More than 80% of palm oil is produced in Imbo Sud (Rumonge and Makamba), and the rest is produced in Imbo Centre (Bujumbura and Bubanza) at 775 to 1000 m of altitude. The determination of the fermentation period was done in the form of a survey. The survey was conducted in the Commune of Rumonge (Imbo Sud) and Mutimbuzi (Imbo Centre). The sample size was chosen based on the formula of François Daniel [21] :

$n=\frac{t^2\cdot N}{t^2+{\left(2e\right)}^2\left(N-1\right)}$

Oil samples were collected, directly after extraction, in four zones (Kigwena, Rumonge, Kizuka, Minago) of the commune of Rumonge and four zones (Kigwena, Rumonge, Kizuka, Minago) of the commune of Mutimbuzi (Rubirizi, Rukaramu, Gatumba and Maramvya zones). In each zone, three hills were randomly selected. In each hill, three artisanal mining units were sampled. Each zone had nine samples and each commune had 32 samples. The acidity was determined according to the method of ISO 660:2009 [22] norms and expressed in mg KOH/g oil.

An experiment was conducted to study the parameters that may be responsible for the acidity of palm oils in Burundi. For this purpose, bunches of two varieties (Dura and Tenera) of Elaeis guineensis were purchased from producers. To identify the factors responsible for the variation in oil acidity, the experiment was performed by analyzing the influence of the following four parameters on oil acidity:

1) Variety;

2) The number of days of fermentation of oil palm fruits;

3) Destemmed and clustered fruits for fermentation; and

4) The impact of leaving oil palm fruits in the open air under a tree, putting them in a shed without covering them with straw or tarpaulin, and putting them in a shed and covering them with tarpaulin has been tested.

After fermentation, according to the four parameters (Section 2.3), artisanal extractions were performed as shown in Figure 1 . The oils obtained underwent acid value analyses like those collected from the producers.

Data analysis was performed using IBM SPSS statistic 20. Results were analyzed using one-way analysis of variance (ANOVA) followed by Tukey’s multiple comparison test. Correlation between various parameters was also investigated. Significance was determined at p < 0.05 level and the results were expressed as mean values ± standard error (SE). All tests were performed in triplicate.

A study was conducted in four zones (Rubirizi, Rukaramu, Gatumba and Maramvya) of Mutimbuzi Commune and four others (Kigwena, Rumonge, Kizuka and Minago) of Rumonge Commune. Through this study, it was found that palm oil producers in Rumonge ferment palm fruit for several days more than those in Mutimbuzi ( Table 1 ). 61.7% of Mutimbuzi producers ferment for 4 days, while in Rumonge commune, only 10.3% of producers ferment during this period. 51% of Rumonge producers ferment palm fruits for more than a week while more than 85% of Mutimbuzi producers do not exceed 5 days.

It was even revealed that in the commune of Rumonge 5% of producers ferment for more than 12 days.

The longer the fruits are stored, the more the endogenous lipases find the opportunity to be in contact with the triglycerides [16] and thus to degrade them, which leads to the acidification of the palm oil [13] [14] . It has been shown that when fruits are stored, fungi secrete exogenous lipases [11] , which are also responsible for the acidification of palm oil. This proves that the oil palm fruits of Rumonge producers are more exposed to acidification factors compared to those of Mutimbuzi.

The analysis was conducted on oils sampled directly from artisanal extraction units in the commune of Rumonge (Kigwena, Rumonge, Kizuka and Minago zones) and the commune of Mutimbuzi (Rubirizi, Rukaramu, Gatumba and Maramvya zones). The oil extracted by Rumonge producers is characterized by a significantly (p < 0.05) higher acidity than that produced in Mutimbuzi, the order being: 14.1 ± 0.8 > 9.7 ± 0.3 mg KOH/g oil.

The acid value of the oils produced at Mutimbuzi varies from 9 to 10 mg KOH/g oil, while that of Rumonge varies from 13 to 15 mg KOH/g oil. The comparison of the results in Table 1 and Table 2 show that the longer the fermentation, the higher the acid value, which greatly affects the quality of the oils.

Note: The values that include these letters a, b, c and d are significantly different in descending order: a > b > c > d.

In the commune of Rumonge, the duration of fermentation was found to be very long: more than 90% exceeds 5 days ( Table 1 ). Previous studies have reported that oil extraction from fruits with little or no fermented yields high quality crude oil of about 0.9 mg KOH/g oil and that the acidity of fruits stored for 6 to 26 days ranged from 12.04 to 64 mg KOH/g oil [23] . It has been reported that industrially produced oil has an acidity of about 9.42 ± 0.12 mg KOH/g oil [24] . Although industrially produced, international standards show that these oils are of very poor quality. Refined oils are ≤0.6 mg KOH/g oil, cold pressed and virgin oils ≤ 4.0 mg KOH/g oil, and virgin palm oils ≤ 10.0 mg KOH/g oil according to the CODEX STAN 210-1999 norms [25] . As the extraction of the oils of these two communes is artisanal and it is the simple pressing after a light heating, it is considered as virgin. Their acid value should be less than 10.0 mg KOH/g, threshold set by CODEX STAN 219-1999 for virgin palm oils [25] . However, in this study, it was found that the acid values (13 - 15 mg KOH/g) of Rumonge oils are higher than the value fixed (10.0 mg KOH/g) by CODEX STAN 219-1999 for virgin oils. Therefore, it is unfit for human consumption. The producers will have to review their preparation methods from fermentation to sale.

The oils produced from Mutimbuzi commune comply with the standards of CODEX STAN 219-1999 (9 - 10 mg KOH/g). Although they comply with the standards, strict measures during their preparation must be observed to maintain these values below the threshold recommended by CODEX STAN 219-1999.

During fermentation, there are producers who collect the palm fruits without destemming them and the others ferment the destemmed fruits. In this study, experimentation was conducted on the varieties and condition of the bunches. Thus, the acid values as a function of varieties and the state of the fruit regime (clustered or not) are depicted in Table 3 .

It was found that the oil acid values of Dura variety and Tenera variety had no significant differences at p > 0.005. After 4 days of fermentation, for clustered fruits, the acid value of Dura variety oil was 5.9 ± 2.3 mg KOH/g oil while that of Tenera oil was 5.8 ± 1.8 mg KOH/g oil. After eight days of fermentation, the acid value of Dura oil was 10.9 ± 4.7 mg KOH/g oil while that of Tenera oil was 12.5 ± 5.4 mg KOH/g oil.

The same trend was also observed between the acid value for Dura and Tenera destemmed fruit oils respectively at the same days of fermentation: for example for 4, 5 and 8 days, the acid value of Dura destemmed were 7.03 ± 3.4, 10.8 ± 3.4 and 12.5 ± 5.4 mg KOH/g oil respectively and those of Tenera were 7.02 ± 3.2, 9.8 ± 3.4 and 12.5 ± 4.1 mg KOH/g oil respectively. This shows that both species have the same physiological properties.

Previous studies have reported that oil palm fruits naturally contain lipases [26] . In the light of these results, it can be deduced that both varieties contain the same amount of endogenous lipases. The acidity can also be caused by the hydrolysis of fatty acids by enzymes secreted by microorganisms [27] through the wounds of the fruits.

Note: The values that include these letters a, b, c and d are significantly different in descending order: a > b > c > d.

Since there were no significant differences at p > 0.05 between the acid value of Dura and Tenera oils and the latter being extended to other provinces, the study continued with Tenera. The fermentation techniques are almost the same in both communes. Some people pile them in a shade around the feet of palm trees or other trees; others put them in the sheds without covering them and others in the sheds and cover them with straw or tarpaulins.

Testing of these techniques through this study revealed significant differences (p < 0.005) of acid values ( Table 4 ) between them. Destemmed and clustered fruit were experimented separately. For destemmed fruit, during four days of fermentation, the acid values of oil fruits stacked in the open air (FFOA), in a shed when fruits uncovered by a tarpaulin (SFNC), and in a shed when a tarpaulin (SFC) covered the fruit were 3.7 ± 0.9 < 6.5 ± 0.3 < 10.8 ± 1.6 mg KOH/g oil, respectively. All these oils are within the limits (10 mg/100g) fixed by CODEX STAN 210-1999 standards [25] . Just after 5 days of fermentation, the FFOA and SFNC oils were still within the limits fixed by CODEX STAN while the SFC oils exceeded it by a large margin. The order respective is as 5.6 ± 0.6 < 10.3 ± 0.3 < 12 ± 2.9 mg KOH/g with significant differences at p < 0.05. After eight days of fermentation only the FFOA, oils were found to be within, the international standards while those of the SFNC and SFC were higher. They are significant at p < 0.05 differences; order being FFOA < SFNC < SFC: their respective values were 9.1 ± 7 < 6 ± 0.8 < 17.7 ± 1.4 mg KOH/g. Based on these data, this study supposes that these differences are due to the humidity conditions created by covering the fruits. These conditions are favorable to the development of microorganisms. Previous investigations have reported that microorganisms (molds and bacteria) produce lipases [11] [12] [28] .

Note: The values that include these letters a, b, c, d, e, f and g are significantly different in descending order: a > b > c > d > e > f > g.

For the clustered fruits, the acidity trends of the oils were the same as those observed in oils from destemmed fruits. They are also significantly (p < 0.05) different in the following order: FFOA < SFNC < SFC. Furthermore, there were no significant differences (p > 0.05) between the acid values of the oils extracted from the fermented fruits with clusters and those from the destemmed fruits. For FFOA, on the third day of fermentation, the acid value recorded was 3.7 ± 0.9 and 3.7 ± 2.6 mg KOH/g, respectively, for destemmed and clustered fruits. After 8 days of fermentation, the acid value of destemmed and clustered fruits was 9.1 ± 0.7 and 9.1 ± 0.7 mg KOH/g, respectively. The trend was the same for the acid value of SFNC and SFC oils. All this had shown that there was no significant difference (p > 0.05) between the acid value of oils extracted from destemmed fruits and those extracted from clustered fruits.