Industrial Measures in Mitigating Glycidyl Esters Formation in Refined Palm Oil

Glycidyl fatty acid esters (GEs) is a heat-induced processing contaminants that most commonly occurred in refined vegetable oils (Ermacora and Hrncirik, 2014; Weißhaar and Perz, 2010). Although harmful effects on humans and animals have not been demonstrated, glycidol, the hydrolysate of GEs have been identified as rodent genotoxic carcinogens that results in the formation of tumours at tissue sites. Therefore, glycidol have been categorised as “probably carcinogenic to humans’’ (group 2A) by the International Agency for Research on Cancer (IARC, 2000).

Precursors of GEs in refined oils have been identified as partial acylglycerols that is, diacylglycerols (DAGs) and monoacylglycerides (MAGs). However, whether they also originate from triacylglycerides (TAGs) is still a topic of controversial debates as several authors noted that pure TAGs were stable during heat treatment and therefore, unlikely to be involved in the formation of GEs (Shimizu et al., 2012a; Destaillats et al., 2012).

As shown in Table 1, GEs can be detected in diverse refined oils. Among these, rice bran oil and palm oil are the most susceptible to the formation of GEs, both exceeding 25 mg/kg of oil. In palm oil, the DAGs contents are particularly high ranging from 4% to 12% with a mean of approximately 6.5% (Long et al., 2005). 

Table 1. Occurrence of GEs in refined edible oils

Types of OilsRange as a sum (mg/kg)
Canola oila0.18-0.48
Coconut oila0.03-1.71
Corn oila0.15-1.57
Cottonseed oila0.47-0.53
Grapeseed oila0.14-3.02
Olive oila0.048-1.10
Palm oilb30.2
Palm oleinc0.24-17
Palm stearina3.58
Rapeseed oild16
Rice bran oild28
Soybean oila0.014-0.50
Sunflower oile1.0

aMacMahon et al. (2013a) bSteenbergen et al. (2013) cCraft et al. (2012) dShiro et al. (2011) eBlumhosrt et al. (2011)

DAGs and MAGs are formed not only through lipase hydrolysis of TAGs during maturing, harvesting and transportation of oil fruits or seeds, but also through pyrolysis of TAGs at high temperatures (Lucas-Torres et al., 2014; Shimizu et al., 2012a). Fruit oils are more susceptible to potential hydrolysis reactions because of a longer water exposure compared to seed oil which explains the high DAGs levels in fruit oils such as palm oil. Thus, high contents of GEs have been reported in refined palm oil (Aniołowska and Kita, 2016a; Pudel et al., 2011). 

As mentioned above, DAGs and MAGs are mainly formed through hydrolysis of TAGs, resulting from the activity of endogenous lipase after maturation of oil plants before inactivation. It has also been reported that bruised oil seeds or fruits display more lipolytic activity than undamaged oil seeds or fruits. Moreover, post mature fruits, processing delay, as well as rough handling of oil plant bunches may contribute to the presence of high DAGs and MAGs concentrations in crude oil (Matthaus and Pudel, 2014; Kopas and Kopas, 2009). 

For oil producers, the elimination of GEs should begin with the selection of plants from different locations and cultivation conditions. Taking some agricultural practices between harvesting and processing of oil fruits or seeds are essential to reduce or eliminate the activity of lipase. These practices include the modification of harvesting conditions, minimizing cracking of oil fruits when transporting to factories, avoiding bruised fruit, reducing the time between harvests to milling, and so on. All these practices taken in concert are aimed to inhibit the formation of DAGs and MAGs.

Craft et al. (2012) reported that the levels of DAGs and FFAs in crude palm oil should be kept below 4% and 2.5% respectively in order to reduce the formation of GEs during deodorization. Malaysia has two different quality of crude palm oil (CPO), namely special and standard quality. The special quality grade CPO has the maximum FFA level of 2.5%. Lower FFA level in CPO will reduce the oil losses throughout the refining process and also the formation of GE in refined oil will be minimal.

The EU’s current maximum limits for GEs and products containing them are tabulated below:

Table 2. EU’s current maximum limits for GEs and products containing them

CategoryMaximum limits (ppm)
Vegetable oils and fats1.0
Vegetable oils and fats for the production of baby food and processed cereal-based food for infants and young children0.5
Powder infant formula, follow-on formula and foods for special medical purposes0.075 (until 30 June 2019)0.05 (from 1 July 2019)
Liquid infant formula, follow-on formula and foods for special medical purposes0.01 (until 30 June 2019)0.006 (from 1 July 2019)

GEs are also formed predominantly during the deodorization step in the oil refining process. Deodorization temperature and time have been shown to represent the most crucial factors for the formation of GEs. Several research groups have reported that GEs level increases with increasing incubation temperature ranging from 140 to 280 °C (Hrncirik and Duijn, 2011; Destaillats et al., 2012). 

GE is generally easier to mitigate because its formation is directly associated with elevated temperatures (with formation beginning at about 200°C and becoming more significant at temperatures >230°C). Oils can be deodorized at temperatures below 230°C to avoid significant GE formation. However, it is not practical to decrease deodorization temperatures below the threshold that would lead to 3-MCPDE formation (160-200°C), as that could affect the quality and safety of the oil.

Processors should consider conducting deodorization of oils at reduced temperatures to reduce the formation of GEs. As an alternative to traditional deodorization, dual deodorization (2-stage deodorization) can be conducted to reduce thermal load in oil to reduce formation of GEs. This includes both a shorter deodorization period at a higher temperature and a longer deodorization period at a lower temperature. Consideration needs to be given to parameters such as temperature, vacuum pressure and time, and variations in equipment design and capability. 

The use of a stronger vacuum can facilitates evaporation of volatile compounds, contributing to decreased deodorization temperatures and reduced formation of GEs. Short-path distillation also has been shown to reduce the thermal load and formation of esters, contributing to lower amounts of GE in comparison to conventional deodorization. However, additional post processing using mild deodorization (e.g. 160- 180°C) is needed to address sensory considerations.

For a good processing economics, majority of palm oils is physically refined. Bleached palm oil is heated to high temperature (260-265°C) to strip off FFAs, followed by retention of 60 minutes for heat bleaching and deodorization. Due to the nature of palm oil having high DAG level compared to other oil, deodorization steps can easily in the format of undesired GE at high temperature. 

Recently, through some innovation, the formation of GE are able to minimize to be less than 1 ppm. For instance, Alfa Laval has introduced dual-temperature-dual strip in which bleached palm oil is heated at 260°C to stripped off FFAs followed by the cooling of oil to <230°C for deodorization. When the deodorization temperature is lowered, the level of GEs in refined oil will be reduced accordingly. However, to compensate less heat bleaching effect at a lower temperature, the retention time must be increased. Their deodorizer column is operating under a very low vacuum with pre- and post-stripping to further improve GEs removal.

In Malaysia, the palm oil industry is very committed to food safety and quality regulations. The industry is subjected to international standards of quality and safety controls. Malaysian palm oil industry took a strong commitment to reduce GE level in palm oil. Although the European Food Safety Authority (EFSA) and WHO-JECFA raised the potential health consequences of GE, the toxicology aspect of it is not fully vetted. Several investigators have noted toxicological data gaps and call for further research. Thus, there is a need for additional research that assesses the potential causes and effects of GE on human health when consumed as typical levels over a lifespan. Most importantly, EFSA and JECFA did not recommend dietary changes based on its findings.

While palm oil may be at the centre of the GEs controversy, the best quality Red Palm Oil (RPO) is processed using molecular distillation techniques and the level of GE is almost nonexistent in such products. RPO has been recognized as a rich source of pro-vitamin A carotenoids for decades. Indeed, it was used to improve vitamin A deficiency in many countries. Regardless of the uncertainty of the health risk assessments, Malaysian palm oil industry is focused on ensuring that palm oil products from Malaysia contain minimum levels of contaminant that comply with the requirement set by importing countries. This is done through introduction of new effective technologies to mitigate the formation of contaminants in refined palm oil and its products and establishing Code of Practice (COP) along the oil palm supply chain for mitigation of GE in refined oil in-line with CODEX Alimentarius Standard. 

References

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Prepared by Sabrina

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