate count
Time  Impregnated-
(days)ControlImpregnatedIrradiated
  1. 1Standard deviation.

  2. a,b,cMeans within a row, which are not followed by a common superscript letter, are significantly different (P < 0.05).

  3. w,x,y,zMeans within a column, which are not followed by a common subscript letter, are significantly different (P < 0.05).

  4. Detection limit = 2.39 CFU/g.

  5. N/A = No values observed, N/D = Under detection limit.

0x 5.653cy 4.891bN/D
 1(0.179)(0.098) 
4y 7.164cx, y 4.980bN/D
 (0.593)(0.458) 
9y, z 7.817cx 6.560bN/D
 (0.404)(0.22) 
12z 8.632bx, y, z 7.431bN/D
 (0.169)(0.728) 
15z 8.693cz 7.984bN/D
 (0.206)(0.041) 
Psychrotrophic plate count
0w 7.016cx 5.437bN/A
 1(0.134)(0.389) 
4x 7.684cx 5.331bN/D
 (0.225)(0.159) 
9y 9.320cy 7.915bN/A
 (0.059)(0.106) 
12y, z 9.433cy 8.203bN/A
 (0.14)(0.616) 
15z 9.774cy 8.815bN/A
 (0.141)(0.379) 
Yeast and molds plate count
0x 3.383bx 2.761bN/A
 1(0.686)(0.111) 
4x 3.620bx 3.043bN/D
 (0.573)(0.245) 
9x 4.412bx, y 3.960bN/D
 (0.206)(0.499) 
12x, y 5.625bx, y 4.621bx, y 2.536a
 (0.206)(0.927)(0.629)
15y 4.458by 4.985by 3.370a
 (0.342)(0.091)(0.438)

Psychrotrophics

The organisms usually responsible for spoilage of mushrooms are Gram-negative, psychrotrophic bacteria, particularly belonging to the Pseudomonae family. These microorganisms are more susceptible to irradiation than other types of spoilage bacteria (Koorapati and others 2004). The combined VI and irradiation treatment was very effective (P < 0.05) in reducing psychrotrophic microbial growth, under the detection limit by day 9 (Table 4). Ascorbic acid impregnation reduced psychrotrophs by 1.5 logs, though this treatment alone is not sufficient to stop their growth.

Yeast and molds

There was a significant (P < 0.05) difference in counts among the control, impregnated, and impregnated-irradiated samples on day 0 (Table 4). After day 12, there was a drastic increase in growth, showing an approximately 2-log counts increase by day 15. This finding suggests that the 1.0 kGy dose may be insufficient to inactivate yeast and molds in sliced mushrooms. It is known that yeasts are more resistant to irradiation than molds (da Silva Aquino 2012).

Effect on sensory attributes

Sensory results indicated that the impregnated, irradiated, and impregnated-irradiated samples were acceptable (P < 0.05) (Figure 3). On day 1, there was no difference (P > 0.05) among the color acceptability of the samples (Figure 3A). The control samples were unacceptable to the panelists by day 15 of storage (scores approximately 4.0), while the impregnated, irradiated, and impregnated-irradiated samples were acceptable (scores approximately 6.0). This finding is supported by the objective color measurements (Table 3). The odor scores of the control were significantly (P < 0.05) different after day 1 (Figure 3B). Since irradiation inhibited microbial growth, no odor changes were observed in the irradiated and impregnated-irradiated samples with scores > 6.0. Mushroom slices lost their original mushroom smell with time, but treated samples did not exhibit unfavorable odor changes.

image

Figure 3. Sensory (A) color scores; (B) odor scores; (C) texture scores; and (D) overall quality scores for control, impregnated, irradiated, and impregnated-irradiated samples stored at 4 °C during 15 d. A score of 1 = dislike extremely, 5 = neither like nor dislike, and 9 = like extremely. A value above 5 is considered acceptable.

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Throughout the evaluation period, the controls were less acceptable in terms of their texture attributes (Figure 3C). Although the objective texture measurements showed clear changes in firmness of the treated samples (Table 2), the panelists found all the samples acceptable throughout storage (scores > 6.0).

The controls exhibited significant (P < 0.05) quality loss and became unacceptable by day 15 of storage (Figure 3D). The impregnated, irradiated, and impregnated-irradiated samples had consistently higher scores than the control group with the impregnated-irradiated samples receiving slightly (though not significant) higher scores ( >6.0).

Product appearance is the most appealing attribute to the consumers. Since irradiation prevented microbial growth and reduced microbial-induced browning, irradiated and impregnated-irradiated samples were consistently rated higher.

Conclusions

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Combination Treatment: VI and Irradiation at 1 kGy
  7. Effect on Microbial Quality
  8. Conclusions
  9. References

Although calcium is a texture enhancer for fresh-cut fruits and vegetables, it was not effective in sliced mushrooms when applied using VI and spoilage by fungi resulted in loss of quality. Irradiation treatment alone caused softening of the sliced mushrooms; however, impregnation with 1 g/100 g calcium lactate–ascorbic acid solution helped to maintain mushroom firmness.

Microbiological analysis demonstrated that e-beam irradiation had an impact on reducing aerobic and psychrotrophic populations, whereas higher doses were required for inhibition of yeasts and molds.

Sensory tests indicated consumers’ acceptance of the impregnated, irradiated, and impregnated-irradiated mushrooms. Product appearance was the most important quality attribute. Irradiated and impregnated-irradiated samples had the highest overall sensory scores because of reduced microbial-induced browning.

Results from this study demonstrated that e-beam irradiation and VI of a solution containing 2 g/100 g of ascorbic acid and 1 g/100 g of calcium lactate can extend the shelf life of sliced mushrooms.

Future work includes evaluation of the effect of impregnation-irradiation on poyphenoloxidase activation to understand the mechanism of browning inhibition; irradiation at doses higher than 1 kGy to reduce growth of yeasts and the effect on quality; evaluation of combination of ascorbic acid and citric acid solutions; and optimization of impregnation solution and irradiation dose combinations.

References

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Combination Treatment: VI and Irradiation at 1 kGy
  7. Effect on Microbial Quality
  8. Conclusions
  9. References
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