Effects of enzymatic treatments and drying methods on the physical properties and rehydration kinetics of instant rice

Abstract Instant rice, also known as pre-cooked rice or quick-cooking, is a convenient food product intended to significantly reduce its preparation time. It undergoes a process of partial or full cooking, followed by dehydration, to produce a shelf-stable product that can be rehydrated and consumed within minutes. Textural properties and rehydration rate are both important characteristics that affect the quality of instant rice and these parameters can be modified through various processing techniques. One technique is through enzyme pretreatment, in which partial hydrolysis alters the starch structure, reducing excessive starch retrogradation and enhancing grain porosity. This modification promotes faster water absorption, making enzyme-treated instant rice more convenient and desirable. The present study therefore aimed to optimise instant rice production by pre-treating cooked rice with α-amylase, followed by employing two distinct drying methods namely oven drying and freeze-drying, utilising Sembada188 rice as the raw material. The evaluation of the resulting instant rice encompassed various parameters, including bulk density, porosity, rehydration ratio, water adsorption rate, swelling rate, volume expansion and texture profile analysis. The study we conducted showed that there is an inverse relationship between bulk density and porosity, the former being higher in oven-than freeze-dried instant rice (<0.28 g/cm 3 and >56% respectively). These findings suggest the formation of bigger holes inside grains which may increase their ability to recover from dryness. This is why swelling rate along with final volume is much lower for FDIR when compared against ODIR (p<0.05). Also, the Peleg model proved effective at predicting water absorption patterns exhibited by different types of quick cook at varied temperatures and times of hydration (R2>0.98). FDIR appeared to exhibit a lower Peleg rate, K 1 than ODIR, indicating that FDIR rehydrates at a much faster rate. Additionally, our results highlighted that pre-treating cooked rice with α-amylase significantly enhances the swelling rate, volume expansion and rehydration ratio of instant rice (p<0.05). In terms of hardness, all instant rice samples exhibited comparable hardness to freshly cooked rice. In summary, the combination of the freeze-drying method with α-amylase pretreatment improves the rehydration rate, cooking, and physical attributes of instant rice.