Abstract:

In recent years, the refrigerator industry has developed an increasing number of independently temperature-controlled products to achieve specific preservation effects for various food items. Different types of fruits and vegetables have distinct optimal storage temperatures, yet there are currently no products specifically designed for optimal temperature storage of fruits and vegetables. This study investigates the preservation effects and taste of non-cold-sensitive and cold-sensitive fruits at various temperatures, concluding that non-cold-sensitive fruits stored near 0°C in a refrigerator and cold-sensitive fruits stored slightly above the critical cold damage temperature can significantly enhance quality. Furthermore, the study compares immature fruits under temperature oscillation conditions with those stored at refrigeration and room temperature, demonstrating that temperature oscillation accelerates ripening and improves taste.

Keywords: Fruits; Temperature; Preservation; Taste; Ripening

Introduction

Household refrigerators typically consist of a refrigeration compartment and a freezing compartment, each maintaining specific temperatures for food preservation. With advancements in the refrigerator industry, many brands have developed products with independently controlled drawers to preserve specific food items, such as maternal and infant foods and meats. These drawers maintain specific temperature environments to meet functional needs, such as nutrient retention for maternal and infant foods and ease of slicing for meats. While technologies for zoned storage and independent temperature control have matured, there are currently no dedicated products for storing fruits and vegetables at optimal temperatures. This is partly due to the low value of many fruits and vegetables, leading to insufficient consumer demand, and partly because optimal temperature storage requires fruit and vegetable variety identification technologies, which are still not fully developed. However, as consumer demand for quality of life increases, interest in healthy and nutritious diets is growing, leading to a rising demand for higher-quality fruits and vegetables. Additionally, advancements in IoT technology have improved the maturity of fruit and vegetable variety identification technologies. This study aims to analyze the differences in preservation effects of fruits stored at various temperatures in household refrigerators, providing a reference for the development of optimal temperature storage technologies for fruits and vegetables.

It is well-known that many tropical fruits are not suitable for refrigerator storage. This is primarily because conventional refrigerator temperatures typically range from 2 to 8°C, while the critical cold damage temperature for many tropical fruits is higher than this range. Storing these fruits in a refrigerator can lead to cold damage, resulting in symptoms such as discoloration, indentations, and water-soaked spots. In fact, approximately one-third of the fruits and vegetables sold in China are cold-sensitive, with losses due to cold damage accounting for nearly 30% of total logistics annually.

In contrast to cold-sensitive fruits and vegetables, many non-cold-sensitive fruits and vegetables are theoretically less affected by cold damage and are better suited for storage near 0°C. Both domestic and international food industries have researched and developed preservation technologies for fruits and vegetables using critical point low-temperature high-humidity storage (CTHH), which maintains storage conditions above the cold damage threshold (approximately 0.5 to 1°C) and relative humidity of around 90-98%.

This study examines the preservation effects of various cold-sensitive and non-cold-sensitive fruits in different temperature environments within refrigerators, analyzing sensory characteristics, soluble solids content, and taste differences. The results indicate that different types of fruits exhibit significant differences in preservation status at varying temperatures, with optimal temperatures yielding better quality.

Materials and Methods

1.Materials: Fruits were purchased from a RT-Mart supermarket in Chuzhou City. The fruits were stored in drawers set to different temperatures, with the experimental period based on the observed quality changes.

2.Instruments and Equipment:

BCD-558 Refrigerator (Anhui Konka Tongchuang Electric Co., Ltd.)

Refractometer PAL-1 (ATAGO, Japan)

MS104TS Analytical Balance (Mettler Toledo)

3.Methods:

Sensory Evaluation Method: Five or more individuals, unrelated to the technical aspects, conducted sensory evaluations according to Table 1. Fruits with scores below three were considered spoiled.

Soluble Solids Content Measurement: NY/T 2637-2014 method for measuring soluble solids content in fruits and vegetables using a refractometer.

Taste Evaluation: Samples were randomly numbered A, B, C, and five or more individuals unrelated to the project tasted the samples and ranked them according to subjective taste preferences.

Results and Analysis

1. Storage Experiments for Cherries and Lychees at 0°C, 3°C, 5°C, and 7°C:

Testing Environment Setup: The temperature control program of the refrigerator’s variable temperature drawer was adjusted to maintain an average temperature within ±0.25°C of the preset temperature, as shown in Table 2. The drawer was a sealed environment with relative humidity exceeding 90% RH.

Spoilage Rate Testing: Approximately 40 cherries and lychees in good initial condition were stored at different temperatures, and spoilage rates were measured after 3, 5, 7, and 10 days, as shown in Tables 3 and 4. Analysis revealed that lower temperatures resulted in lower spoilage rates and longer preservation periods.

Soluble Solids Content Testing: Three samples of cherries and lychees were randomly selected from each group, homogenized, and soluble solids content was measured after 0, 5, 7, 10, and 15 days. Results indicated that higher temperatures correlated with higher soluble solids content and greater polysaccharide hydrolysis. The best preservation effect was observed at 0°C, extending storage time by approximately double compared to 7°C with the same spoilage rate.

2. Storage Experiments for Bananas and Mangoes at 5°C, 12°C, and 28°C:

Testing Environment Setup: The heating wires and corresponding temperature control programs were adjusted in the refrigerator’s humidity-controlled drawer to maintain an average temperature within ±0.5°C of the preset temperature.

Banana Storage Status Tracking: High-quality bananas (one bunch, about 7 pieces) were stored at different temperatures and monitored over the first 5 days.

1.Bananas stored at 5°C easily developed cold damage, with mature bananas lasting only about 3 days.

2.Bananas at 28°C quickly became overripe and spoiled within about 2 days.

3.Bananas stored at approximately 12°C maintained better appearance and quality, lasting over 5 days, nearly doubling the storage period.

Mango Storage Status Tracking: Five high-quality mangoes were stored at different temperatures and monitored over the first 7 days. Results showed that mangoes at 5°C experienced cold damage, while those at 28°C spoiled easily. Mangoes stored at 12°C had a spoilage rate of 20% after 5 days, improving fourfold compared to room temperature.

Soluble Solids Content: The initial and final soluble solids content of bananas and mangoes was measured. Results indicated a general decline in soluble solids content, with higher temperatures leading to faster declines.

3. Storage Experiments for Unripe Kiwifruit and Green Bananas at 5°C, 28°C, and Temperature Fluctuations (10-15°C):

Testing Environment Setup: The heating wires and corresponding temperature control programs were adjusted in the refrigerator’s humidity-controlled drawer to create temperature fluctuations approximately every hour, maintaining relative humidity above 90% RH.

Kiwifruit Ripening Effect Tracking: Six uniformly hard unripe kiwifruits were stored at different temperatures and monitored over the first 7 days. Results indicated that kiwifruits stored at 5°C did not ripen after 7 days, while those at 28°C began to ripen after about 6 days. In contrast, kiwifruits at (10-15°C) started to ripen after just 3 days, significantly faster than at 28°C.

Green Banana Ripening Effect Tracking: One bunch of uniformly green bananas was stored at different temperatures and monitored over the first 4 days. Results showed that bananas at 5°C experienced cold damage and could not ripen, while those at 28°C ripened quickly but also spoiled rapidly. Bananas at (10-15°C) ripened at a similar rate to those at 28°C but had a longer preservation period.

Soluble Solids Content: The initial and final soluble solids content of kiwifruit and green bananas was measured. Results indicated that soluble solids content ranked from high to low as follows: (10-15°C) fluctuations were superior to 28°C, which were superior to 5°C.

Taste Evaluation: At the end of the experiment, kiwifruit and bananas from three storage environments were randomly numbered A, B, C, and five individuals unrelated to the project tasted the samples. Results showed that (10-15°C) significantly outperformed the other two groups in color, aroma, and flavor.

This study concludes that:

1.Comparing the results of cherries and lychees stored at 0°C, 3°C, 5°C, and 7°C reveals that lower temperatures correlate with lower spoilage rates and longer preservation periods, while higher temperatures lead to higher soluble solids content.

2.Comparing the results for bananas and mangoes at 5°C, 12°C, and 28°C shows that bananas and mangoes at 5°C are prone to cold damage, exhibiting symptoms such as blackening and dark spots. At 28°C, they spoil rapidly, while at 12°C, their appearance and quality are superior. However, the soluble solids content of ripe bananas and mangoes generally declines, and higher temperatures accelerate this decline.

3.Comparing the ripening effects of unripe kiwifruit and green bananas at 5°C, 28°C, and (10-15°C) fluctuations shows that neither ripens properly at 5°C, while those at 28°C spoil quickly. The (10-15°C) fluctuating environment balances ripening and preservation, with improved soluble solids content and taste confirmed through blind testing. It is important to note that this study only compared the effects of the (10-15°C) fluctuating environment; the ripening effects should not be limited to this temperature range.

Based on the analysis of the results from these three parts, it is concluded that non-cold-sensitive fruits such as cherries and lychees are best stored near 0°C, extending storage time by approximately double compared to higher temperatures with the same spoilage rate. Cold-sensitive fruits should be stored slightly above the critical cold damage temperature to avoid cold damage, extending storage time by 2 to 4 times compared to room temperature at 28°C with the same spoilage rate. Immature fruits benefit from temperature oscillation environments that accelerate ripening, reducing ripening time and significantly improving edibility. When designing storage areas for fruits and vegetables in refrigerators, appropriate storage temperatures should be selected based on the type of produce: non-cold-sensitive fruits should be stored near 0°C, cold-sensitive fruits should be stored at temperatures above conventional refrigeration levels but slightly above the cold damage threshold, and immature fruits such as unripe kiwifruit and green bananas can benefit from temperature oscillation methods. The conclusions drawn from this study aim to provide a reference for the application of optimal temperature storage technologies for fruits and vegetables in the refrigerator industry.