Effect of packaging methods and storage conditions on quality characteristics of flour product naan (2024)

Materials

The Xinjiang naan used in the study were procured from local market in Jinan, China. All chemicals and microbiological culture media were of analytical grade.

Packaging and storage of naan samples

Cool naan was selected, which the shape, size, thickness and weight were uniform. “naan” was packaged as follows: (1) ordinary plastic packaging (OPP), (2) vacuum packaging (VP) and (3) deoxygenation packaging (DP) for storage over 0–49days at room temperature 25, 4 and − 20°C, respectively. Outer packaging materials were polyamide/polyethylene (PA/PE) composite material. Three packaging methods and three kinds of storage methods were divided into 9 groups and each group was triplicate. Samples were analyzed every 2days for sensory evaluation, acid value, moisture content and microbial count.

Proximate analysis

The lipid content of naan was determined by the method of by the Association of Official Analytical Chemist (AOAC 2012) method. The total crude protein was determined by the micro Kjeldahl method (AOAC 2012). The ash content was measured by total carbonization of the sample using a muffle furnace at 550°C for 6h to incinerate until the sample was free of carbon particles (AOAC 2012). Moisture was determined in samples by the Association of Official Analytical Chemist (AOAC 2012) method. The triplicates for each sample were carried out.

Acid value determination

Acid values of naan samples were measured according to AOCS methods Da 14–48 (AOCS 2006). 100g of naan flour crushed was placed in 500mL Erlenmeyer flask, 300mL of petroleum ether was added, and sealed with plastic wrap. The mixture was leached for about 12h at room temperature. Then the supernatant was obtained by filtering with a fast filter paper. The solvent was removed by rotary evaporator. The separated fat for acid value determination was gained. 3g of oil samples was placed in 250mL conical flask with adding 100mL of neutral mixture of ether and ethanol with the ratio of 2: 1 (v/v), which was shaken to make the grease fully dissolved. 2 or 3 drops of phenolphthalein indicator was added. Finally, 0.1M KOH solution was used to titrate the sample until the solution appeared reddish and did not fade within 30s, which was as the titration end point. The volume of consumed KOH standard solution was recorded. Acid value (AV) of naan was calculated by the following Eq.(1).

where V1 and V2 were the volume of KOH standard solution and the blank, respectively; C was the concentration in moles per liter of the KOH standard solution; 56.1 was number of milligrams of KOH per 1mL of KOH standard solution; m was the mass in grams of the test sample.

Microbiological analysis

Microbiological changes of naan during storage were analyzed by Khoshakhlagh et al. (2014) method with slight modifications. A 25g of the sample was hom*ogenized in a sterile blender with 225mL of sterile peptone water (0.1% sterile peptone, w/v) containing sterilized 0.9% NaCl for 1min. The sample solution was diluted according to 10⁻¹. Then, 1.0mL dilution and 15mL plate count agar (PCA) were poured onto Petri dishes (9cm diameter). Bacterial colonies were counted after the plates were cultured at 37°C for 48 ± 2h. The microbiological numbers in the “naan” samples were expressed as log 10 colony forming units (CFU) per gram. The triplicates for each sample were carried out.

Sensory evaluation

Sensory determinations on naan were performed by the sensory assessment scheme of Licciardello et al. (2017) and Özogul et al. (2004) with small modifications. The samples were analyzed everyday during storage. Table1 shows the index of sensory evaluation. Quantitative Descriptive Sensory Analysis of each sample was carried out by a minimum of 6 trained members in the conditions described in a work of Licciardello et al. (2017). The list of sensory terms included descriptors of appearance (color), texture characteristics (hard and flexible), and odor (smell and taste). Each assessment was carried out by a minimum of six trained panellists. When a demerit score was 8, the acceptable quality was found. Triplicate samples from each of the storage conditions were determined at regular intervals.

Ultraviolet and microwave treated “naan” before packaging

In order to extend the shelf life of naan, it was treated by the ultraviolet (UV) and microwave (MW) treatment before DP. For the UV-treated naan preparation, the cool naan was divided into five groups (each group 200g), which were irradiated by for 0, 5, 10, 20 and 30min at room temperature, respectively (Jongyingcharoen and Cheevitsopon 2016). After the UV (power 30W) treatment, the naan was sealed by DP in the radiation chamber. After the packaging process was completed to ensure the aseptic condition inside the chamber, the UV lamp was turned off. For the MW-treated naan preparation, the cool naan was divided into five groups (each group 200g), which were treated by MW (power 500W) for 0, 2, 3, 4 and 5min at 50°C. Then, the MW treated naan were packaged and stored at 25°C according to methods of 2.2 part under aseptic operation. To investigate the quality changes of UV and MW treated naan during storage, the sensory evaluation, acid value, moisture content and microbial count were determined throughout its shelf life. All experiments were triplicate.

Statistical analysis

For experiment data analysis, the Student t test, standard deviation and coefficient of variance were used. The significance of the difference was defined as p < 0.05. Statistical comparison was based on three samples for each treatment.

Proximate analysis

Table2 shows the main chemical components, standard deviation and coefficient of variation of the naan. You (2017) reported that the chemical composition of naan was 11.98–13.96% moisture, 12.9–137% protein and 1.5–2.3% fat. In this study, the chemical composition was 20.89% moisture, 8.12% crude protein, 15.07% fat and 1.58% ash. The variation in the chemical composition of naan was closely related flour type, man-made practices, geographical changes, ingredient changes, and different process equipment (You 2017). Variation chemical components would result in the changes of naan taste, flavor and preservation quality (You 2017).

Sensory evaluation

Figure1 shows the pattern of increase in the demerit score from day 0 to 10days at 25°C, for naan kept under three different storage conditions. As shown in Fig.1, the rate of increase of demerit points was fairly linear with extension of storage time in three cases, which indicated that the quality of naan would decline. There were significant differences (p < 0.05) in the level of demerit points between naan in ordinary plastic packaging (OPP) and vacuum packaging (VP), particularly in deoxygenation packaging (DP). The observed shelf life of naan was found to be 4days (demerit score: 16) in VP, 6days (demerit score: 16) in OPP and 10days (demerit score: 16) for naan stored in DP. In general, some studies have reported that the VP could increase the shelf life of food products during storage (Clingman and Hooper 1986). However, in this study, it was interesting noted that VP did significantly extend the sensory shelf life of naan as compared to storage in OPP (Fig.1). The results were not agreement with the investigation of references. It was assumed that the vacuum packaging could make the porous fermentation “naan” products lost sponge-like structure by extrusion, which would exclude the internal air of naan and exude oil. So, the rancidity and mildew of nana with forming a dense and hardened texture would be accelerated. Özogul et al. (2004) observed that the sensory quality of sardines deteriorated faster in vacuum packages than modified atmosphere packages. Briefly, sensory attributes of naan were significantly affected by the packaging methods and storage time at 25°C (p < 0.05).

Moisture content

A most sensitive quality factor during bakery product storage is the moisture content, which is significantly affected by storage conditions (Novotni et al. 2011). Figure2 shows the effect of temperature storage time on moisture content of naan in OPP, VP and DP. The moisture content of naan in different packaging methods stored at 4 and − 20°C during days 48 was significantly lower (p < 0.05; Fig.2a, b) compared with initial moisture content. It could be attributed that the water of naan in freezing and cold storage process was slowly sublimated, which would lead to the loss of moisture. The moisture content in DP at 25°C was not significantly affected through storage time (p > 0.05) (Fig.2c). The moisture content in VP at 25°C during days 48 was slight lower compared with initial moisture content, while in OPP was higher (Fig.2c). In addition, the moisture content during storage at − 20°C for days 48 was significantly lower than other temperature (p < 0.05). These results indicated that packaging methods and storage temperature had the major effects on naan moisture loss. On the other hand, the effect of the difference between water activity of the naan and relative humidity of the ambient atmosphere was not noticeable, which was not accordance with Novotni et al. (2011) finding. The reasons might be the difference texture between naan and bread.

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Fig.2

Changes of moisture content of naan stored in OPP, VP and DP at 4 (a), − 20 (b) and 25°C (c), respectively; Changes of acid values of naan stored in OPP, VP and DP at 4 (d), − 20 (e) and 25°C (f), respectively; Changes of surface appearance of naan stored in OPP (h), VP (i) and DP (j) at 25°C, respectively; Changes of microbiological count (MC) of naan with different packaging methods during storage at 25 (k), 4 (l) and − 20°C (m), respectively (OPP ordinary plastic packaging, VP vacuum packaging, DP deoxygenation packaging). Each point was shown by the mean value of three determinations for each sample. Bars represented the standard deviation

Acid value

As shown in Fig.2d–f, the acid values of samples in different packaging methods, storage temperature and time were detected. The acid values at 25°C increased rapidly with the increase of storage days. The acid values of naan stored for 12days has exceeded the consumption standard (5mgKOHg⁻¹) (Guo et al. 2009), and reached 64.43, 27.44 and 21.27mgg⁻¹ in OPP, VP and DP samples after 18days. The result showed the increase of acid values in naan DP was significantly smaller than in OPP and VP (p < 0.05). It was due to the addition of the deoxidizer, which consumed part of the oxygen in the bag and reduced the oxidation of the oil. The acid values of naan in VP still increased during storage time at 25°C, which was probably attributed that the part air in the pores of naan caused the oxidation of the oil.

The acid values in OPP, VP and DP samples slightly increased for 38days of storage at 4°C, which was maintained to be the range from 0.2 to 0.9mgg⁻¹. The acid values of naan during storage exceeded the standard at 4°C for 45days. Meanwhile, there was significant difference among three packaging method (p < 0.05). The acid value of naan in DP was the lowest, while in VP was the largest. However, the rancidity was very slow at − 20°C, and there was no significant difference between the acid value of OPP, VP and DP samples (p > 0.05).

Microbiological assessment

The bakery products presented high initial microbiological quality, considering the microbiological maximum limit (3logCFUg⁻¹) as recommended by the International Commission on Microbiological Specifications for Foods (ICMSF 1998). This limit was also considered for total aerobic mesophilic count as a microbiological criterion to indicate the shelf life of naan during the storage time.

Figure2g–i shows microbiological data of naan packaged under different storage conditions. Naan in OPP, VP and DP at 25°C showed the surface mold growth at day fourth, day sixth and tenth of storage, respectively (Fig.2j–l). Total microbiological count (MC) in naan at day fourth started to show gradual increase, reaching levels of 2.25logCFUg⁻¹ in OPP samples, 1.99logCFUg⁻¹ in DP samples and 3.04logCFUg⁻¹ in VP samples for 5days, respectively (Fig.2j). Significant different of MC was found among OPP, VP and DP (p < 0.05). MC in VP samples was significantly higher than in OPP and DP (p < 0.05), which had exceed the standard of microbiological maximum limit (ICMSF 1998). In general, the shelf life of food by vacuum packing was extended by taking off the oxygen, which could inhibit the growth of bacterial (Kung et al. 2017). It was interesting that the results in this study were not agreement with the references. It was related to oxygen production in air by microorganisms such as lactic acid bacteria, bacillus and yeast etc., which would cause the dominance of anaerobic microorganisms over the aerobes (Khoshakhlagh et al. 2014). The reason in detail need to be further studied. The MC values in OPP, VP and DP reached the 3.83, 4.39 and 3.31logCFUg⁻¹ on the 9th day of storage, respectively. It suggested that the storage time of naan at 25°C in OPP, VP and DP was about 5, 3 and 5days, respectively. The DP was more helpful to extend the shelf life of naan.

Figure2k, l show MC for naan stored in OPP, VP and DP during 40days at 4 and − 20°C. Comparing these results with the ones from Fig.2k, l, it could be observed that the MC of naan in DP at 4°C during storage 11days was not determined, while MC in OPP and VP started to increase, reaching 1.47 and 1.00logCFUg⁻¹, respectively. The MC of naan from three packages at − 20°C was not determined on the 16th day of storage. It was noted that the MC of naan in three packages during storage 40days at − 20°C still remained under 3logCFUg⁻¹. On the other hand, the results showed MC from naan at stored 4 and − 20°C lower than those from naan at 25°C. These results were supported by the study of Leuschner et al. (1997), who observed that the microbial growth of Irish brown soda bread at 5°C could be observed after 15days. Karaoglu et al. (2005) also found that the surface mold growth at 9th of storage at 7°C and did not show surface mold growth during 28days of storage at 1°C. The analysis of microorganism growth indicated that packaging and storage conditions would affect the shelf life of naan. Therefore, the lower temperature could retard the growth of microbial.

Effect of ultraviolet (UV) and microwave (MW) treatment on shelf life of naan

Changes in sensory values, moisture content, acid values during storage of UV and MW treated naan are showed in Fig.3. In the following days of storage time, the demerit score of UV and MV treated nana with different time was increased significantly (p < 0.05; Fig.3a, d). The demerit score 16 of UV-treated naan was found to be 12days for UV 5min, 17days for UV 10min, 21days for UV 20min and 23days for UV 30min stored in DP, respectively. While the demerit score 16 of MW-treated naan was found to be 13days for MW 2min, 14days for MW 3min, 14days for MW 4min and 19days for MW 5min stored in DP, respectively. The results indicated that UV and MW treatment could extend the sensory quality of naan compared to the control naan (Fig.1). It was attributed that the UV and MW radiation could effectively inhibit the microorganism growth on the naan surface (Auksornsri et al. 2017; Roig-Sagués et al. 2018). Sensory evaluation showed that the MW treated naan had better sensory characteristics compared to the UV treated ones. Some references have reported that the longer time at lower power levels of microwave would preferably not damage the organoleptical quality of the final product (Valero et al. 2014).

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Fig.3

The changes in sensory values, moisture content, acid values of UV (a–c, respectively) and MV (d–f, respectively) treated naan with DP during the storage period at 25°C, respectively. Each point was shown by the mean value of three determinations for each sample. Bars represented the standard deviation. DP deoxygenation packaging, UV ultraviolet, MW microwave

Figure3b, e show the effect of UV and MW treatment on moisture content of naan. As shown Fig.3b, e, the moisture content was not significantly (p > 0.05) affected by UV and MW. So, the effects of UV and MW on moisture content were not noticeable.

Changes in acid values are shown in all UV and MW treatment with different time (Fig.3c, f). It could be observed that the initial acid value of UV and MW treated naan with different time were almost the same comparing to the control (Fig.3c, f). The acid values of UV and MW treated naan with different time for 7days were still below 3mgKOHg⁻¹. The acid values of naan submitted to DP + UV and DP + MW began to significantly increase from the 12th day of storage compared to the control naan (p < 0.05). And there was also significantly difference for the increase of acid values between UV and MW treated naan (p < 0.05; Fig.3c, f), the UV treated naan was higher. While the acid value with UV treatment for 30min at 12th day of storage was the lowest, which was lower than 3mgKOHg⁻¹, the acid values of samples with UV treatment for 5 and 10min became higher than 3mgKOHg⁻¹ (Fig.3c). For MW treated naan, the lowest acid value was for 2min at 12th day of storage, which was lower than 1mgKOHg⁻¹, the acid values of samples with MW treatment for 3 and 4min were lower than 3mgKOHg⁻¹, for 5min became higher than 5mgKOHg⁻¹ (Fig.3f). During storage time from the 15th day to 46th day, the acid values of UV and MW treated naan with different time were greatly higher than 3mgKOHg⁻¹, especially the acid value for UV treatment for 30min at 46th day of storage was the highest (97mgKOHg⁻¹), while the acid value for MV treatment for 5min was 74.29mgKOHg⁻¹. These indicated that the ultraviolet and microwave radiation could trigger free radicals, prompting hydroperoxide decomposition only at the 12th of naan storage and the quality of naan. With extending the UV and MW radiation time, the acid values would become more and more lager, which was due to be related to oxygen solubility and absorption in naan (Jongyingcharoen and Cheevitsopon 2016). There were the significant differences in acid values (p < 0.05) for UV and MW treated samples, throughout storage at 25°C. These results indicated that MW treatment method by using a lower process time could better prolong the shelf life of naan.

Table3 shows the effect of UV and MW treatments on the MC of naan.MC of naan with UV radiation for 5min and MW radiation for 2 and 3min at 25°C during the 3th day of storage did not exhibit significant changes (p > 0.05; Table3). For the UV and MW treated naan, a longer UV and MW exposure led to a greater reduction of MC (Jongyingcharoen and Cheevitsopon 2016; Benlloch-Tinoco et al. 2014). It was in agreement with the work of Tremarin et al. (2017) and Benlloch-Tinoco et al. (2014). This was attributed that the ultraviolet and microwave radiation could trigger free radicals, prompting hydroperoxide decomposition (Iwaguch et al. 2002). The greatest differences of MC among these treatments were observed at day 3 of storage. However, MC increased in all treatments during storage, which was significant of control naan (p < 0.05). The reason was probably that the UV and MW treatment reduced significantly the MC of naan surface or inside. Comparing to control naan (1.97logCFUg⁻¹) at day 3 of storage, the number of microorganism of UV treatment for 20min and MW treatment for 3min (p < 0.05) reduced by 1.48 and 1.34CFUg⁻¹, respectively. For UV treated naan for 20 and 30min and MW treated naan for 4 and 5min, the MC of were undetectable, respectively. After at the 8th day storage, the MC of naan by using UV and MW treatment was significantly lower compared to control (p < 0.05; Table3). But there was not significant decrease among the UV and MW treated naan (p > 0.05; Table3). These results suggested that the UV and MW processing was much more effective in destroying the growth of microorganism and improved naan quality (Jongyingcharoen and Cheevitsopon 2016; Iwaguch et al. 2002; Valero et al. 2014), especially MW treatment.

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Xiaoyan Zhao and Han Sun have contributed equally to this work.

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