Effect of pretreatment on physicochemical, microbiological, and aflatoxin quality of solar sliced dried ginger (Zingiber officinale Roscoe) rhizome

Abstract Pretreatment of fruit and vegetables is necessary to reduce microbial proliferation and to preserve color of the produce. The effect of drying and pretreatment with potassium metabisulfite (KMBS) of concentrations 0.0%, 0.1%, 0.15%, 0.2%, and 1.0% and blanching at 100°C and 50°C using a tent‐like concrete solar (CSD) dryer as compared to open‐sun drying (OSD) of yellow ginger rhizomes was investigated using routine methods. The total color change and residual sulfur dioxide (SO2) were analyzed. KMBS reduced the yeast and mould load significantly from 3.6 × 104 ± 1.4 × 103 CFU/g in 0.0% (control) to <10 CFU/g in 1.0% KMBS and 100°C blanched fresh samples. Drying of the fresh samples for 5 days increased the yeast and mould load of all the treatments to as high as 1.15 × 105 ± 2.12 × 104 CFU/g for the 1.0% KMBS. Overall, the CSD had fewer microbial loads than the OSD but it was not significant. Aflatoxins and Salmonella sp. were not detected in any of the samples. The sulfur dioxide residue (SO2) for KMBS pretreated samples increased as the concentration of KMBS increased with the CSD retaining slightly higher amount than the OSD. The total color change index increased with increase in KMBS, and drying further increased the total color change index. On the whole, the blanched samples had the least color change among the pretreatments with 100°C CSD showing the least change among the dried samples.

Physical approaches may include various forms of thermal blanching (hot water, steam, super-heated steam etc) and nonthermal treatments (ultrasound and freezing) (Lizhen et al., 2017). Drying is the process of reducing the moisture content of an agricultural produce to an acceptable limit by the application of heat to reduce microbial proliferation and increase shelf-life of the produce (Sansaniwal & Kumar, 2015). Several drying methods are used for the drying of different produce Ding, An, Zhao, Guo, & Wang, 2012).
However, in tropical regions, open-sun drying is the most widely used (Fudholi, Ruslan, Othman, Zaharim, & Sopian, 2013) due to its economic viability (Sansaniwal & Kumar, 2015). Open-sun drying however has many disadvantages such as high microbial load, vast color changes to produce, and contamination by foreign matter such as sand and pieces of twigs and leaves from neighboring flora. Solar drying on the other hand could be a good alternative (Deshmukh, Varma, Yoo, & Wasewar, 2014;Fudholi et al., 2013). Studies have shown that different solar dryers with different designs affect the drying time and color of a particular produce. Ginger rhizome (Zingiber officinale Roscoe) is an important cash crop of the world that can be used in the fresh and dried form, both as spice and for its medicinal properties on a daily basis. It has been reportedly used, as a pain relief for arthritis, muscle soreness, chest pain, lower back pain, stomach pain, and menstrual pain (Gümü Say, Borazan, Ercal, & Demirkol, 2015;Shukla & Singh, 2007). The importance of this spice underscores it drying for year-round availability. Prasad, Prasad, and Vijay (2006)  in color showed that the solar dried ginger rhizome had less color change than the open-sun dried. To improve quality of produce, pretreatment of produce prior to drying or processing is considered as an alternative. This process helps to reduce microbial load, improves organoleptic properties, and has a synergetic effect in improving the shelf-life of the produce. Pretreatments using potassium metabisulfite (KMBS) and blanching have been reported to reduce microbial load and also preserve the color of ginger. Sulfite is used as an additive in the food industries for its numerous benefits such as inhibiting enzymatic and nonenzymatic browning, increasing antioxidant properties of the food product to prevent oxidative spoilage; (oxygen scavenger and reducing agent) and inhibiting the action of enzymes such as proteases, oxidases, peroxidases, as well as an antimicrobial and a fungistat. Sulfite also plasmolyzes cells which facilitate drying (Latapi & Barrett, 2006a, 2006bOnyemaobi & Williams, 2012;Sangwan, Kawatra, & Sehgal, 2012).
Campylobacter and Escherichia coli O157:H7 outbreaks have been reported as well as Salmonella sp. contamination of fresh herbs (Li et al., 2017). Research has also shown that fresh ginger contains high loads of moulds such as Aspergillus flavus (Ramesh & Santoshkumar, 2013;Singh, Gitansh, & Bhadauria, 2013;Toma & Abdulla, 2013) which has the potential to produce aflatoxins in the dried ginger product. This thus requires proper treatment and monitoring of aflatoxin development at different doses (Jeswal & Kumar, 2015;Rajarajan, Rajasekaran, & Devi, 2013). Even though studies have established the benefits of pretreatments and solar drying, it is produce specific in terms of the type and conditions of the solar dryer and pretreatment. The objective of this study was to assess the effect of pretreatment and drying using a tent-like concrete solar dryer on the physicochemical and microbiological quality of sliced solar dried ginger rhizome compared to open-sun drying.

| Source of raw materials
Eighty kilograms (80 kg) of fresh ginger rhizomes of 9 months maturity were purchased from an out-grower in the ginger-producing areas of the Ashanti region of Ghana. The fresh ginger rhizomes were transported to the laboratory packed in perforated nylon sack (aeration).

| Washing and pretreating the fresh ginger
Fresh raw ginger was soaked in water for 2 hr to remove the adhering sand/debris and washed vigorously three times each time with fresh water. The washed ginger rhizomes were sliced manually with a kitchen knife to a thickness of 3-5 mm. The sliced ginger was washed again in water. The sliced ginger was divided into seven parts; four parts were soaked separately in 0.1%, 0.15%, 0.2%, and 1.0% potassium metabisulfite (KMBS) concentration for 5 min. Another one part was soaked in water for 5 min serving as the control (0.0% KMBS concentration). The final two parts were soaked in water at 50°C and 100°C temperature for 5 min and 60 s, respectively. All the pretreated sliced ginger rhizomes were drained separately and dried.

| Drying of the fresh washed sliced ginger rhizome
The different pretreatments were drained separately and sampled for physicochemical, microbial and aflatoxin analysis as well as sulfur dioxide (SO 2 ) and color determination. The remaining sliced fresh ginger of KMBS and blanching pretreatment were divided into two.
One part was dried with the concrete solar dryer (CSD) and the other one part dried using the open sun (OSD) for 5 days and subjected to same analysis as in the fresh. Temperature and humidity of the environmental conditions were recorded using a digital temperaturehumidity data logger (HOBO pro v2 digital logger (Model U23-001)).

| Milling of ginger samples
The samples (fresh washed or dried ginger) were pulverized into paste and powder, respectively, using a Philips mill (HR 2113/05).

| Moisture content
The moisture content determination followed the method of AOAC (1980). Moisture cans were heated in the hot air oven for 1 hr, cooled in a desiccator and weighed. Five grams of the pulverized/milled fresh washed and dried ginger was weighed into the conditioned moisture dish and spread evenly. The moisture dish containing the sample was placed in the oven and heated for three (3) hours at 105°C. The dish containing the sample was placed in the desiccator to cool and then reweighed. The percentage moisture content was calculated as below:

| Total ash/acid-insoluble ash content
The analysis was done according to AOAC (1980). Two grams of the milled fresh washed or dried ginger sample was weighed into a previously preheated, cooled, and weighed crucible. The sample was then decarbonized on a Bunsen burner. The crucible containing the sample was placed in the furnace at 600°C for 3 hr. After 3 hr, the crucible was cooled in a desiccator and weighed again and difference in weight was calculated as total ash content. To the total ash obtained as described above, 20 ml of 10% hydrochloric acid (HCl) was added and boiled gently for 5-10 min on a hot plate. The sample was quantitatively transferred into a filter paper in a funnel on a beaker. The residue from the crucible was washed with boiling distilled water into the filter paper. This was washed severally with hot distilled water until the filtrate was free of chloride ions. The filter paper was dried in the oven at 105°C ± 2°C for 30 min. The filter paper containing the acid-insoluble ash was decarbonized on a Bunsen burner and placed in a furnace at 550°C for 1 hr. The crucible was cooled in a desiccator and weighed to the nearest 0.1 mg.

| Sulfur dioxide residue analysis
The sulfur dioxide residue (dry weight basis) was determined according to the modified method by Reith Williams (FAO, 1986; Owureku-Asare, Oduro, Saalia, Tortoe, & Ambrose, 2018). Twenty-five grams of fresh ginger or dried milled ginger was dispersed in 20 ml of water and diluted with 25 ml of dilute sodium hydroxide. It was allowed to stand for 5 min and diluted with 10 ml sulfuric acid. The mixture was allowed to stand for another 5 min, and 1 ml of starch indicator added. It was titrated with standard iodine solution to a permanent purple color.

| Quantitative estimation of fungal population
The fungal population of the fresh and dried ginger samples was determined using the spread plate method described in ISO 21527: (2008)-1&2. Thirty grams of the ginger sample was transferred into 500-ml conical flasks containing 270 ml of 0.1% peptone water as diluent. Each flask was shaken at 140 rpm for 20 min on an Orbital shaker. Serial dilution up to 1:10 9 was made, and 0.1 ml aliquots were inoculated in sterile Petri dishes containing already poured Dichloran Rose Bengal Chloramphenicol agar (DRBC) for fresh ginger and Dichloran-18-Glycerol agar (DG 18) for dried samples. All the DRBC and DG18 plates were incubated at 28 ± 2°C for 5-7 days.
Plates containing fungal colonies were counted, and the population expressed as CFU/g sample.

| Determination of Salmonella sp. contamination
This followed the procedure described in ISO 6579: (2002). Twentyfive grams of ground ginger (fresh or dried) was added to 225 ml of peptone water and shaken for 20 min. The culture was incubated at 37°C ± 1°C for 24 ± 3 hr. An aliquot of 0.1 ml of the culture was taken and added to 10 ml of Rappaport-Vassiliadis Soya Peptone (RSV) broth. The mixture was incubated at 41.5 ± 1°C for 24 ± 3 hr. This culture was then plated on bismuth sulfite agar at 37°C ± 1°C for 24 ± 3 hr.
Colonies with black centre and a lightly transparent zone of reddish color or pink with a darker centre were subcultured in nutri- with the maximum concentration of 12.5 ng/ml in methanol. Limit of detection and limit of quantification for total aflatoxin were established at 0.1 µg/kg, and a recovery rate of G1, G2, B1, and B2 was 64%, 81%, 65%, and 82%, respectively.

| Statistical analysis
The data for all analysis conducted in triplicates were subjected to analysis of variance (ANOVA), SAS ® JMP Pro 13 test at p ≤ .05 to determine significant differences between treatments for the samples. Multiple range analysis was done using Tukey's HSB.

| Effect of potassium metabisulfite and blanching on physicochemical quality of ginger
The results of the physicochemical analysis are shown in Table 1 The cosmetic quality of food is influenced by color which is a primary evaluating attribute accessed by consumers (Calvo, 2004).
The color in a food product during handling is influenced by naturally occurring pigments resulting from both enzymatic and nonenzymatic reactions (Marshall, Kim, & Wei, 2000). In this study, the change in color of the treated samples was 1. fresh ginger which may be attributed to the ability of the treatment regimens to delay or prevent the oxidative reaction initiated by the polyphenol oxidase in enzymatic browning as well as the type of drying (Ioannou & Ghoul, 2013). These results also showed that drying itself affected the color; the fresh 100°C with the initial total color change of 1.03 ± 0.39 had increased to 3.44 ± 0.30 for CSD and 5.87 ± 0.36 for OSD. Generally, the CSD samples had minimum total color change as compared to the OSD irrespective of the treatment employed. The 1.0% KMBS treated samples recorded a total color change index of 5.53 ± 0.15 for the CSD and 11.25 ± 0.77 for the OSD which was significant. This finding agrees with the findings of Blanching inhibits enzyme activity and, subsequently, browning.
This was evident in this study as blanched samples for the fresh and both drying methods except the 100°C OSD had less total color change as compared to the KMBS treated samples.

| Effect of potassium metabisulfite and blanching on microbial quality of ginger
The microbial quality of the samples is shown in Note: Different alphabets as superscript in the same column denote significance (p ≤ .05).
For the fresh samples, there was a significant general decrease in microbial load from 3.60 × 10 4 for the control (0.0% KMBS) to less than 10 for the 1.0% KMBS (Table 2). Even though the general trend was significant, this was not the trend for the extent of decrease between 0.1% and 0.15% KMBS concentration. The same pattern is observed for the solar dried samples with a general decrease in yeasts and moulds from a load of 9.10 × 10 4 (control-0.0% KMBS) to 4.75 × 10 4 CFU/g (1.0% KMBS) but not significant.
The same observation is made with the open-sun drying (OSD) showing a general decrease in microbial load from 2.95 × 10 6 CFU/g (control) to 1.15 × 10 5 CFU/g (1.0% KMBS). However, for OSD, there was a slight variation between 0.2% and 1.0% where 0.2% had a slightly lower microbial load (8.08 × 10 4 CFU/g) compared to 1.15 × 10 5 CFU/g for 1.0% KMBS. The general observation would have been to have a lower value for 1.0% and a higher value for 0.2%. However, statistical analysis indicates that the difference was not significant (p > .05) ( Table 2). In terms of load content, the fresh samples had relatively lower microbial load compared to the dried samples. Thus, the period of drying could have increased the microbial load of the sliced ginger irrespective of the drying and pretreatment methods used. The fresh control (0.0%) with an initial load of 3.60 × 10 4 increased to 9.10 × 10 4 CFU/g for the CSD which was not significant (p > .05) and to 2.05 × 10 6 CFU/g for the OSD which was significant (p < .05). Thus, the CSD had less load (9.10 × 10 4 CFU/g) compared to OSD (2.05 × 10 6 CFU/g). Blanching at 50°C for 5 min did not have a significant reduction in microbial load when compared to the control (0.0%). However, the 100°C blanched fresh ginger for 60 s recorded almost no growth (  (Table 2) (Latapi & Barrett, 2006a, 2006bOnyemaobi & Williams, 2012) and the high relative humidity of both drying methods (Appendix 1, Appendix 2, and Appendix 3). This is because studies have shown that a relative humidity higher than 70% allows growth of most moulds (Fact Sheet, 2019;Ibrahim, Rabah, Liman, & Ibrahim, 2011). This notwithstanding, the CSD samples showed less microbial population than the OSD samples. This agrees with the findings of Eze and Agbo (2011), and that solar dried samples had lower microbial load (2.18 × 10 5 CFU/g) than open-sun dried samples (2.6 × 10 5 CFU/g). However, the yeast and mould load recorded in this study were higher than reported by Eze and Agbo (2011). It is also higher than the mould load recorded by Addo (2005), and Ahene, Even though this work did not specifically identify the type of mould species present, reports from other studies by Ramesh and Santoshkumar (2013), Singh et al. (2013), and Jeswal and Kumar (2015) have shown that the most dominant genera of mycoflora with mycotoxigenic potential in their studies of five different spices was Aspergillus (7 species) followed by Penicillium with 3 species, Fusarium with 2 species, and Mucor with a species. In their studies, the Aspergillus species had the aflatoxin producing A. flavus and A. parasiticus and A. niger, A. ochraceus which produces ochratoxin.
Results of this study showed the absence of aflatoxin (Table 2) The high aflatoxin contamination by these researchers may be due to prolonged drying period, high humidity during drying, or improper storage facilities which increases the relative humidity and moisture content to levels for mould growth and aflatoxin production.

| CON CLUS ION
The drying methods reduced the moisture of the ginger rhizome from an initial level of 80% wet base to 10% d.b in 5 days. Increasing the concentration of KMBS did not affect the moisture content in this study as expected or the total ash content of ginger. Total color change, however, increased with increasing KMBS concentration, and 0.1% KMBS preserved the color of the fresh samples better because high concentrations of KMBS increase bleaching and consequently a higher total color deviation. However, 100°C blanching was best for all pretreated ginger in preserving color because it inactivated the enzymes that causes browning. Among the dried samples, 100°C blanching and CSD had less total color change. The effect of potassium metabisulfite (KMBS) application for the sliced fresh ginger rhizome reduced significantly the yeast and mould load as the concentration of (KMBS) increased. For the dried samples, there was an increase in the yeast and mould load. However, the solar dried samples (CSD) had fewer loads than the open-sun dried (OSD) samples.
Even though the yeast and mould load were relatively high, aflatoxins were not detected in any of the dried samples for both CSD and OSD which will prevent a possible aspergillosis and mycotoxin infections.