The Effects Of Hot-air And Infrared Drying On The Physiochemical Properties And Quality Of Coffee Leaves

Coffee leaves are one of the by-products continuously produced during the cultivation of coffee trees,but their utilization rate is currently low.Coffee leaves are rich in a wide range of bioactive components,including alkaloids,phenols,flavonoids,plant sterols,tannins,flavonoids,amino acids,terpenes,and carotenoids,which offer numerous health benefits to humans.Therefore,in recent years,there has been an increasing amount of research on coffee leaves,especially in the field of coffee leaf tea beverages.During the processing of coffee leaf tea,drying is a critical step influencing the quality of coffee leaf tea;however,there is still a lack of research on the effect of drying,especially high-temperature drying,on the quality of coffee leaves.In the present study,the effects of hot-air and infrared drying on the drying characteristics,proximate composition,phytochemical components,antioxidant activity,color change kinetics,harmful Maillard reaction products(MPRs),and surface microstructure of coffee leaves were evaluated.Then,using freeze-dried leaves as a control,the volatile compounds in two selected samples of heat-dried coffee leaves were qualitatively and quantitatively analyzed,and the key aroma compounds and their contribution were compared.Thereafter,sensory analysis was used to evaluate the two heat-dried coffee leaves.Finally,the “biomimetic in leaf model” was used to further explore the mechanism of Maillard reaction products and flavor formation during hot-air drying.The main conclusions are as follows:(1)The effect of hot-air drying on coffee leaves was as follows.The drying kinetic fits the Page model,and the kinetics of color change follows the first-order kinetic model.Soluble protein(TSP)and total free amino acids(TAA)gradually decreased as the drying time extended,while the total soluble sugars(TSS),monosaccharides,and disaccharides showed a first increasing and then decreasing temporal pattern.In addition,the content of chlorogenic acid(5-CQA)continuously decreased during the drying process.The antioxidant activity of coffee leaves declined conconmitantly with reductions in total phenols(TPC)and 5-CQA.The browning intensity of coffee leaves was positively correlated with the drying temperature.The levelss of 5-hydroxymethylfurfural(5-HMF),glyoxal(GO),and methylglyoxal(MGO)exhibited an initial increase followed by a subsequent decrease during the drying process.Notably,the content of MGO was about 8 to 20 times higher than that of GO.The rate of 5-HMF accumulation was positively correlated with the drying temperature.Drying can gradually wrinkle the surface of the coffee leaves and close the stomata.Based on a comprehensive evaluation,the results showed that coffee leaves exhibited better nutritional composition and quality when subjected to hot-air drying at 160 ℃ for 24 min.(2)The effect of infrared drying on coffee leaves was as follows.The drying kinetics fits the Page model,and the trend of color index change was similar to that of hot-air drying,but the total color change(ΔE)was significantly lower than that of hot-air drying.TSP showed a gradual decrease,whereas TAA and TSS exhibited an initial increase followed by a subsequent decrease.A reverse trend was found for TPC and antioxidant activities,and there was a strong correlation between them.With the increase in drying power,the browning of coffee leaves intensifies.Compared with hot-air drying,infrared drying produced more intermediate MRPs and less browning late-stage MRPs such as melanoidins.Infrared drying significantly increased the contents of 5-HMF,GO,and MGO,and the accumulation rate of 5-HMF increased with the increased drying power.However,compared with hot-air drying,infrared drying produced a lower level of MRPs in coffee leaves.The surface structure of the infrared-dried leaves was similar to that of hot-air drying.In addition,the efficiency of infrared drying was significantly higher than that of hot-air drying,and increasing the drying power can significantly shorten the drying time.Therefore,the selection of an appropriate drying power can be used to retain nutrients in coffee leaves.Based on a comprehensive analysis,the results showed that coffee leaves exhibited higher quality when subjected to infraed drying at 1600 W for 10 min.(3)Electronic nose(E-nose)analysis showed that hot-air and infrared drying increased the production of short-chain alkanes,sulfur compounds,terpenes,alcohols,aldehydes,ketones,and aromatic compounds in coffee leaves,whereas infrared drying also caused the reduction of some aromatic compounds.Then,the volatile compounds in freeze-(a control),hot-air-,and infrared-dried coffee leaves were identified and compared using headspace solid-phase microextraction gas chromatography.A total of 296 volatile compounds were identified in these three coffee leaves,145 in freeze-dried leaves,169 in hot-air-dried leaves,and 157 in infrared-dried leaves.In freeze-dried coffee leaves control group,the main volatile compounds were alcohols,aldehydes,alkanes,olefins,and acids.However,in hot-air-dried and infrared-dried leaves,the alcohol content significantly decreased,while ketones and heterocyclics(pyrazine,pyridine,furan,and other compounds)significantly increased,becoming the predominant volatile compounds in heat-dried leaves.A total of 70 differential volatile compounds were screened from the three types of coffee leaves.The analysis of relative odor activity values(ROAV)of the differential volatiles in different coffee leaves demonstrated that D-limonene contributed the most to the aroma of freeze-dried leaves,which had grassy,floral,and citrus flavors.In hot-air drying coffee leaves,the ROAV of α-ionone,nonanaldehyde,2,4-heptadienal,phenylacetaldehyde,trans-2-hexen-1-ol,and linalool were large,which contribute to the fatty,nutty,floral,sweet,and fruity aroma of these leaves.The aroma characteristics of infrared-dried leaves were grassy,citrus,sweet,fruity,and floral,which were attributed to the high ROAV of nonanaldehyde,phenylacetaldehyde,n-octanaldehyde,D-limonene,trans-2-hexen-1-ol and linalool in infrared-dried leaves.Hot-air-dried leaves have a richer flavor than infrared-dried leaves.The sensory evaluation also concluded that hot-air-dried leaves have better aroma,taste,and liquor color than infrared-dried leaves.(4)Finally,the effects of Maillard reaction,caramelization,amino acid degradation,and polyphenol degradation on the chemical composition and flavor of coffee leaves were studied using the “biomimetic in leaf model”.It was found that ΔE,antioxidant activity,browning intensity,and harmful MRPs significantly increased in the coffee leaf Mallard reaction model containing both sugars and amino acids.The polyphenol degradation model system with only 5-CQA had high antioxidant activity.5-CQA significantly promoted the production of 5-HMF in the Maillard reaction system,while having little effect on the contents of GO and MGO.Compared to the Maillard reaction,caramelization had a lesser impact on the formation of 5-HMF,GO,and MGO,but it affected the flavor significantly.The Maillard reaction promoted the production of sulfur-containing compounds,terpenes,short-chain alkanes,alcohols,aldehydes,ketones,aromatic compounds,and other volatile substances,which plays a key role in the flavor of coffee leaves.