Synthesis of adsorbent from food industry waste for purification of used cooking oil

Research sample

UCO samples were randomly taken from three SMEs foodstalls in Samarinda, Indonesia. All samples were then mixed together and tested in duplicate. As the control, we used the most widely oil brand used by them. Banana peels were collected from fried banana traders in the same location. Samples were collected on August 8, 2018, before the SME closed their stalls in the night.

Materials and equipment

The tools used in this study were an oven, furnace, sieve, burette, Erlenmeyer flask, stirring rod, pipette, measuring glass and hotplate. The materials used were distilled water, acetic acid, chloroform, KI, 0.01 M Na₂S₂O₃, starch, phenolphthalein, ethanol, 0.5 M KOH, 0.5 M HCl, iodine-bromide solution, and NaOH.

Optimization of temperature and time of carbonation

The dried banana peel was furnaced for 3 hours at 200°C, 400°C or 600°C until the materials turned into charcoal. Subsequently, the temperature of the charcoal was allowed to cool, and the charcoal was immediately sieved with a 200-mesh-size sieve and then activated using 1 M NaOH solution at ambient temperature. The charcoal was stirred for 2 hours and then neutralized by washing with distilled water until the pH is neutral. The obtained charcoal was then dried in the oven at 80°C.

Sample preparation

Some impurities and foreign materials were initially removed in the precipitation and separation process. Samples were put in a glass tube (30 cm height, 5 cm diameter) and the impurities were left to settle at room temperature by gravitation. This process was performed without reducing the amount of free fatty acids in the UCO.

Refining process

In the UCO refining process, 10 g of BPAC was added to 200 ml UCO and stirred with magnetic stirrer at the room temperature. After 24 hours, the UCO was filtered out using a paper filter.

Oil quality testing

The color of the oil was analyzed using a scoring sensory test by 16 untrained panelists, using a scale of 1 (very dark brown), 2 (dark brown), 3 (rather dark brown), 4 (clear yellow) and 5 (very clear yellow, (equivalent to fresh oil)). Judgement of oil color was performed using UCO filtered with charcoal heated to 600°C for 3 hours. Peroxide value (PV), free fatty acid value (AV) and iodine value (IV) were analyzed using the official methods of analysis of the Association of Official Analytical Chemists (AOAC)¹¹. PV, AV and IV were evaluated using charcoal heated at each temperature for each length of time.

PV was determined as follow. Approximately 5.0 g of UCO sample was placed into iodine flask and was dissolved in the solution mixture of 50 mL glacial acetic-acid isooctane (3: 2, v/v). The solution was added with 0.5 mL saturated solution of KI. The mixture was then shaken vigorously for 0.5 min and allowed in the dark condition for another 3 min. The solution was added with 30 mL distilled water and was titrated using sodium thiosulphate 0.01 N using 1 mL of starch indicator 0.05%. Titration was stopped if blue colour of solution just disappeared. The blank titration was also carried out under similar condition without addition of samples. PV was calculated as:

where PV is peroxide value (in meq/kg), Vs is volume (in mL) of thiosulphate used for sample titration, Vb is volume (in mL) of thiosulphate used for blank titration, Nthio is normality of thiosulphate.

Acid value (AV) was determined using a titration method. A 10.0 g of samples were accurately weighed and dissolved in 100 mL ethanol-ethyl ether mixture (1:1 v/v). This solution was then titrated using standardized KOH-ethanolic solution using phenolphthalein as indicator until pink-violet color was observed. AV was expressed as the number of mg KOH needed to neutralize free fatty acids in 1 g of sample. AV was calculated as the volume of KOH needed per gram sample multiplied by N KOH and 56.1.

Determination of iodine value. A-1.0 g of samples was added with 20 mL cyclohexane-acetic acid mixture (1:1 v/v). The solution was added with 25 mL of Wijs solution (iodine monochloride, ICl) and was kept in the dark condition for 1 h. The mixture was added with 20 mL saturated KI solution and 150 mL distilled water, shaken homogenously, and titrated with 0.1 N sodium thiosulphate 0.1 N using 1 mL of starch indicator 0.05% until the color became clear. The blank titration was also carried under similar condition without addition of FO samples. IV was calculated as:

Vb is volume (in mL) of thiosulphate used for blank titration and Vs is volume (in mL) of thiosulphate used for sample titration.

Color of filtered vs. unfiltered cooking UCO

Table 1 presents the results of the score observations of color. Initially, the color of fresh cooking oil is very clear yellow due to the presence of α and β-carotene or xanthophyll⁴. This color and clarity becomes darker and brown, respectively, after several uses for frying as a result of the degradation of natural dyes and oxidation¹². Some foodstuffs with high protein and sugar contents will initiate browning reactions as a result of Maillard and caramelization reactions. During the refining process, significant color changes occurred. Banana peel is able to absorb dark colors in UCO because of its lutein content.

Chemical analysis

Figure 1 presents the results analysis on the quality of samples before and after the treatments using BPAC which were made from different temperatures and time. Increasing the temperature and time in the charcoal production process has a positive effect on the quality of cooking oil refinery. Higher temperatures and longer casting processes make the activated carbon more refined to make the material surface wider. Surface area is one factor that affects the rate of adsorption of dirt in used cooking oil.

Figure 1. Sample quality parameters.

Free fatty acid content

The level of free fatty acids is an important parameter when determining the quality of cooking oil since it could determine the age, purity and the level of oil hydrolysis. Free fatty acid content exceeding 0.30% of the oil mass result in an undesirable flavor⁴. Based on the results of UCO sample analysis, the level of free fatty acids was quite high (1.23%). However, after soaking with BPAC the AV levels decreased to 0.71%, but this number still does not conform to Indonesia’s standard (0.3%) of oil quality¹³. The high levels of free fatty acids in UCO indicate the degree of oil damage⁴. In this research, soaking with BPAC gives a quite efficient effect in reducing about 40–50% damage.

Saponification number

In this study, the saponification number of UCO cannot be improved to the level of recommended quality standard. Instead, the value tends to decrease. It was also found that UCO has a higher molecular weight than fresh oil, meaning that it has bound many other foreign materials other than oil; so that it is no longer suitable for use.

Peroxide values

The peroxide compound concentration in oil indicates the level of cooking oil degradation due to oxidation in heating process⁴. The content of peroxide compounds in cooking oil can accelerate rancidity and drive the presence of undesired flavor.

Interestingly, peroxide values significantly dropped during soaking with BPAC. Yet, the peroxide value still does not conform to the quality standard which is at maximum of 2 mEq/ kg. Immersion with BPAC reduced these values by about 50–60%.