Fruit and vegetable fermentation refers to juice drinks obtained by fermenting for a certain period of time under the action of single or complex microorganisms by using one or more fresh fruits, vegetables, edible fungi, etc., also known as enzymes. The fruit and vegetable products prepared by the fermentation process not only increase the content of the original nutrients but also introduce new functional components and endow them with a new taste. In terms of efficacy, it can also enhance the body's immunity, regulate intestinal flora, enhance antioxidant capacity, lower blood sugar, and lower blood lipids. In terms of form, with the improvement and innovation of packaging technology and materials, many new end product styles have appeared on the packaging of fruit and vegetable products, which are more in line with the living habits and needs of modern people.
◢Common strains used in fruit and vegetable fermentation
Bacteria commonly used in fruit and vegetable fermentation include lactic acid bacteria, yeast and acetic acid bacteria. These strains can use fruit and vegetable juice raw materials in the fermentation process to generate a variety of intermediate organic substances and metabolites through autogenous metabolism.
Lactic acid bacteria (LAB) is a general term for a class of spore-free, Gram-positive bacteria that ferment sugars (such as glucose, lactose, etc.) into lactic acid. There are many types of lactic acid bacteria, including Lactobacillus, Leuconostoc, Streptococcus, Bifidobacterium, Pediococcus, and Lactobacillus sporogenes.
Yeast is a facultative anaerobic single-celled fungus, which can reproduce in large numbers under aerobic conditions, and can metabolize sugars into alcohol and carbon dioxide under anaerobic conditions. Yeast can use the sugar in the fruit juice for its own growth and reproduction, and at the same time produce alcohol through anaerobic fermentation to obtain fruit wine. Fruit wine can also be further aerobically fermented to obtain fruit vinegar. Polysaccharides, organic acids, phenolic substances and other components in fruit wine are functional compounds, which have antioxidant activity, lower cholesterol, and prevent cardiovascular diseases.
◢Nutritional and flavor changes in fruit and vegetable fermentation
Probiotic fermented fruit and vegetable juice can not only exert the effect of probiotics but also change the nutritional components in the fruit and vegetable juice. After the fermentation of probiotics, some nutrients are changed and transformed into nutrients that are more easily absorbed by the body. For example, probiotics can convert proteins into small molecular polypeptides and macromolecular polysaccharides into oligosaccharides. Probiotic fermented fruit and vegetable products can biotransform or biodegrade the nutrients of fruits and vegetables, produce nutrients that are easier for the body to absorb, increase the function of probiotics to improve the intestinal tract, and make fermented fruit and vegetable products more nutritious.
Some functional components in probiotic fermented fruit and vegetable products will be transformed, especially the transformation between some phenolic substances. Fruits and vegetables contain a large number of phenolic substances, which have antioxidant activity. Most of the phenolic compounds can reach the colon. Under the action of intestinal microorganisms, the phenolic compounds will be transformed to produce metabolites, which will be absorbed by the body to produce physiological effects.
Fruits and vegetables contain a lot of flavor substances, which will also produce new fermented flavors after being fermented by probiotics. The type of strain, fermentation time, temperature, substrate type and concentration will all affect the production of flavor substances in probiotic fermented fruits and vegetables. These flavor substances mainly include acids, alcohols, aldehydes, ketones, and terpenes. Due to the different types of probiotics and fruits and vegetables, aroma substances with different characteristics will be produced after fermentation.
◢Problems in fruit and vegetable fermented products
1. Product stability
Probiotic fermented fruit and vegetable products contain not only the nutrients of the fruit and vegetable itself but also the metabolites produced during the fermentation process and the somatic cells of probiotic bacteria. Improper processing of probiotic fermented fruit and vegetable products will cause precipitation, browning, and acidification of the product during the shelf life, resulting in a decrease in product quality. Various components in fruit and vegetable juices also interact during fermentation, reducing their stability. Most fruit and vegetable juices will change color during their shelf life. The main reason is that pigments degrade during the storage of fruit and vegetable juices. The substances that affect the degradation of pigments in fruit and vegetable products are Vc, reducing sugars, and total phenols.
In addition, for fermented fruit and vegetable products with live bacteria, since probiotic fermented fruit and vegetable products contain live bacteria, the quality of the product has been changing dynamically during the shelf life. Maintaining a high concentration of live bacteria in probiotic fermented fruit and vegetable products is a difficult problem in fruit and vegetable fermentation research. At present, the shelf life of probiotic fermented fruit and vegetable juice live bacteria products sold on the market is relatively short, and they are gradually inactivated during the shelf life due to the influence of storage temperature, time, pH and the nature of the strain itself. In addition, due to the wide variety of fruits and vegetables, some high-acid fruit and vegetable products cannot provide an optimal growth environment to ensure the activity of probiotics, resulting in the inactivation of probiotics during the fermentation process.
2. Flavor problem
During the process of fermenting fruit and vegetable juice with probiotics, some strains will produce a fruity aroma, but some will also produce unpleasant special flavors, and the formation of these flavors is relatively complicated, and many flavors have not been clearly identified so far. In probiotic fermented fruit and vegetable juices, the types of volatile flavor substances increased, among which alcohols and esters increased significantly, which was conducive to the prominent fruity and floral aromas of ester-flavored juices, such as apple juice and pear juice. Probiotics play an important role in the sensory, nutritional and hygienic qualities of food and beverages. During the fermentation process, they will produce aromatic compounds related to the bacteria to show the flavor of the food.
The characteristic flavor substances of probiotic fermented fruit and vegetable juices depend not only on the type and content of flavor substances but also on their flavor characteristics and the threshold value. The production of flavor has a certain relationship with the selection of fruit and vegetable varieties and strains. In addition, the generation of probiotic odor is also related to storage time, substrates and fermentation strains, and the mechanism is relatively complicated.