24
Table 8. Mean water activity a
w
of dried bamboo shoots [Na
2
S
2
O
5
aq] used in blanching
Mean a
w
50 ⁰C
60 ⁰C
70 ⁰C
0.411 ± 0.006 0.401 ± 0.004
0.405 ± 0.012 0.5
0.406 ± 0.001 0.404 ± 0.007
0.402 ± 0.018 1.0
0.406 ± 0.009 0.406 ± 0.001
0.403 ± 0.012 1.5
0.407 ± 0.006 0.406 ± 0.002
0.398 ± 0.009
Figure 12. Chemical and biochemical rates as functions of water activity Chen 2008
As shown in Figure 12, it is well known that each microorganism has a critical a
w
below which growth can not occur. Pathogenic bacteria can not grow at a
w
0.86, yeast and moulds are more tolerant and usually can not grow at a
w
0.62. The dried bamboo shoots had low water activity, ranged from 0.398 to 0.411, thus the microorganism deteriorations
can not occur within the products. Yet, it should be noted that at the level of a
w
, probability of non-enzymatic browning, enzyme activity, and loss of lysine increases.
4.2.2 Drying Time
Drying temperature highly significant affected p0.01 drying time, referred to Appendix 6. Drying time decreased with increase in drying temperature, as shown in Figure
12. When a wet sample is contacted with a flowing of heated air, heat transfer occurs on surface of the sample, free water within the sample moves to the surface and evaporates due
to latent heat of vaporization. Water vapour formed diffuses through a boundary air and is carried away from the drying surface by the moving air. It forms a region of lower water
vapor pressure on the surface of sample. At higher drying temperature, a higher water vapor pressure gradient is formed between the sample surface and the heated air, thus results a
25 larger driving force for water removal from the sample that lead to a higher rate of moisture
evaporation in the sample Francis 2000; Cabrera 2008.
Figure 13. Mean drying time of untreated and treated bamboo shoots
Result, as seen in Figure 13, also showed that the untreated sample needed longer dying time than the pretreated one. A role of blanching in inceasing drying rate was observed
as decrease in drying time. Hot water blanching has been reported that it increases drying rates of green peppers, banana slices, and oyster mushroom as decrease in drying time
Kaymak-Ertekin 2002; Taiwo and Adeyemi 2009; Srivastava et al. 2009. On the contrary, a different result was reported by Trongpanich et al. 1993. The authors observed that
unblanched sample had higher than blanched sample.
4.2.3 Color of Dried and Rehydrated Bamboo Shoots
Dried bamboo shoots produced were shown in Figure 14 and results of color measurements of dried bamboo shoots were shown in Table 9. The results showed that there
was no significant effect p0.05 of interaction between concentration of sodium metabisulfite used in blanching solution and drying temperature on L, a, and b values,
while each factor had significant effect on some parameters, referred to Appendix 16, Appendix 17, and Appendix 18. The L value of the sample blanched in 1.5 Na
2
S
2
O
5
solution was highly significant higher p0.01 than the sample blanched in water without
addition of Na
2
S
2
O
5
and blanched in 0.5 Na
2
S
2
O
5
solution, while it was not significantly different p0.05 with the sample blanched in 1.0 Na
2
S
2
O
5
solution. On the contrary, L value was not significantly affected p0.05 by drying temperature. Moreover, the similar
result was found on a value. The a value was significantly affected p0.05 by concentration of sodium metabisulfite, yet it was not significantly affected p0.05 by
drying temperature. In contrast to b value, both factors highly significant affected b value of dried
bamboo shoots. The sample dried at 70 ⁰C
had the lowest p0.01 b value as well as the unblanched sample. Mizobutsi et al. 2010 reported that peroxidase of licthi pericarp became
Untreated sample Treated sample
26 completely inactive when maintained at 90
⁰C for 10 minutes or 1 minutes at 100 ⁰C, and PPO activity was inactivated after 10 minutes at 60
⁰C. However, the application of heat treatment was inappropriate to control the browning reaction due to thermal stability of both
enzymes Shin et al. 1997; Jiang 2001. Thus, it is required to apply antibrowning agents to prevent browning reaction during drying and to keep light color of product. Moreover, sulfur
dioxide as an active form of sodium metabisulfite may aid retention of ascorbic acid and carotene
Sudheer and Indira 2007; Brennan and Grandison 2012. The results also showed
that t he b value of the sample blanched in 1.5 Na
2
S
2
O
5
solution was highly significantly
higher p0.01 than the sample blanched in water without addition of Na
2
S
2
O
5
and blanched in 0.5 Na
2
S
2
O
5
solution, while it was not significantly different p0.05 with the sample blanched in 1.0 Na
2
S
2
O
5
solution. It can be suggested that blanching the sample in
1.0 Na
2
S
2
O
5
solution prior to drying was appropriate to keep light-yellow color of dried bamboo shoots by retarding activity of browning enzymes and loss of carotene contained
within the sample.
Table 9. Mean L, a, b values of dried bamboo shoots
[Na
2
S
2
O
5
aq] used in
blanching L
a b
50 ⁰C
60 ⁰C
70 ⁰C
50 ⁰C
60 ⁰C
70 ⁰C
50 ⁰C
60 ⁰C
70 ⁰C
39.31 ± 4.57
Aa
44.13 ± 9.23
Aa
36.40 ± 12.91
Aa
12.22 ± 5.26
Aa
11.42 ± 1.77
Aa
13.45 ± 6.66
Aa
18.53 ± 1.29
Aa
16.30 ± 6.36
Aa
10.75 ± 4.27
Ba
0.5 43.98 ±
2.89
Aa
45.57 ± 7.83
Aa
35.64 ± 1.58
Aa
8.83 ± 2.04
Aa
9.59 ± 2.40
Aa
12.73 ± 3.32
Aa
23.31 ± 1.60
Ab
22.26 ± 5.14
Ab
14.66 ± 4.47
Bb
1.0 52.92 ±
6.17
Ab
54.90 ± 3.85
Ab
48.86 ± 3.21
Ab
10.20 ± 3.63
Aa
6.49 ± 0.26
Aa
9.89 ± 0.71
Aa
28.56 ± 0.94
Ac
24.87 ± 2.62
Ac
20.68 ± 1.43
Bc
1.5 55.02 ±
8.11
Ab
55.95 ± 9.19
Ab
53.08 ± 1.25
Ab
5.80 ± 1.75
Aa
6.74 ± 0.03
Aa
9.42 ± 3.34
Aa
28.50 ± 1.72
Ac
26.73 ± 6.23
Ac
25.89 ± 2.14
Bc
Values in the same row having the same letters A, B for each parameter were not significantly different p0.05. Values in the same column having the same letters a, b, c for each parameter were not significantly different p0.05.
a
b
c
Figure 14. Dried bamboo shoots left-right: blanched in 0, 0.5, 1.0, 1.5 of Na
2
S
2
O
5
solution; a dried at 50 ⁰C, b dried at 60 ⁰C, and c dried at 70 ⁰C
27 Rehydrated bamboo shoots produced were shown in Figure 15 and results of color
measurements of rehydrated bamboo shoots were shown in Table 10. Referred to Appendix 22, Appendix 23, and Appendix 24, both concentration of sodium metabisulfite and drying
temperature highly significant affected p0.01 L and a values. The L value of the sample blanched in 1.5 Na
2
S
2
O
5
solution was highly significantly higher p0.01 than the
sample blanched in water without addition of Na
2
S
2
O
5
and blanched in 0.5 Na
2
S
2
O
5
solution, while it was not significantly different p0.05 with the sample blanched in 1.0 Na
2
S
2
O
5
solution. In the other side, the lowest L value was found in sample dried at 70 ⁰C.
Table 10. Mean L, a, b values of rehydrated bamboo shoots
[Na
2
S
2
O
5
aq] used in
blanching L
a b
50 ⁰C
60 ⁰C
70 ⁰C
50 ⁰C
60 ⁰C
70 ⁰C
50 ⁰C
60 ⁰C
70 ⁰C
69.05 ± 4.39
Aa
60.80 ± 14.96
Aa
49.45 ± 1.14
Ba
4.39 ± 2.32
Aa
5.13 ± 2.03
Aa
11.91 ± 1.23
Aa
21.69 ± 2.70
Aa
24.68 ± 8.20
Aa
26.55 ± 4.84
Aa
0.5 71.44 ±
4.58
Aa
69.32 ± 9.92
Aa
53.00 ± 2.16
Ba
2.24 ± 0.92
Ab
5.15 ± 3.39
Ab
7.13 ± 0.20
Ab
22.92 ± 4.37
Aa
27.04 ± 5.32
Aa
22.90 ± 2.49
Aa
1.0 74.81 ±
5.98
Ab
74.38 ± 2.15
Ab
73.11 ± 9.41
Bb
1.92 ± 0.52
Ab
1.76 ± 0.36
Ab
7.10 ± 1.24
Ab
22.75 ± 4.89
Aa
27.54 ± 6.10
Aa
29.82 ± 1.20
Aa
1.5 79.81 ±
2.73
Ab
76.08 ± 4.39
Ab
78.21 ± 5.92
Bb
1.45 ± 0.75
Ac
1.78 ± 0.60
Ac
1.09 ± 0.73
Ac
28.98 ± 6.84
Aa
26.55 ± 6.22
Aa
30.31 ± 3.57
Aa
Values in the same row having the same letters A, B for each parameter were not significantly different p0.05. Values in the same column having the same letters a, b, c for each parameter were not significantly different
p0.05.
In contrast to result of L value, the sample blanched in 1.5 Na
2
S
2
O
5
solution had
the lowest p0.01 a value, while the highest L value was found in sample dried at 70 ⁰C.
A different tendency was found on b value of rehydrated sample. It was not significantly
affected p0.05 by both factors, the concentration of sodium metabisulfite and the drying temperature. Generally, rehydrated bamboo shoots had higher L value, lower a value, and
higher b value when compared with the dried one.
a b
c
Figure 15. Rehydrated bamboo shoots left-right: blanched in 0, 0.5, 1.0, 1.5 of
Na
2
S
2
O
5
solution; a dried at 50 ⁰C, b dried at 60 ⁰C, and c dried at 70 ⁰C
4.2.4 Rehydration Ratio
