Effect of thermal and Pulsed Electric Field treatments on carrot texture

Abstract

The purpose of the current research project was to study the effects of thermal and pulsed electric field (PEF) processing on carrot texture with the ultimate aim to assess the chewing behaviour in vivo and the resulting β-carotene bioaccessibility in vitro. Three trials were conducted. Firstly, the effect of thermal and PEF treatments on carrot texture and carotenoids content was studied. Thermal treatment at elevated temperatures up to 90oC could enhance the release of β-carotene which led to an increase in the content of β-carotene, however the texture (cutting force and hardness) of carrots was reduced greatly. By applying PEF treatment, the cutting force of carrots could be maintained to some extent or slightly improved depending on the cutting position towards the epidermis of carrot. This slight texture improvement was most likely due to the effect of epidermis (in this study, it was called as ‘vertical cutting’ with the perpendicular cutting orientation towards epidermis). When thermal pre-treatment at moderate temperatures (20, 40, 50 and 60oC) was conducted prior to PEF treatment, the vertical cutting was not much affected but it decreased the hardness and the cutting force at horizontal position (parallel cutting orientation towards epidermis). In this study, β-carotene content of carrots was not much affected by PEF treatment with electric field strength from 0.30±0.00 to 1.93±0.06 kV/cm.

During the second stage of the study, calcium chloride solution was used for soaking and as PEF medium, in order to investigate the combined effect of thermal, PEF treatment and calcium infusion. When calcium chloride solution with different concentrations (0.03 and 0.5% w/v) was used for overnight soaking, a slight increase in carrot calcium content was found, but not hardness. Thermal pre-treatment at 20 and 40 oC followed by PEF treatment with 0.03% CaCl2 solution as treatment medium at energy input of 10 and 70% decreased the hardness of carrots and the third subsequent thermal treatment at 20, 40, 60 and 100oC resulted in a further slight decrease in hardness. Interestingly, thermal pre-treatment at 60 oC could preserve the hardness of carrots after subsequent two treatments of PEF treatments using 0.03% CaCl2 solution as treatment medium at energy input of 10 and 70% and thermal treatment (20, 40, 60 and 100 oC). The hardness of carrots pre-treated with thermal treatment temperature at 20 and 40 oC (30 min) decreased when the energy input of the subsequent PEF treatment using 0.03% CaCl2 solution as treatment medium was increased, i.e. 10, 30, 50 and 70%, however, the carrots became firmer during storage up to 48 hours (4 oC). Interestingly, when the 60 oC pre-treated carrots were subsequently PEF-processed with 0.03% CaCl2 solution as treatment medium, the firmness of carrots immediately after treatments was decreased by 70% of their original value. However during storage at 4 oC for up to 48 hours the hardness became improved until reaching the original hardness of the carrot without any treatments.

For the last trial of the study, untreated carrots, carrots with PEF pretreatment using 0.03% CaCl2 solution as treatment medium at energy input of 70% and carrots with traditional calcium soaking and low temperature blanching (i.e. calcium soaking (0.5%) combined with thermal pretreatment at 60oC for 30 min) were cooked at 100oC whereas raw carrots were used as control. The aim of the third study were to evaluate whether the carrot texture obtained from PEF, thermal and calcium infusion could affect the in vivo chewing behaviour and the β-carotene bioaccessibility using in vitro digestion. Seven eligible participants were recruited. The combined PEF and calcium pre-treatment gave similar texture properties (i.e. hardness, adhesiveness and cohesiveness) and similar β-carotene content with no increase in bioaccessibility. The carrots without pre-treatments had the softest texture and the control had the highest firmness. The results showed that the average of chewing time for carrots without pre-treatment and PEF pre-treatment was shorter than traditional calcium soaking and low temperature blanching. The β-carotene bioaccessibility for carrots without pre-treatment and PEF pre-treatment was lower, on average, than raw carrots and traditional calcium soaking and low temperature blanching. In this study, it was found that chewing behaviour also played an important role in β-carotene bioaccessibility.

The current study demonstrated new potential application of PEF processing as a fast and efficient alternative to traditional calcium soaking combined with low temperature blanching. In this respect, PEF processing could be potentially considered as a pre-treatment to maintain the carrot texture when combined with thermal processing.