Microencapsulation instant coffee by spray drying using hydrolysed konjac glucomannan

Abstract

This research is aimed at finding a suitable wall material from konjac glucomannan to shield antioxidant compounds in coffee extract along with appropriate condition of spray drying to produce microencapsulated instant coffee. There are two steps of study. In the first step, two types of konjac powder (A. muelleri and A. bulbifer) and different concentrations of mannanase enzyme were used to investigate the viscosity reduction of konjac glucomannan hydrolysate (KGMH). It is found that mannanase at 38,000 units per gram substrate was able to lower the viscosity of A. muelleri solution to less than 100 m.Pa.s, while A. bulbifer remained above 100 mPa.s at the chosen concentration. In the second step, concentrated coffee extract (20 °Brix) was mixed with KGMH (5, 15 and 20% w/w) as coating material. Maltodextrin (MD) (15, 5 % w/w) was added to adjust total solids of coating material and used as control (20% w/w). The mixture of coating material and coffee extract at solids ratio 2.2:1 (w/w) was then spray dried at 160, 170 and 180 °C inlet temperature and at 85-90 °C outlet temperature. Instant coffee produced from 20% KGMH (without MD) and dried at 160 °C presented the highest apparent retention of phenolic compounds (129%), DPPH scavenging activity (131%), and antioxidant activity of FRAP (122%) (p<0.05). Meanwhile, the highest apparent retention of chlorogenic acid was provided by instant coffee of 20% KGMH and dried at 180 °C (130%). Apparently, KGMH was contributing to the antioxidant properties and the coffee solids were retained nearly 100% in microcapsule. Increasing KGMH concentration leads to higher dietary fiber both soluble and insoluble dietary fiber (p<0.05). Meanwhile, in physical properties, increasing KGMH concentration was correlated to lower production yield and lower water solubility index but higher water absorption index (p<0.05). However, it should be noted that the lowest water solubility index was still above 88% which can be classified as good solubility of powder. Moisture content and water activity of all formulations of instant coffee ranged between 3.37-3.75% and 0.21-0.25 (p>0.05). To sum up, this study revealed the potential use of KGMH solution as wall material in spray drying microencapsulation. Instant coffee alone as coating material presented better antioxidant activity and dietary fiber than microcapsule of MD alone.