Changes in microbial and postharvest quality of shiitakemushroom Lentinus edodes treated with chitosan–glucose complex coating under cold storage Tianjia Jiang, Lifang Feng, Jianrong Li⇑
Trang 1Changes in microbial and postharvest quality of shiitake
mushroom (Lentinus edodes) treated with chitosan–glucose
complex coating under cold storage
Tianjia Jiang, Lifang Feng, Jianrong Li⇑
College of Food Science and Biotechnology, Zhejiang Gongshang University, Food Safety Key Lab of Zhejiang Province, Hangzhou 310035, PR China
Article history:
Received 13 July 2011
Received in revised form 23 August 2011
Accepted 23 August 2011
Available online 19 September 2011
Keywords:
Chitosan-glucose complex
Shiitake mushroom
Microbiological quality
Sensory evaluation
Storage life
a b s t r a c t
The effect of chitosan, glucose and chitosan–glucose complex (CGC) on the microbial and postharvest quality of shiitake (Lentinus edodes) mushroom stored at 4 ± 1 °C for 16 days was investigated Mushroom weight loss, respiration rate, firmness, ascorbic acid, total soluble solids, microbial and sensory quality were measured The results indicate that treatment with CGC coating maintained tissue firmness, inhib-ited increase of respiration rate, reduced microorganism counts, e.g., pseudomonads, yeasts and moulds, compared to uncoated control mushroom The efficiency was better than that of chitosan or glucose coat-ing treatment In addition, CGC coatcoat-ing also delayed changes in the ascorbic acid and soluble solids con-centration Sensory evaluation proved the efficacy of CGC coating by maintaining the overall quality of shiitake mushroom during the storage period Our study suggests that CGC coating might be a promising candidate for maintaining shiitake mushroom quality and extending its postharvest life
Ó 2011 Elsevier Ltd All rights reserved
1 Introduction
Shiitake (Lentinus edodes) mushroom is highly perishable and
tends to lose quality immediately after harvest Its shelf life is short
because of its high respiration rate, tendency to turn brown and
having no physical protection to avoid water loss or microbial
at-tack (Simón, González-Fandos, & Tobar, 2005) Bacteria, moulds,
enzymatic activity and biochemical changes can cause spoilage
during storage Gram-negative microorganisms, such as
Pseudomo-nas tolaasii, PseudomoPseudomo-nas fluorescens and yeasts, such as Candida
sake, have been associated with mushroom spoilage (Masson,
of mushroom is an impediment to the distribution and marketing
of the fresh product
The use of modified atmosphere packaging as an adjunct to low
temperature storage has been extensively reported to extend the
shelf-life of shiitake mushrooms (Ares, Parentelli, Gámbaro, Lareo,
gamma-irradiation in combination with MAP can extend the storage life
of shiitake mushroom up to 20 days
Chitosan [b-(1,4)-2-amino-2-deoxy-D-glucopyranose], which is
mainly made from crustacean shells, is the second most abundant
natural polymer in nature after cellulose (Shahidi, Arachchi, & Jeon,
1999) Due to its non-toxic nature, antioxidative and antibacterial activity, film-forming property, biocompatibility and biodegrad-ability, chitosan has attracted much attention as a natural food additive (Majeti & Ravi, 2000) Chitosan has been used in foods,
as a clarifying agent in apple juice, and antimicrobial and antioxidant in muscle foods (Gómez-Estaca, Montero, Giménez, &
chito-san also has potential for food packaging, especially as edible films and coatings (Tual, Espuche, Escoubes, & Domard, 2000) It has been used to maintain the quality of postharvest fruits and vegetables, such as citrus (Chien, Sheu, & Lin, 2007), tomatoes (El
2001), peach, pear and kiwifruit (Du, Gemma, & Iwahori, 1997) Several researchers have developed methods to improve the properties of chitosan using chemical and enzymatic modifica-tions However, chemical modifications are generally not preferred for food applications because of the formation of potential detrimental products Chitosan–lysozyme conjugates have been reported to have better emulsifying properties and bactericidal action (Song, Babiker, Usui, Saito, & Kato, 2002)
The Maillard reaction, resulting from condensation between the carbonyl group of reducing sugars, aldehydes or ketones and an amine group of amino acids, proteins or any nitrogenous compound, is one of the main reactions taking place in food Mail-lard reaction compounds contribute to flavour formation, antioxi-dative and antimicrobial effects and improvement of functional
0308-8146/$ - see front matter Ó 2011 Elsevier Ltd All rights reserved.
⇑Corresponding author Tel./fax: +86 571 88071024.
E-mail address: li58516@sohu.com (J Li).
Contents lists available atSciVerse ScienceDirect
Food Chemistry
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / f o o d c h e m
Trang 2properties (Chevalier, Chobert, Genot, & Haertle, 2001) It is
desir-able to modify chitosan so that it attains excellent antioxidant
activity without affecting its antimicrobial activity Chitosan has
an amino group which can react with the carbonyl group of a
reducing sugar Hence, chitosan was heated with glucose to form
a Maillard reaction product.Kanatt, Chander, and Sharma (2008)
found that chitosan–glucose complex (CGC), a modified form of
chitosan prepared by heating chitosan with glucose, showed
excel-lent antioxidant activity, while chitosan or glucose alone did not
have any significant activity On the other hand, the antimicrobial
activity of CGC was similar to chitosan against Escherichia coli,
Pseudomonas, Staphylococcus aureus and Bacillus cereus, and it can
increase the shelf life of pork cocktail salami to 28 days
However, research on the application of CGC to fruits and
veg-etables is limited The objectives of this work were to evaluate
the effect of CGC on the microbiological and postharvest quality
of shiitake mushrooms during cold storage
2 Materials and methods
2.1 Materials
Shiitake (Lentinula edodes) mushrooms used in this study were
harvested from a local farm in Hangzhou, China Mushrooms were
picked from the same flower and from the same area of the shed so
as to reduce possible variations caused by cultivation and
environ-mental conditions The mushrooms were transported to the
labo-ratory within one hour of picking, under refrigerated conditions,
then stored in darkness at 1 ± 1 °C and 95% relative humidity (RH)
2.2 Preparation of chitosan–glucose complex (CGC) solutions and
application of treatments
Chitosan (deacetylated P95%, and viscosity 630 mPa s) was
purchased from Zhejiang Xuefeng Calcium Carbonate Co., Ltd
(Zhejiang, China) One percentage of chitosan was prepared in 1%
glacial acetic acid Chitosan glucose complex (CGC) was prepared
by autoclaving chitosan (1%) and glucose (1%) for 15 min
Mush-rooms were divided into four samples of 60 each Four different
treatments were used: (1) control; (2) 1% glucose coating; (3) 1%
chitosan coating, and (4) CGC coating Mushrooms were dipped
into the solution for 5 min Samples dipped in distilled water were
used as control Treated samples were kept over a plastic sieve for
30 min and a fan generating low-speed air was used to hasten
dry-ing Then a tissue paper was used to absorb excess solution from
the surface The treated samples were placed and sealed in
18 cm 20 cm bags of low density polyethylene (PE) (0.04 mm
thickness) in the laboratory; the PE gas transmission rates were
1078 1018mol m1s1Pa1 for O2, 4134 1018mol m1
s1Pa1for CO2(both at 20 °C and 100% RH) and 2.8 105–6.5
105g m2s1for H2O (at 37 °C and 90% RH) They were then
stored for 16 days at 4 ± 1 °C and 95% relative humidity (RH)
Fifteen replicates were included in each treatment group, and
sub-sequently every 4 days, three replicates from each treatment group
were analysed
2.3 Respiration rate
Respiration rate was determined according to the method ofLi,
res-piration rate of the product At each storage time, approximately
50 g of mushrooms from the four groups were placed under
nor-mal air for 1 h Then, mushrooms were stored at 20 °C for 1 h in
a closed container, which contained 15 mL 0.05 M Ba(OH)2 Then,
2 drops of phenolphthalein were added, and titrated with 1/44 M
oxalate Measurements were replicated three times Respiration rates of samples were (expressed as CO2production rate) calcu-lated with the following formula:
RI ¼ðV1 V2Þ c 44
W t
In the formula, V1is the volume of oxalate titrated for the control (mL); V2is the volume of oxalate titrated for the samples (mL); c
is the concentration of oxalate (M); 44 is the molecular weight of
CO2; W is the weight of samples (g); t is the test time (h)
2.4 Weight loss Weight loss was determined by weighing the whole mushroom before and after the storage period Weight loss was expressed as the percentage of loss of weight with respect to the initial weight
2.5 Texture measurement
A penetration test was performed on the shiitake mushroom cap using a TA.XT Express-v3.1 texture analyser (Stable Micro Sys-tems, Godalming, UK), with a 5 mm diameter cylindrical probe Samples were penetrated 5 mm in depth The speed of the probe was 2.0 mm s1during the pretest as well as during penetration Force and time data were recorded with Texture Expert (Version 1.0) from Stable Micro Systems From the force vs time curves, firmness was defined as the maximum force used
2.6 Total soluble solids and ascorbic acid content Mushrooms were ground in a mortar and squeezed with a hand press, and the juice was analysed for total soluble solids (TSS) TSS was measured at 25 °C with a digital refractometer (Atago, Tokyo, Japan) The determination of total ascorbic acid was carried out as described by Hanson et al (2004), on the basis of coupling 2, 4-dinitrophenylhydrazine (DNPH) with the ketonic groups of dehy-droascorbic acid through the oxidation of ascorbic acid by 2,6-dichlorophenolindophenol (DCPIP) to give a yellow/orange colour
in acidic conditions Mushroom tissues (10 g) were blended with
80 mL of 5% metaphosphoric acid in a homogeniser and centri-fuged After centrifuging, 2 mL of the supernatant were poured into
a 20 mL test tube containing 0.1 mL of 0.2% 2,6-DCIP sodium salt in water, 2 mL of 2% thiourea in 5% metaphosphoric acid and 1 mL of 4% 2,4-DNPH in 9 N sulphuric acid The mixtures were kept in a water bath at 37 °C for 3 h followed by an ice bath for 10 min Five millilitres of 85% sulphuric acid were added and the mixtures were kept at room temperature for 30 min before reading at 520 nm
2.7 Microbiological analysis All samples were analysed for the mesophilic, psychrophilic, pseudomonad, and yeasts and moulds bacteria counts Twenty-five grams of mushrooms were removed aseptically from each pack and diluted with 225 mL 0.1% peptone water The samples were homogenised by a stomacher at high speed for 2 min Serial dilu-tions (101–10v9) were made in serial dilution tubes by taking 1.0 mL with 9.0 mL of 0.1% peptone water Aerobic counts were determined on plate count agar (PCA; Merck, Darmstadt, Germany) following incubation at 35 °C for 2 days for mesophilic bacteria, and at 4 °C for 7 days for psychrophilic bacteria Pseudomonas was counted on cephaloridin fucidin cetrimide agar (CFC; Difco;
BD, Franklin Lakes, NJ), with selective supplement SR 103 (Oxoid, Basingstoke, UK) The incubation temperature was 25 °C and plates were examined after 48 h Yeasts and moulds were estimated on
Trang 3potato dextrose agar (PDA; Merck) and incubation conditions were
28 ± 1 °C for 5–7 days
2.8 Sensory evaluation
The sensory attributes that characterised mushroom
deteriora-tion were determined These attributes were: off-odour, gill colour,
gill uniformity, cap surface uniformity, and presence of dark zones
on the cap (Ares et al., 2006) Samples were evaluated by a sensory
panel of 10 trained assessors Mushrooms were served in closed,
odourless plastic containers at room temperature After opening
polyethylene bags, mushrooms were placed in plastic containers
and evaluations were performed within 2 h, in order to avoid loss
of off-odours A balanced complete block design was carried out for
duplicate evaluation of the samples For scoring, 10 cm
unstruc-tured scales anchored with ‘‘nil’’ for zero and ‘‘high’’ for 10 were
used, except for the gill colour descriptor, for which the anchors
were ‘‘white’’ and ‘‘brown’’
2.9 Statistical analysis
Experiments were performed using a completely randomised
design Data were subjected to one-way analysis of variance
(ANO-VA) Mean separations were performed by Tukey’s multiple range
test (DPS Version 6.55) Differences at p < 0.05 were considered
significant
3 Results and discussion
3.1 Effect of CGC coating on respiration rate
The main characteristics of the respiration rates of the shiitake
mushrooms treated with different kinds of coatings are shown in
the respiration rates of coated mushrooms significantly decreased
(p < 0.05) These values were 78.2–92.6% of those of the control
samples at the beginning of the cold storage period By Day 16,
the respiration rates of the control samples were 1.23–1.37 times
higher than those of the coated mushrooms Internal gas
atmo-sphere modification has been suggested to be the cause of reduced
CO2production by coated fruits and vegetables In this regard the
gas barrier properties and permselectivity of the edible coating
ap-plied to the skin surface and their dependence on relative humidity
and temperature will play an important role in the changes in
endogenous O2 and CO2 levels It is well known that excessive
restriction of gas exchange can lead to anaerobiosis and the
devel-opment of off-flavour Chitosan coating has been reported to
modify the internal atmosphere of tomatoes (El Ghaouth et al.,
1992), Japanese pear (Du et al., 1997) and apples (Gemma & Du,
1998) by depletion of endogenous O2and a rise in CO2, without achieving anaerobiosis In our study, CGC coating is more effective
in reducing the respiration rates of shiitake mushroom although the difference between the three coating treatments was not sig-nificant (p > 0.05) This could be because CGC coating is more effi-cient in restricting the gas exchange between mushroom and the atmosphere during storage
3.2 Effect of CGC coating on weight loss Compared with the control samples, the coated mushrooms showed a significantly (p < 0.05) reduced weight loss during stor-age (Fig 2) After 16 days of storage, the mushrooms coated with CGC and chitosan showed 2.41% and 2.71% weight loss, respec-tively, as compared to 3.71% and 3.13% weight loss in control and glucose-coated mushroom Mushroom weight loss is mainly cause
by water transpiration and CO2loss during respiration The thin skin of shiitake mushrooms makes them susceptible to rapid water loss, resulting in shrivelling and deterioration The rate at which water is lost depends on the water pressure gradient between the mushroom tissue and the surrounding atmosphere and the storage temperature Low vapour pressure differences between the mushroom and its surroundings and low temperature are rec-ommended for the storage of mushrooms Edible coatings act as barriers, thereby restricting water transfer and protecting mush-room epidermis from mechanical injuries, as well as sealing small wounds and thus delaying dehydration Chitosan coatings have been effective at controlling water loss from some commodities, including cucumber, pepper and longan fruit (El Ghaouth, Arul,
lower weight loss in CGC coated mushrooms contributed to main-taining better quality of mushroom during cold storage
3.3 Effect of CGC coating on texture The texture of shiitake mushroom is often the first of many quality attributes judged by the consumer and is, therefore, extremely important in overall product acceptance Shiitake mush-room suffers a rapid loss of firmness during senescence which con-tributes greatly to its short postharvest life and susceptibility to fungal contamination.Fig 3shows that CGC and chitosan coatings significantly (p < 0.05) reduced the loss in firmness of shiitake mushroom during storage There was no significant (p > 0.05) difference in the firmness of the control mushrooms and those
60
80
100
120
140
160
180
200
Storage time (days)
2 kg
1 h
1 )
Control Glucose Chitosan CGC
Fig 1 Effect of CGC coating on respiration rate changes of shiitake mushrooms
stored at 4 °C for 16 days Each data point is the mean of three replicate samples.
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Storage time (days)
Control Glucose Chitosan CGC
Fig 2 Effect of CGC coating on weight loss changes of shiitake mushrooms stored
at 4 °C for 16 days Each data point is the mean of three replicate samples Vertical
Trang 4glucose coated The maximum retention in firmness was obtained
by CGC and chitosan coating, with 2.80 N and 2.76 N firmness
values, respectively, at the end of storage Softening can occur
be-cause of the degradation of cell walls in postharvest mushrooms by
bacterial enzymes and increased activity of endogenous autolysins
Pseudomonas degrade mushrooms by breaking down the
intracel-lular matrix and reducing the central vacuole, resulting in partially
collapsed cells and a loss of turgor This kind of bacterial-induced
softening was observed in control samples but was inhibited by
chitosan and CGC coating treatments The maintenance of firmness
in the mushrooms treated with CGC and chitosan coatings could be
due to their higher antifungal activity, and covering of the cuticle
and lenticels, thereby reducing infection, respiration and other
senescence processes during storage, according to previous reports
in sweet cherry coated with aloe vera gel (Martínez-Romero et al.,
3.4 Effect of CGC coating on total soluble solids and ascorbic acid
content
Changes in the soluble solids content (SSC) of shiitake
mush-rooms over storage are shown inFig 4A The SSC of control
mush-rooms increased after 4 days of storage whilst coated mushmush-rooms
experienced a slight increase during the same period The lowest
levels of SSC were recorded in CGC and chitosan-coated mushroom
at end of the storage.Tao, Zhang, Yu, and Sun (2006)have reported
an increase in SSC in button mushrooms stored at 4 ± 1 °C and 75%
RH The effect of chitosan in reducing the increase in SSC during
storage of shiitake mushroom was probably due to the slowing
down of respiration and metabolic activity, hence retarding the
senescence process Indeed, the greater changes in SSC occurred
in those mushrooms which suffered the greatest water loss The
solubilisation of the cell wall polysaccharides and hemicelluloses
in senescent mushroom might also contribute to the increase in
SSC It is well documented that the filmogenic property of chitosan
results in an excellent semi-permeable film around the vegetable
and fruit, modifying the internal atmosphere by reducing O2and/
or elevating CO2, and suppressing ethylene evolution (Dong, Cheng,
down the synthesis and the use of metabolites, resulting in lower
SSC, due to the slower hydrolysis of carbohydrates to sugars (
rise in SSC was recorded in mango and banana treated with
chito-san However, other studies have indicated that the SSC of chitosan
dipped papaya and zucchini were the same as in the untreated
fruits (Bautista-Baños, Hernández-López, Bosquez-Molina, &
un-coated shiitake mushrooms during 16 days storage The initial ascorbic acid content of shiitake mushrooms was 41.6 mg/kg Although ascorbic acid of both coated and uncoated samples de-creased throughout storage, the use of CGC coating significantly re-duced the loss of ascorbic acid in mushroom samples After 16 days
of storage, ascorbic acid retention of mushroom treated with glu-cose, chitosan and CGC coating was 19.3, 23.5 and 25.9 mg/kg, respectively, whereas control samples maintained 17.7 mg/kg of initial ascorbic acid content Since ascorbic acid loss can be greatly favoured by the presence of O2, the incorporation of chitosan to coating formulations may reduce O2diffusion, slow down the res-piration rate and consequently better preserve ascorbic acid con-tent and delay senescence of shiitake mushroom Similar results were obtained by Ayranci and Tunc (2003), who reported that methylcellulose-based edible coating reduced ascorbic acid loss
in both button mushrooms and cauliflower The ascorbic acid con-tent in the CGC coated samples was higher than that in the samples coated with chitosan It has been suggested that edible coatings containing chitosan promote ascorbic acid loss by acting as an abi-otic elicitor, generating reactive oxygen species (ROS), which are scavenged by antioxidant compounds, such as ascorbic acid CGC coating could inhibit ascorbic acid loss, due to the protection ef-fected by its superior antioxidant activity, as compared to chitosan
or glucose (Kanatt et al., 2008)
0
0.5
1
1.5
2
2.5
3
3.5
4
Storage time (days)
Control Glucose Chitosan CGC
Fig 3 Effect of CGC coating on firmness changes of shiitake mushrooms stored at
4 °C for 16 days Each data point is the mean of three replicate samples Vertical
bars represent standard errors of means.
0 1 2 3 4 5 6 7 8 9
Storage time (days)
Glucose Chitosan CGC
0 5 10 15 20 25 30 35 40 45
Storage time (days)
Control Glucose Chitosan CGC
A
B
Fig 4 Effect of CGC coating on total soluble solids (A) and ascorbic acid (B) change
of shiitake mushrooms stored at 4 °C for 16 days Each data point is the mean of three replicate samples Vertical bars represent standard errors of means.
Trang 53.5 Effect of CGC coating on microbiological quality
Mesophilic bacteria, pseudomonads, yeasts and moulds
pre-dominated during storage in all the analysed samples It is evident
from this study that CGC coating was more effective in reducing microbial counts than other treatments (Table 1) In any of the studied treatments, the psychrophilic bacteria counts increased less than two orders during the entire storage period All samples
Table 1
Effect of CGC coating on microbial counts (log 10 cfu g 1 ) change of shiitake mushrooms stored at 4 °C for 16 days a , b , c
Mesophilic
4.10 ± 0.08 aE
4.16 ± 0.04 aE
4.13 ± 0.11 aD
5.25 ± 0.17 aC
4.76 ± 0.16 bC
4.57 ± 0.08 cC
6.10 ± 0.05 aB
5.12 ± 0.07 cB
4.84 ± 0.14 dB
6.82 ± 0.13 aA
5.47 ± 0.14 cA
5.22 ± 0.24 dA
Psychrophilic
2.12 ± 0.05 abC
2.27 ± 0.16 aBC
1.98 ± 0.18 bC
2.59 ± 0.09 aB
2.58 ± 0.12 aB
2.47 ± 0.09 aB
2.92 ± 0.14 aAB
2.90 ± 0.09 aAB
2.87 ± 0.10 aAB
3.20 ± 0.24 aA
3.17 ± 0.14 aA
3.18 ± 0.17 aA
Pseudomonad
5.35 ± 0.06 aE
5.34 ± 0.13 aE
5.38 ± 0.07 aE
6.11 ± 0.08 bD
5.75 ± 0.22 bD
5.57 ± 0.14 cD
7.21 ± 0.10 aC
6.34 ± 0.18 cC
5.74 ± 0.27 dC
7.90 ± 0.07 aB
6.68 ± 0.27 bB
6.12 ± 0.24 cB
Yeasts and moulds
3.82 ± 0.07 aE
3.86 ± 0.05 aE
3.80 ± 0.13 aE
4.36 ± 0.13 aD
4.11 ± 0.09 bD
4.15 ± 0.06 bD
6.68 ± 0.14 aA
5.76 ± 0.21 bA
5.05 ± 0.26 cA a
Mean of three replications ± standard deviation.
b
Means in same row with different small letters are significantly different (p < 0.05).
c
Means in same column with different capital letters are significantly different (p < 0.05).
Table 2
Effect of CGC coating on sensory attributes change of shiitake mushrooms stored at 4 °C for 16 days a , b , c
Off-odour
1.51 ± 0.06 abD
1.54 ± 0.03 abD
1.41 ± 0.05 bD
2.20 ± 0.11 bC
2.16 ± 0.07 bC
2.19 ± 0.08 bC
5.05 ± 0.07 bB
4.22 ± 0.08 cB
3.83 ± 0.12 dB
6.93 ± 0.15 bA
5.82 ± 0.05 cA
5.52 ± 0.16 dA
Gills colour
1.23 ± 0.05 bcD
1.10 ± 0.06 cD
1.87 ± 0.11 aD
2.81 ± 0.12 abC
2.55 ± 0.06 cC
2.63 ± 0.08 bcC
Gill uniformity
5.17 ± 0.12 bC
5.72 ± 0.12 aC
5.87 ± 0.08 aC
4.32 ± 0.10 cD
5.08 ± 0.05 bD
5.32 ± 0.13 aD
Cap uniformity
8.44 ± 0.26 bA
8.60 ± 0.07 bA
8.85 ± 0.30 aA
7.42 ± 0.20 bB
7.83 ± 0.21 aB
7.91 ± 0.21 aB
6.57 ± 0.27 bC
7.21 ± 0.14 aC
7.16 ± 0.17 aC
5.22 ± 0.16 bD
6.21 ± 0.15 aD
6.11 ± 0.15 aD
Dark zones
1.17 ± 0.04 aD
0.88 ± 0.07 bD
1.07 ± 0.02 aD
2.22 ± 0.07 aC
1.73 ± 0.11 bC
1.53 ± 0.04 cC
3.33 ± 0.12 bB
2.55 ± 0.02 cB
2.12 ± 0.10 dB
a Mean of three replications ± standard deviation.
b
Means in same row with different small letters are significantly different (p < 0.05).
c
Means in same column with different capital letters are significantly different (p < 0.05).
Trang 6had counts below 104cfu g1 This contamination level suggests
that shiitake mushrooms in the studied coating conditions did
not favour the development of this type of bacteria Mushrooms
from the control treatment exhibited tiny brown spots on Day 4
that developed into dark zones, characteristic of Pseudomonas
spoilage by Day 8 Mushrooms were highly decayed at this point
and the end of shelf-life was due to microbial spoilage The
CGC-coated samples did not exhibit these characteristics of microbial
degradation even on Day 12 The organisms usually responsible
for spoilage of mushrooms are gram-negative, psychrotrophic
bac-teria, particularly belonging to the Pseudomonadacae family,
be-cause of contamination of the product from compost Kanatt
similar to chitosan against E coli, Pseudomonas, S aureus and B
cer-eus, the common food spoilage and pathogenic bacteria However,
in our experiment, we found that CGC exhibited superior
antimi-crobial activity to chitosan in coating shiitake mushrooms during
storage Therefore, microbial degradation resulting in changes such
as browning and softening was clearly delayed in CGC-coated
samples
3.6 Effect of CGC coating on sensory attributes
As expected, mushroom off-odour, gill colour, gill uniformity,
cap uniformity and dark zones significantly (p < 0.05) changed with
storage time, supporting the validity of using these parameters as
indicators of mushroom deterioration Average values for the
sen-sory attributes are shown inTable 2 Off-odour intensity
signifi-cantly increased after 8 days of storage in control samples The
colour of mushroom gills gradually became browner and less
uni-form with time for all the evaluated conditions The gills of control
mushrooms showed a colour intensity of 5.4 and uniformity near to
5.0 at the 12th day of storage However, the gills of chitosan coated
mushrooms reached these intensities after 4 days of storage A
bet-ter trend was observed for the uniformity of the cap surface and the
presence of dark stains on the cap in CGC-coated samples These
re-sults suggest that CGC and chitosan coating were more effective in
retarding mushroom sensory deterioration The browning of
mush-rooms is attributed to the action of polyphenol oxidase (PPO) and
the action of bacteria and mould on the mushroom tissue CGC
and chitosan coating cause reduction of spoilage organisms, such
as Pseudomonas, responsible for oxidation of phenolic compounds
to form brown-coloured melanins, and thus prevent the formation
of brown patches, hence improving the appearance and colour
Considering the development of the evaluated sensory attributes,
mushrooms coated with CGC showed the lowest deterioration rate,
followed by those coated with chitosan and finally those coated
with glucose and the control treatment Although CGC coating
showed negative effect on gills colour because of the browning
col-our itself, it could maintain sensory characteristics of shiitake
mushrooms for the longest time This could be related to the fact
that CGC had superior antioxidant and antimicrobial activity as
compared to chitosan or glucose
4 Conclusions
Our research showed that the senescence inhibition of
cold-stored shiitake mushroom by the CGC coating treatment involved
the maintenance of tissue firmness and sensory quality, inhibition
of respiration rate, and reduction of microbial counts compared
with the control In addition, CGC coating also delayed changes
in the ascorbic acid and soluble solids concentration during the
storage period These results suggest that CGC is promising as an
edible coating to be used for maintaining shiitake mushroom quality and extending its postharvest life
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