Ultrasound treatment used with edible coatings may extend peanut shelf life

08 Nov 2010

Ultrasound treatment (sonication) and an edible coating including jujube and pomegranate extracts can be used for delaying the oxidation of roasted peanuts, and eliminates the need for modified atmosphere packaging, claims new research from Brazil.

Lipid oxidation is a problem in roasted peanuts and peanut product safety, and also reduces their shelf life and nutritional quality. About 80 per cent of the fat in the peanut kernels comprises unsaturated fatty acids, making it prone to oxidative and hydrolytic changes, thus affecting its quality and desired nutty flavour.

Oxidation inhibition

The authors of this study, who published their findings in the Journal of Food Process Engineering, claim that the combination of sonication and edible coatings mixed with natural antioxidants on peanuts will not only prevent penetration of oxygen onto the peanuts, but will also inhibit further lipid oxidation reactions.

“This process can be used for the preparation of shelf-stable peanut products for the purpose of reaching distant markets, to be used during extended storage periods.

Likewise, the peanuts can be packed without the need to modify the packaging atmosphere or use of expensive packages,” commented the research team.

Alternatives sought

Different means, such as the use of synthetic antioxidants, modified atmosphere (MAP) and UV proof packaging have been reported to extend the shelf life of roasted peanuts to various degrees, said the researchers.

But they point to safety issues with the synthetic antioxidants such as butylated hydroxyanisole and butylated hydroxytoluene and claim MAP and UV proof packaging is costly.

Research objectives

Thus, the authors said that they evaluated the effects of carboxymethyl cellulose (CMC) coatings mixed with jujube extract, pomegranate extract and synthetic tocopherol in delaying lipid oxidation of roasted and roasted–sonicated peanut samples; and examined the colour changes of the coated samples during storage.

Sonication, they explain, enables minimal processing, because transfer of acoustic energy to the food product is instantaneous. “This means a reduction of the total processing time, higher throughput and lower energy consumption.

According to the authors, laboratory- and pilot-scale ultrasound equipment is available and has been used for different processes like emulsification and deactivation of enzymes. “In many cases, combinations of conventional techniques with ultrasound give the best results,” they add.

The study

The researchers generated three different types of samples as part of their investigation including peanuts that were

roasted–coated with CMC mixed with jujube extract, pomegranate extract and tocopherol,
roasted–sonicated–coated with CMC mixed with jujube extract, pomegranate extract and tocopherol and
roasted uncoated as a control.

Some of the peanut samples were randomly taken immediately following roasting and cooling, placed in a glass jar filled with 100 mL n-hexane and subjected to sonication, said the team.

Immediately following sonication, the peanuts were separated from the n-hexane, and warm air was blown over the peanuts for approximately 1 min to remove n-hexane residuals under a hood, they continued.

The authors said that after the cooling process, the peanut skin was easily removed by rubbing peanuts against each other manually. The skin was then separated from the kernels using an air blower, the said.

According to the article, pomegranate extract (1,000 ppm), jujube extract (1,000 ppm) and tocopherol (250 ppm) was each added separately to 1,000 mL of prepared CMC solution. The authors said that the peanut samples were immersed in a vessel containing the coating formulations for 30 seconds and then dried at room temperature for 24 hours.

The samples, they added, were placed in separate polyethylene bags and stored in a controlled atmosphere chamber which was set at a relative humidity of 45 ± 5 per cent and a storage temperature of 35 °C ± 2°C throughout the entire storage period. “This high temperature was used as per experimental design for accelerating shelf life tests,” explained the researchers.

Oxidative stability of the samples was monitored during the 12 weeks of storage by measuring the oxidative stability index (OSI) to estimate the shelf life of the peanut samples, commented the team.

Results

The researchers found that the roasted and roasted–sonicated–coated samples with the CMC and natural extracts of jujube and pomegranate fruit combination had a higher superior oxidative stability during storage than the control.

“Pomegranate extract was the most effective, followed by jujube extract,” they said.

The researchers concluded that the sonication treatment significantly improved storage stability of roasted peanuts.

“After 12 weeks of storage at 35°C, the reduction in oxidation was 14.5 and 19.7 per cent for samples roasted–coated with CMC added with extracts of jujube and pomegranate extract, respectively, as compared to uncoated sample,” said the team.

“Meanwhile, the oxidative stability of samples roasted–sonicated coated with CMC added with extracts of jujube and pomegranate was significantly improved by 24.8 and 31.8 per cent, respectively, in relation to the control,” said the authors.

The natural extracts, because of their natural pigment, caused some colour differences between coated and uncoated, but storage conditions and durations did not have significant effects on the coated samples, they added.

Source: Journal of Food Process Engineering