Worldwide, around 1.12 million tons of crude protein are produced annually from rapeseed oil. Although farmers have long used this so-called rapeseed cake as a protein feed for animals, it has not played a role as a protein source in human nutrition.
One reason is the very bitter-tasting secondary plant constituents contained in rapeseed that strongly impair the taste of the protein isolates.
Now, a team led by Thomas Hofmann, food chemist and director of the The Leibniz-Institute for Food Systems Biology, has identified a substance that is pivotal for making this rapeseed cake a viable source of protein for human nutrition.
Hofman's team found the source of the bitter taste using mass spectrometric analysis - a means of working out the structure and composition of materials in a sample - and taste tests.
The lead researcher says it is important to develop new plant protein sources for human nutrition as the population continues to grow and he says rapeseed is a good local source.
Co-author Corinna Dawid, who heads the Phytometabolomics research group at TUM says this is the first step to making rapeseed a source of protein for humans.
He says: "Since we now know the cause of the bitter off-taste, it is much easier to develop suitable technological processes or breeding strategies that can be used to produce tasty, protein-rich foods from rapeseed."
The bitter bit
The researchers investigated three different protein isolates that could have been causing the bitter flavour. The first isolate was an extract of all the proteins contained in rapeseed meal.
The second isolate predominantly contained cruciferin and the third napin, which are the rapeseed's two main storage proteins. All three isolates had a protein content of 80 to 90 percent.
The investigations showed, for the first time, that a compound called kaempferol 3-O-(2‘‘‘-O-sinapoyl-ß-sophoroside) is the key substance that makes protein extracts from rapeseed inedible.
The cruciferin isolate in particular contained a large amount of this bitter substance with 390 milligrams per kilogramme.
The rapeseed meal and napin isolate had less than a tenth of the quantity, but still tasted bitter in the sensory test.
Turning waste into protein
Nutraingredients reported in 2016 that using enzymes to extract protein from rapeseed press cakes could transform 34 million tonnes of waste product into a valuable protein source.
Katarina Rommi, doctoral researcher at the VTT Technical Research Centre in Finland, said rapeseed proteins contain bioactive peptides and high levels of essential amino acids cysteine and methionine, compared to soy or pea protein.
She also pointed out that sertain rapeseed proteins act as effective emulsifiers and have foaming properties.
However, she said the health benefits of the proteins could not yet be enjoyed via food due to their limited digestibility, undesirable taste and colour.
Meat and baking
Rommi says that the high fat absorption and water holding properties of rapeseed protein is a bonus for cooking stability of meat products meaning that partially replacing meat protein with rapeseed protein in sausage could improve taste and aroma.
Rapeseed protein isolates have also been tested in baking experiments with them being added to wheat flour at up to 18% protein concentration without detrimental effects.
Source: J Agric Food Chem, 67: 372–378
Published online, doi: 10.1021/acs.jafc.8b06260
“Kaempferol 3-O-(2'''-O-sinapoyl-β-sophoroside) causes the undesired bitter taste of canola/rapeseed protein isolates”
Authors: T. Hofmann et al