Konjac glucomannan : interactions between milk and starches and applications in dairy products

Abstract

Konjac glucomannan (KGM) is a food hydrocolloid with numerous functional benefits, and has potential to be used in foods. The interactions of KGM with other components in food systems need to be studied to give guidance for its further utilization. In this study, interactions of KGM with milk and starches, as well as its applications in low-fat/skimmed yogurts and Mozzarella cheeses were investiugated. Stability and phase behavior of KGM-milk mixed systems were investigated to assess the interactions between KGM and milk system. The phase separation diagram showed that a low level of KGM (0.2%, w/v) induced segregative phase3 separation in KGM-milk systems. Stability measured by turbidity, precipitation, particle size and zeta potential increased with increasing milk percentage, but decreased with KGM concentration. When KGM solution with concentration higher than 0.5% was mixed with milk, aggregate structures were observed by confocal laser scanning microscopy (CLSM). Some KGM-milk system exhibited metastable structures even though they showed no visual phase separation. The stable KGM-milk mixed system (0.5% KGM solution mixed with milk at a ratio of 5:95, v:v) was used to manufacture low-fat (skimmed) yogurts and Mozzarella cheeses, and the properties of yogurts and Mozzarella cheeses with KGM were compared with the low-fat (skimmed) yogurts and Mozzarella cheeses without KGM, as well as the full-fat products during 0, 7, 14, 21 and 28 days refrigerated storage. Addition of KGM to low-fat (skimmed) yogurts and Mozzarella cheeses did not have any effect on the composition of yogurt and Mozzarella cheese, while it increased their whiteness, greenness and yellowness because of the formation of casein and peptides and due to Maillard reaction. Serum expulsion in low-fat (skimmed) yogurt decreased by KGM because of its water-holding capacity and high viscosity. Yogurts with KGM showed more stable and stronger gel microstructures than in those without KGM. Addition of KGM improved the textural characteristics (softer and stickier texture) of low-fat (skimmed) Mozzarella cheeses. In addition, KGM improved the cheese functional properties with more4 complete shred melt, less scorching and lower browning on the cheese surface when they were baked as a pizza topping because of the lubricant role of KGM. Overall, KGM addition in low-fat (skimmed) yogurts and Mozzarella cheeses improved their characteristics and gave them properties closer to full-fat products, indicating KGM could be a potential fat replacer for use in dairy food. The effects of different KGM levels in starch on thermal, swelling, pasting, textural and microstructural properties of potato and rice starches, as well as their blends were investigated. With KGM addition, the onset temperature of the starch was increased while the enthalpy was decreased. KGM addition increased swelling powder, solubility index and peak viscosity of the starch/stach blends, as well as hardness and stickiness of their gels, while it decreased their pasting time and temperature. Starch/starch blend showed more homogenous and more complete swelling microstructures with KGM addition. All these changes were related to the water-holding capacity and high viscosity of KGM that promoted the inner-adhesive interactions among the starch granules.