The customer expectations and information included in the EU Commission Regulation changed the criteria of the quality evaluation of products. The approval of products has influence on, among other things, the looks [1
], taste, aroma and texture [2
], safety and nutritional values of products [3
]. It is not allowed to accept an inadequate quality of fresh fruits, which can be, among other things, immature, overripe, moldy or too dirty. Consumers are becoming increasingly aware and are beginning to choose a balanced diet. They nourish themselves increasingly healthily, which leads to an increased demand for healthy snacks. An alternative to crisps, salty sticks or puff snacks can be dried fruits and vegetables. By selecting a proper method for the control of parameters during the drying process, products can keep their attractive sensory features and nutritional values.
The fruit presented in the work is popular in Poland and in the world with regard to its sensory [5
] and health [6
] values. Strawberries are characterized by a pleasant sweet taste and firm, easy to consume pulp. They are consumed in the form of raw fruits. Strawberry preserves such as jams, mousses and sorbets are getting increasingly popular. It was also confirmed that strawberries have a positive influence on health, because they contain a complex of vitamin B, vitamin C and trace amounts of vitamin E and A, and micro-elements such as calcium and phosphorus [8
]. They can be treated as a low-calorie snack, because 100 g of strawberries contain only 27 kcal. Strawberry contains anti-oxygen substances, which take part, among other things, in reducing the risk of cardiovascular diseases [9
Dried fruits, which play a substantial role in the food industry, are becoming increasingly popular. Heat treatment processes, which are one of the most important methods of preserving the aforementioned food, ensure microbiological durability and reduce water activity by its elimination [13
]. Water in food has a substantial influence on the safety, quality condition [15
] and physical properties of food products [16
], especially strawberry. Dried fruits must be free of contamination by, among other things, stones, insects and foreign bodies. Dried fruits can become only partially dried, burned or over-ripe as a result of chemical reactions [17
In the age of technological development, techniques that support decision-making processes during production processes are being searched for. Acoustic signals have aroused interest [19
], for example, in the analysis of acoustic wave shifts, which were used as a basic non-invasive method of evaluating the quality condition of watermelons. The effective analysis method and the low cost of the mentioned evaluation allowed the efficient determination of the quality class of watermelons with acoustic resonance. As a result of the conducted research, the effectiveness of identifying watermelon ripeness was determined at 95% [20
]. Scientists noticed a substantial correlation between acoustic features and watermelon properties; among other things, the speed of acoustic wave dispersion seems to be lower as those fruits ripen [21
]. Other experts proposed that the impulse of the acoustic spectrum reacted if there were any internal damage to the watermelons. It was determined that the adequate band of the amplitude spectrum frequency of the acoustic signal demonstrates the highest capability for detecting internal structure disorders [22
During the research, the use of an acoustic signal to evaluate crispy or crunchy processed food, which includes dried fruits and vegetables, was taken into consideration. It has been found that food sound subject to external force has an influence on the material, causing deformation, damage or tearing. The release of the stored energy occurs when the research material cracks. Experts have evaluated the degree of carrot hardness on the basis of its firmness, and the results allowed carrot to be grouped according to the degree of hardness on account of positive and negative related acoustic waves [23
The aim of this work was to draw up a classification method of dried strawberry fruits on the basis of parameters obtained from the acoustic spectrum, recognizing the degree of crispness and taking into consideration the degree of ripeness. In the research, two classes of dried strawberry fruits were used, derived from ripe and over-ripe fruits. The use of an acoustic signal was supported by machine learning in order to quickly evaluate the quality of dried strawberry juice in the production process, which is considered to be an innovation in the research.
Neural networks, which are part of artificial intelligence, are increasingly being used to support the optimization, control or implementation of different production processes. They allow the complete automatization and control of decision-making processes in food production. The research demonstrated that the MLP 2:14:1 for the Z2 type, which obtained a RMS error at the level of 0.09 and MAPE at the level of about 10%, is characterized by the highest classification ability. The printed neural model determined that the second method allows a better recognition of quality condition in dried strawberry fruit as opposed to the other methods.
The use of acoustic sound backed up by the method of machine learning allowed the fast and effective recognition of the class of research material, also with respect to its level of crispness with different quality conditions. The acoustic signals can support the evaluation of the quality condition of dried strawberry fruit, as conducted in the case of other products; among other things, watermelons or carrots. They can also serve as an objective evaluation of crispness. Using acoustic signals can also eliminate human error, which is usually due to individual preferences or tastes disturbing the objective values of the crispness of products.
In the described research on drying strawberry fruits, it was also noticed that, on account of the difference of the initial humidity value in the two trials of the material, an unfavorable change in color, texture and shape in the trials of the over-ripe class was observed. Changes observed in the ripe strawberries were much less distinct. Drying is one of the safest methods of preserving food. This method is very popular because of the simplicity of building dryers. Thanks to a wide variety of convection dryers, one can dry almost any product with this device, irrespective of the shape or state of matter (e.g., whole fruits, juice, granules, or paste).