| As the main by-product of coffee processing,fresh coffee peel accounts for about45-55% of the fresh weight of the whole coffee fruit.Often,large amounts of coffee peel are discarded without treatment,which not only causes serious environment al problems in coffee-growing countries,but also results in an enormous waste of resources.Coffee peel has been proven to be a safe food ingredient with many high-value ingredients.However,the moisture content of fresh coffee peel is very high(about 80%),which poses great challenges to the storage and transportation of articles,and significant restricts the deep processing of coffee peel.Therefore,removing moisture from coffee peel by drying is essential in producing and processing high-value-added products.At present,the literature on coffee peel drying is very limited.Choosing suitable drying technology according to the characteristics of coffee peel,with low cost,low energy consumption,and high efficiency drying coffee peel is an urgent problem to be solved.Five conventional drying techniques,namely,hot air drying(HAD),sun drying(SD),microwave drying(MD),vacuum drying(VD)and vacuum freeze-drying(VFD),were used in this paper to analyze the effects of different drying methods on drying characteristics,physicochemical properties,non-volatile flavor compounds and volatile compounds.In addition,the relationship between the economic benefits of coffee peel in industrial applications and drying methods was evaluated.The main research contents and results are as follows:(1)The drying characteristics of various drying methods during the drying process were compared,and the physical properties(water content,water activity,color,microstructure),bioactive components(total polyphenols,total flavonoids,total anthocyanins,single phenolic compounds),and antioxidant activities of the dried coffee peel were analyzed.The results showed that the coffee peel of the five drying treatments was typically dried at a reduced rate overall,i.e.,the drying rate gradually decreased with an increase in drying time.MD had the fastest drying rate;The SD rate was the slowest,and the drying rate was greatly affected by natural environmental factors.The VFD treatment could better maintain the original colour,internal structure,total polyphenols,total flavonoids,total anthocyanins,single phenolic compounds and antioxidant activity of coffee peel.In the hot drying treatment,the color,microstructure,bioactive components and antioxidant activities of the coffee peel were significantly damaged by HAD and SD treatments,and the coffee peel’s water activity and water content were significantly higher than those of other treatments(P < 0.05).In contrast,VD and MD treatments had better bioactive ingredients and antioxidant capacity,and relatively little color and internal structure,especially MD treatments.Correlation analysis showed that total polyphenols,total flavonoids,total anthocyanins,trigonelline,chlorogenic acid,protocatechins,neochlorogenic acid and cryptophyllic acid were highly correlated with the antioxidant activity of coffee peel.(2)The effect of different drying methods on the volatile substances in coffee peel was investigated by a headspace solid-phase microextraction gas chromatograph,and the volatile compounds were analyzed by orthogonal partial least squares discriminant analysis(OPLS-DA)for principal component analysis and screening of main flavor substances,considering the complexity of the data.In addition,nonvolatile flavor-presenting substances(free amino acids,soluble sugars,organic acids,and fatty acids)related to flavor were measured to analyze the effects of non-volatile components on volatile components.The results showed that the VFD sample showed similar types and contents of volatile compounds as the fresh samples.The HAD treatment had more volatile substances and contents that imparted different flavors to the coffee peels,especially the MD treatment.The SD sample had the most obvious soil and herbaceous odor.HAD,MD,and VD samples all exhibit ed distinct caramel,fat,honey,and sweet aromas.In contrast,the caramel and fat flavors of HAD samples were the most prominent,while MD samples have the most significant honey and swe et aromas.The results of the non-volatile presenting substances analysis showed that the drying treatment decreased the content of soluble sugars and fatty acids,but increased the content of free amino acids and organic acids.In contrast,HAD,SD and VD treatments could maximize the production of free amino acids and organic acids,which also caused significant degradation of soluble sugars and fatty acids during drying,and glucose,xylose,heptadecanoic acid,and 11-octadecenoic acid even disappeared in the samples.Combined with the results of volatile compounds,we conclud that prolong thermal drying can cause the excessive degradation of non-volatile compounds in coffee peel,which is detrimental to the quality of coffee peel.(3)The relationship between the economic benefits of coffee peel in industrial application and drying methods was analyzed.The results showed that MD was the most economically attractive of all drying methods,and VFD was the least economically viable.SD uses solar energy,and although there is no production cost,the drying cycle is too long.It is subject to natural conditions such as the weather,making it unsuitable for large-scale industrial production.VD was more expensive to operate than HAD,but VD significantly reduced drying time,brought in more bioactive ingredients,and produced a more volatile flavor.In actual production,VFD can be selected in actual production to obtain more bioactive ingredients to produce high-value-added products products.Considering equipment investment and production costs,MD is more suitable for a wide range of industrial production than other drying methods.The results of this study can provide options in drying technology for further processing of coffee peel and may also provide a scientific basis for future large-scale utilization of coffee peel. |
