Study on the Prediction Models for the Browning Degree of the Postharvest Agaricus Bisporus based on Color Parameters

Browning is one of the most important factors affecting the quality of postharvest agaricus bisporus. In the study, the changing rules of L*, a*, b* value and Browning Degree (BD) were studied during the browning of agaricus bisporus mushroom cover. The correlation of these changes and browning was discussed and equations between color indices and browning degree were developed. Based on it, the prediction models for the browning degree of the postharvest agaricus bisporus was established. The results indicated that, color indices of L*, a*, b* could be used to quantify the browning degree and L* value was most highly correlated to the browning degree and equations between L value and browning degree were developed under different temperatures. Based on ∑R, the zero-order kinetics model is used as the prediction model for the browning degree of the postharvest agaricus bisporus based on color parameters: Through comparing correlation between predicted values and actual values, the prediction model for the browning degree of the postharvest agaricus bisporus was validated.


INTRODUCTION
Agaricus bisporus takes advantages of white color, crisp texture, delicious taste and high nutritional value, which has a large area of cultivation in our country.However, Agaricus bisporus is watery and the mushroom cover is unprotected, so it is easy to quality deteriorate after harvest and the sales time and radius of agaricus bisporus are limited (Tian et al., 2012;Aiping, 2010).Agaricus bisporus is highly susceptible to cover browning and it can intuitively reflect the shelf quality and shelf life of agaricus bisporus (TianJia, 2010).Browning reaction in fruits and vegetables is recognised as a serious problem in the food industry.The browning of mushrooms upon storage is a rather complex process (Mac Canna and Gormley, 1968).So the determination of the browning degree is very important to determine the shelf quality of postharvest agaricus bisporus.Traditionally, people evaluate the degree of browning with naked eyes and chemical test.This, however, may not reflect the real situation of browning because the sensibility of the eyes varies with persons and with light density and thus the results may differ greatly (Wu et al., 2006).Loredana et al. (2011) developed a multispectral vision system to evaluate enzymatic browning in fresh-cut apple slices.The chemical method is destructive and labor-consuming.Thus, developing a rapid simple and accurate browning determination method is necessary.
Enzymatic browning reactions limit the commercial shelf life of apple juice, so that color preservation during storage is one of the main objectives of fruit processors (López-Nicolás et al., 2007).Hunter is a new way to evaluate the color changes of fruits and vegetables.With a colorimeter, colors can be quantified in the values of L*, a*, b*, C* and h.The way can accurately reflect the color changes of fruits and vegetables and has been successfully used in a variety of browning degree testing.
In this study, on the basis of pre-research results, using the color parameters of agaricus bisporus to reflect the browning degree can provide a new way for the browning degree detection.

Materials:
Agaricus bisporus were harvested from cooperation in Xingyue, Tianjin.Agaricus bisporus were stored in a cold room at (4±1) °C.Fresh-keeping packaging films in thickness as 0.04 mm were bought from modified atmosphere packaging lab in Tianjin.The foamed PP tray in size as 15*11*2 cm also was bought from modified atmosphere packaging lab in Tianjin.

Methods:
Treatment of experiment: Agaricus bisporus uniform in size and color were selected, air-dried, quartered and packaged in low density plastic pallet with 90±5 g in each bag before they were stored in temperature humidity chamber at 2, 10, 16, 22°C.And the color indices were detected at the same time.

Indices measurement:
The grading of browning: Refering to (Fei et al., 2005) method and slightly modifying, color evaluation criteria is established as Table 1.(Wu et al., 2006).

Detection of the BD:
Samples is mixed with boiling water in mass ratio as 1:20.Soak time was 30 s and the speed of translation stage was 1000 r/min for the whole centrifugalization at 5 min.Then, determine absorbance of the supernatant at the wavelength of 410 nm and calculate the browning degree as followed: Graphing and analysis: SPSS v19.0 for Windows was used for data treatment and difference analysis.Matlab 2012 was used for correlation analysis and model fitting.

RESULTS AND ANALYSIS
Changes in the grading of browning: As shown in Table 2, with the increase in degrees of cover browning, the browning grade of agaricus bisporus changes obviously in 16 and 22°C.Based on Table 1, agaricus bisporus over grade 5 in sensorial evaluation is losing sale value.

Changes in color indices during mushroom cover browning:
As shown in Fig. 1, with the increase in degrees of cover browning, the L* value all decreased and the downtrend was tardy at low temperature.The As shown in Fig. 5, C* values rised rapidly at high temperature.The C* values of agaricus bisporus were respectively 27.5 and 32.0 at 16, 22°C after 5 d.And The C* value of agaricus bisporus was as high as 32.0 and the sensorial evaluation of agaricus bisporus is losing sale value.

Changes in BD during mushroom cover browning:
Browning degree is the most important indicator and it can result in other sensory qualities.As indicated in Fig. 6, BD values all increased and the browning was more serious at the higher the temperature.BD had been on a low level at 2°C.BD changed slightly at beginning from 10 days in storage and rised rapidly after 14 days.BD reached the maximum at 19 th day. Figure 4, the temperature could influence the h value to few extent and the changing trend is irregular.Thus, h value cannot reflect the browning degree during different temperature.
Table 3 shows that the browning degree of all the temperatures was highly correlated with L*, a*, b* and C* (p<0.05).L* among all color indices was most significantly correlated to browning degree in all temperatures (0.9435, 0.9523, 0.9612, 0.9400).L* value can reflect the browning degree as (Liu, 2010) indicated.
Equations describing the relationship between browning degree and L* , a*, b* and C* values are listed in Table 4.
Establishment of the prediction models for the browning degree: Table 5 shows that rate constant k for the zero prediction model is higher than the first prediction model.The zero prediction model is more accurate than the first prediction model.So the zero prediction model based on L* value is used as the prediction models for the browning degree of the postharvest agaricus bisporus.
According to regression equation and rate constant k calculated by Arrhenius equation, the prediction model for the browning degree of the postharvest agaricus bisporus based on L* value was established as followed: Verification of the prediction model: predicted L* values calculated by L* model and actual values (Fig. 7).The result shows, the determination coefficient R 2 based on predicted values and actual values is 0.9690 and average less than 10%.Thus, the model can predict the browning degree.

DISCUSSION
Agaricus bisporus cover displays a bright white color at maturity, the cover gradually becomes brown and dark during storage at room temperature, resulting in reduction in nutritive value and deterioration in quality (Liu and Wang, 2011).The existence of damaged areas on mushroom surface tissue caused by browning is the most common and challenging quality defect encountered in the mushroom industry.
So far most studies on the browning degree are largely based on visual observation.Vanoli Verification of the prediction model: Compare value prediction (Fig. 7).The result shows, the based on predicted values average relative error is n 10%.Thus, the model can predict the cover displays a bright white color at maturity, the cover gradually becomes brown and dark during storage at room temperature, resulting in reduction in nutritive value and deterioration in The existence of mushroom surface tissue caused by browning is the most common and challenging quality defect encountered in the mushroom industry.
So far most studies on the browning degree are largely based on visual observation.Vanoli et al. (2014) aimed at studying the feasibility of Time Reflectance Spectroscopy (TRS) to nondestructively detect Internal Browning (IB) in 'Braeburn' apples through the development of classification models based on absorption (µa) and scattering (µ pulp (Vanoli et al., 2014).Others use chemical methods (Wu et al., 1995;Yueming, 2000), which involves the extraction of the brown substances from the pericarp and the measurement of the absorption of the long wavelength light.But this method is complex and has great error.
Color indices have been used in many fruits and vegetables to reflect the processes of ripening and senescence with changes in color.Ren reported that the b* value is corrected to the first dynamical model during packaged and ex for green cauliflower.As it was significantly correlated with the grading of etiolation, b* value could be used as the quality model.Zheng and Xu (2011) ., 1995;Yueming, 2000), which involves the extraction of the brown substances from the pericarp and the measurement of the absorption of the long wavelength light.But this method is complex and has Color indices have been used in many fruits and vegetables to reflect the processes of ripening and senescence with changes in color.Ren et al. (2005) reported that the b* value is corrected to the first dynamical model during packaged and exposed storage for green cauliflower.As it was significantly correlated with the grading of etiolation, b* value could be used as Xu (2011) reported that, a*/b* can reflect the color changes of samples.
(1999) observed changes in the values of L*, a* and b* during the development of litchi fruits.However, there have been few reports so far on the relationship between the color indices and the browning degree of mushroom cover.
This study shows that all the color indices except h were significantly correlated with browning degrees in agaricus bisporus during different temperature.The L* value was most significantly (p<0.05)correlated with browning degrees among all color indices.A linear equation between L* value and the browning degree was thus worked out and the prediction model for the browning degree of the postharvest agaricus bisporus based on L* value was established.The prediction model for the browning degree based on L* value was very close to the actual browning degree.
In actual production, using colorimeter to detect the color indices can conveniently reflect the color changes during storage for agaricus bisporus.The prediction model for the browning degree of the postharvest agaricus bisporus based on L* value can predict the browning degree and the model may have a potential use for the commercial production on freshing-line.

CONCLUSION
Color indices of L*, a*, b* and C all can be used to indicate the color changes of agaricus bisporus and L* value is most significantly correlated to the browning degree during storage for agaricus bisporus at different temperatures.The prediction model for the browning degree of the postharvest agaricus bisporus based on L* value is available for the prediction of agaricus bisporus browning.

ACKNOWLEDGMENT
First of all, I would like to extend my sincere gratitude to my supervisor, HuYunfeng, for her instructive advice and useful suggestions on my thesis.I am deeply grateful of her help in the completion of this thesis.
High tribute shall be paid to Ms. Li, whose profound knowledge of English triggers my love for this beautiful language and whose earnest attitude tells me how to learn English.
I am also deeply indebted to all the other tutors and teachers in Translation Studies for their direct and indirect help to me.
Special thanks should go to my friends who have put considerable time and effort into their comments on the draft.
Finally, I am indebted to my parents for their continuous support and encouragement.
: A automatic CR colorimeter was used for determination of color indices.The color indices L*, a*, b* and h were recorded at 5 spots for each agaricus bisporus cover with at least 4 for each treatment.The color space was defined by L*, a*, b* (CIELAB), where L* stands for brightness and a* and b* are the color coordinates, positive value of a* stands for red color direction and minus value for green; positive value of b* represents the achromatic area

Fig. 6 :
Fig. 6: Changes in browning degree of agaricus bisporus during storageTable 3: The correlation of BD and color parameters Temperature °C BD and L* BD and b* BD and C* 22 0.9435 0.9295 0.9933 16 0.9523 0.9109 0.9752 10 0.9612 0.8218 0.8925 2 0.9400 0.8093 0.8869The correlation between different color indices and the browning degree: As indicated in results above, a* and b* value alues all increased during different temperatures, but b* values changed significantly, so the correction a* between with BD is less than b*.Figure4, the temperature could influence the h value to few extent and the changing trend is irregular.Thus, h value cannot reflect the browning degree during different temperature.Table3shows that the browning degree of all the temperatures was highly correlated with L*, a*, b* and C* (p<0.05).L* among all color indices was most significantly correlated to browning degree in all temperatures (0.9435, 0.9523, 0.9612, 0.9400).L* value can reflect the browning degree as(Liu, 2010) indicated.Equations describing the relationship between browning degree and L* , a*, b* and C* values are listed in Table4.

Table 1 :
The grading standard of browning of Agaricus bisporus at