Changes in Moisture Content, Aw, pH, and Color
Moisture content of dried chili fish paste with/without 0.1% SB and stored in different packaging containers showed a slightly increasing trend (
Table 2). Moisture content of 17.45% at day 0 of storage continually increased as storage time increased and reached 19.25–20.65% at the 20th week of storage. The increase in moisture content results in the increase in A
w with increasing storage time (data not shown). Normally, high moisture content or high water activity plays a role to limit the shelf life of dried or semi-dried products. The standard of chili paste in Thailand [
12] regulates the moisture content of this product at not more than 20% (
w/
w); therefore, this parameter can be used for judging the shelf life of the product. Among all samples, PET had the highest rate of increasing moisture, and the moisture content reached 20% at 12th week of storage. Keller and Kouzes [
22] reported that the water permeability of PET at room temperature was 130 cm
3 × cm × cm
−2 × S
−1 × cm-Hg
−1, which was higher than PP and LLDPE (material for zip-lock bag) (35 and 68 cm
3 × cm × cm
−2 × S
−1 × cm-Hg
−1, respectively). The higher water permeability of plastic/material of packaging favored moisture migration from the environment through the packaging, resulting in the increase in moisture content of the product. It was also noted that dried chili paste added with 0.1% SB showed lower rate of moisture increase during storage compared with the samples without added SB. This corresponded well with the lower rate of A
w of those samples as well. At the 20th week of storage, samples having 0.1% SB, including PP+SB, PET+SB, and ZL+SB, had a moisture content of 19.78%, 19.25%, and 19.87%, respectively, which was in the range of the limitation of the standard. However, the samples without 0.1% SB, including PP, PET, and ZL, had a moisture content over 20% within weeks 14, 12, and 20, respectively. This indicated that dried chili fish paste without added 0.1% SB had a shelf life lower than 20 weeks at room temperature due to the excess moisture content.
The pH of dried chili fish paste, which was 5.72 at day 0, gradually decreased as storage time increased (data not shown). The pH dropped to 4.91–5.39 when stored for 20 weeks. The rate of pH change was similar with the changes in other characteristics. Samples with added 0.1% SB had a slower rate in quality change compared with the samples without preservatives at all periods of storage time. The decrease in pH may relate to the increase in total microbial populations (Table 4). During storage, some microorganisms can use carbon sources, small peptides, or amino acids as nutrients for growth and can produce acid as products, such as lactic acid bacteria [
23], resulting in the decrease in pH of the product. This phenomenon was also in agreement with the slower rate of pH decrease in samples without added preservatives. Normally, SB has antimicrobial properties preventing the growth of bacteria and mold [
3]. The inhibition of some microorganisms by SB may delay the production of acid by those microorganisms to some extent.
Changes in color of dried chili fish paste stored in different packaging containers during storage at room temperature were observed (
Table 3).
L*-value of all samples slightly decreased as storage time increased (
p < 0.05). The lightness in color decreased from 25.92–28.42 at day 0 to 19.63–24.08 at the 20th week of storage, indicating the significantly darker color of samples when the storage time was extended. The change in
a* -value or redness was slightly fluctuant throughout storage; however, most samples had no differences in
a* -value when stored for 20 weeks compared with day 0 (
p > 0.05). This indicated that the red color of the product, which mainly come from dried chili (used as 7.5% of overall ingredients), was not changed during storage for 5 months. However,
b*-value was in contrast. The
b*-value or yellowness of dried chili fish paste was obviously increased as storage time increased (
p < 0.05). The increase in
b*-value as well as the decrease in
L*-value revealed that the samples turned darker or browner in color. The decrease in moisture content during prolonged storage time may result in the darker or more intense color of the products. Change in color was also in agreement with the increase in browning index (
Figure 1). Pomsa et al. [
24] reported that the decrease in
L* -value of roasted chili paste mixed with mung bean hull was observed when storage time increased, indicating the darker color of chili-paste product when the storage time was extended. Normally, color is one of the most important factors affecting consumers’ acceptability. However, change in color of all samples in this study did not affect sensory scores (in the aspects of color-liking score) during storage for 20 weeks, as shown in Table 5.
Changes in Browning Index (A420)
Figure 1 reported the change in browning index of dried chili fish paste during storage at room temperature. Generally, browning index indicated the browning development in the final stage of the Maillard reaction [
25]. It was noted that the browning index of all chili-paste samples continually increased as storage time increased up to 20 weeks. The increase in browning index was in agreement with the changes in color (
Table 3). This change was also associated with the increase in moisture content (
Table 2) and water activity as well. The result indicated that extended storage time could favor Maillard browning reactions. Normally, water activity above 0.3 is known to cause non-enzymatic (Maillard) browning if the product is susceptible to such reactions [
19]. The rate of nonenzymatic reaction increases with increasing water activity, reaching a maximum at water activity (A
w) ranging from 0.6 to 0.75 [
19]. During storage, A
w of all samples increased from 0.72 (day 0) to 0.79 (week 20), which is suitable for this reaction. Moreover, the ingredients used to produce chili paste, especially carbonyl group from sugar or palm sugar, as well as carbonyl groups from grilled catfish can be well served as substrates for the Maillard reaction. In addition, the high relative humidity of the storage environment was therefore assumed to have influenced the rate of browning reactions of samples stored in different type of packaging/plastic. A relationship between color development, especially changing to darker or browner, resulting from Maillard reaction products, pH, temperature, and packaging material, has been demonstrated by much previous research [
19,
21,
26,
27].
Changes in Lipid Oxidation Products
Lipid oxidation products of dried chili fish paste were monitored throughout 20 weeks of room-temperature storage (
Figure 2 and
Figure 3). At day 0, PV of chili paste was 0.1762 mg/kg sample. PV of all samples increased as storage time increased (
p < 0.05), especially at the first period of storage. PV of most samples was constant or slightly decreased when extending the storage time except for the ZL+SB sample. PV of ZL+SB continually increased throughout 20 weeks and reached the maximum at 0.1994 mg cumene/kg sample. Normally, PV is a good indicator of lipid oxidation under normal conditions or room temperature [
28]. Peroxides are the initial reaction products of lipid oxidation and responsible for primary oxidation. However, the hydroperoxide subsequently breaks down and forms secondary oxidation products during storage. The decrease in PV in later weeks was well in accordance with the increase in TBARS value (
Figure 3). Dried chili fish paste had the TBARS value of 0.6502 mg MDA/kg sample at day 0 of storage. The increase of TBARS value as storage time increased was monitored in all samples (
p < 0.05). TBARS value of all samples reached the maximum level at 20th week of storage (1.0024–1.3067 mg MDA/kg sample). Changes in PV and TBARS indicated well that lipid oxidation occurs in all samples during storage. These compounds may cause bad characteristics, especially flavor and odor of the product, which may relate to the decrease in flavor-liking scores when extending the storage time of samples tested by panelists (Table 5). It was also observed that the rate of the increase in PV and TBARS value was governed more by preservative added than by the packaging container. Compared between samples stored in the same packaging container, higher lipid oxidation products were mostly found in samples with added 0.1% SB throughout the storage (
p < 0.05). At the end of storage, both PP+SB and PET+SB had significantly lower PV and TBARS than those without preservative. This indicated that adding 0.1% SB can retard lipid oxidation damages effectively, which may be associated with the limitation of some bacterial growth, especially lipase-producing bacteria, etc.