Crispy Duck Products: Mechanism Of Crispness Deterioration And Techniques For Longer Crispness Retention

Crispy duck products with excellent sensory quality,such as crumb meat,crispness skin,rich taste,and non-greasy,in recent years are popular by consumers.However,the crispness deterioration of crispy duck products is often presented in the process of storage and transportation,resulted in much poorer taste.In this paper,moisture content,water activity（a_w）,and moisture type of crispy duck products were measured at different storage temperatures and humidities to investigate the law of moisture migration,while the microstructure,shear force,and sensory score of crispy duck products were also determined.The mechanism of crispness deterioration was illustrated according to the law of moisture migration and crispness changes.Furthermore,the techniques for longer crispness retention were developed on the basis of the mechanism of crispness deterioration.The findings will provide scientific guidance and technical support for the production of crispy duck products.The main contents and conclusions are as follows:1.Crispness deterioration mechanism of crispy duck products.Crispy duck legs were put into the constant temperature and humidity box and stored at different temperatures（5,30℃）and humidities（50%,70%,90%）for 0,2,4,6,8,and 10 h,respectively.Afterwords,moisture content,water activity,moisture type,shear force,and microstructure of crispy duck legs were measured.Influence of moisture migration on the crispness of crispy duck legs was analyzed and the mechanism of crisp deterioration was investigated.The results showed that for 5℃,a decrease of the moisture content in skin was firstly presented and then increased,after 6 h with a slow increase.Water activity of skin increased firstly and then tends gentle after 4 h.However,water activity of meat decreased at the rapid speed from 0 to 2 h and then a slow decrease was observed.The proportion of free water in skin increased,while that in meat decreased from 0 to 4 h,after 4 h with a slow decrease.Shear force of skin increased from 0 to 6 h and then decreased quickly.For 30℃,moisture content of skin increased at first and then decreased.The change for water activity and moisture type,and shear force of skin was similar to 5℃.Moisture content and water activity of skin increased with an increase of humidity,while shear force increased and crispness decreased.Few and big pores in skin after being fried were observed,and the amount of pores with smaller diameter increased with the migration of water.The findings indicated that temperature and humidity significantly influenced moisture migration between skin and meat during storage and transportation of crispy duck legs.There is a slow moisture migration rate at 5℃resulted in low moisture content and water activity in crispy duck legs.Moisture content and water activity in crispy duck legs increased with an increase of humidity.Furthermore,microstructure of skin was destroyed due to the continuous moisture migration among skin,meat,and environment in box,resulted in a decrease of crispness.2.Techniques for longer crispness retention of crispy duck products.Crispy duck legs,which were fried using four kinds of frying oil,were put into the constant temperature and humidity box and stored at 20℃and 50%of relative humidity for 4 h,respectively,and the color,shear force,moisture content,and sensory score of crispy duck legs（0,2,4 h）were measured.Moisture content of skin from high to low followed by soybean oil group,rice oil group,duck oil group,and palm oil group,while the crispness followed by palm oil group,duck oil group,rice oil group,and soybean oil group.The redness（a*value）of palm oil group and duck oil group were moderate.However,a*value of crispy duck legs from soybean oil group and rice oil group was slightly higher than that of other two groups and the light paste taste was also presented.The highest sensory score of crispy duck legs was duck oil group.Of all,the duck oil group stored for 2 h had the lowest moisture content,the best crispness,the highest sensory score,and moderate color.Therefore,duck oil was preferred to produce the crispy duck products.Pot-stewed duck legs were fried using duck oil at different frying temperatures（135,140,145,150℃）and times（8,9,10,11min）,respectively,and the color,shear force,moisture content,and sensory score of crispy duck legs were measured.With an increase of frying temperature and time,the moisture content and shear force of skin decreased,while the crispness,a*value,and b*value obviously increased.However,there were no difference for L*value.Frying temperature of 145℃and time of 10 min was preferred since the qualities of crispy duck legs including crispness,color,and sensory score were better than other kinds of frying conditions.Pot-stewed duck legs,which were fried at 145℃for 10 min and were cooled for5 min,then coated using four kinds of sugar（glucose syrup,fructose syrup,maltose syrup,isomaltose syrup）and three kinds of concentration（70%、80%、90%）,were put into the constant temperature and humidity box and stored at 20℃and 50%of relative humidity for 2 h,respectively,and moisture content,shear force,color,and sensory score of crispy duck legs（0,2 h）were measured.For the same concentration,moisture content from high to low followed by isomalt syrup group,maltose syrup group,glucose syrup group,and fructose syrup group.Moisture migration speed of crispy duck legs stored for 2 h from quick to slow followed by maltose syrup group,isomaltose syrup group,glucose syrup group,and fructose syrup group.Moreover,moisture content of crisp duck legs decreased,while crispness and L*value increased when the concentration of fructose syrup increased from 70%to 90%.Furthermore,90%group had the best sensory score among three kinds of concentration of fructose syrup.Therefore,fructose syrup with the concentration of 90%was used to produce crispy duck legs.Pot-stewed duck legs,which were fried using duck oil at 145℃for 10 min and coated by fructose syrup with the concentration of 90%,were cooled by four kinds of cooling methods（5℃for 30 min,20℃for 30 min,blast cooling 20℃for 30 min,and blast cooling 30℃for 30 min）,respectively,and the color,shear force,moisture content,and sensory score of crispy duck legs were measured.Compared to the other two kinds of cooling methods（5℃for 30 min,20℃for 30 min）,the blast cooling group with 20℃and 30℃had the lower moisture content and shear force,while higher sensory score and L*value were also presented.If the economic benefits can be regard as an important factor,blast cooling group with 20℃will be preferred（blast cooling group with 30℃will be preferred in summer）.3.Techniques for storage and packaging of crispy duck products.crispy duck legs were packed using atmosphere packaging method with four kinds of the ratio of CO₂ to N₂（60%CO₂+40%N₂,40%CO₂+60%N₂,20%CO₂+80%N₂,and100%N₂,then they were put into the constant temperature and humidity box and were stored at 5 or 30℃）and 50%humidity for 0-5d,respectively.Afterwords,moisture content,color,shear stress,sensory score,and total colony count.The results showed that the ratio of CO₂ to N₂ significantly affected the colony count,while the effects of moisture content,shear stress,and color was not observed.Furthermore,the microorganism growth rate of crispy duck legs with 60%CO₂+40%N₂ stored at5℃is the slowest among four kinds of the ratio of CO₂ to N₂,resulted in the lowest total amount of colonies.The shelf-life of crispy duck legs were obviously extended at the optimized ratio of CO₂ to N₂ and 5℃,and can be used as a better storage and transportation method of crispy duck products.