At the fruit-specific level, ABA maintained a higher number of functional xylem vessels at early stages of growth and development, reducing the resistance to Ca2+ movement in the fruit and thus allowing Ca2+ to be translocated towards the blossom-end tissue. Such Ca2+ accumulation in the blossom-end tissue was enhanced in response to whole-plant ABA treatment, compared with fruit-specific ABA treatment, possibly due to the additional effects on increasing xylem sap flow rate into the fruit, and Ca2+ concentration in the xylem sap taken up by the fruit. Blossom-end tissue has the lowest concentration of Ca2+ in the fruit, and for that reason is the most susceptible tissue in the fruit to Ca2+ deficiency disorders . Studies suggest that Ca2+ deficiency symptoms are triggered by a depletion of the apoplastic pool of Ca2+ required to bind to phospholipids and proteins on the plasma membrane . Under conditions of low apoplastic Ca2+, the plasma membrane can become leaky, leading to cell plasmolysis and eventually death . Accordingly, the results show that higher Ca2+ accumulation in the blossom-end tissue in response to ABA treatment resulted in higher water-soluble apoplastic Ca2+ concentration, lower membrane leakage, and reduced fruit susceptibility to BER development. In this context, grow bags for gardening new tomato cultivars with higher ABA biosynthesis could be selected not only for water saving purposes, but also for reduced fruit susceptibility to BER. However, it is possible that fruit water content will be higher in high ABA genotypes, which may reduced the post-harvest life of fresh fruit.Low-moisture foods and dried fruits.
Water activity is a ratio between the vapor pressure of the food itself, and the vapor pressure of distilled water under identical conditions. It is a representation of how much “free” water is available in a food. A low-moisture food is a food that has a water activity of 0.85 or lower . When there is limited free water, microbes are unable to access water for use in biological processes, such as cellular growth . While the food industry often uses the terms moisture content and aw interchangeably, moisture content alone does not dictate a microbe’s ability to grow. Moisture content is the total amount of water in a food regardless of whether the water is free or bound. Thus, two foods of the same moisture content could have dissimilar water activities, making one more susceptible to bacterial growth than the other. Fresh fruits are good sources of nutrients and their consumption can contribute to a decreased risk of obesity, diabetes, heart disease, and cancer . However, common fruits are mostly produced on a seasonal basis. Fresh fruits contain more than 70% of water and they are considered perishable food with a limited shelf life . The estimated total loss of fresh fruits at retail and consumer levels were 13.9 billion pounds in 2010, contributing to 37% of the total food supply of fresh fruits . Therefore, fruits are dried with various techniques to extend shelf-life, retrain nutritional values, decrease packaging and shipping cost, and ultimately reduce food waste. The global production of dried fruits increased from 2,246,739 metric tons in 2009/2010 to 3,22,767 metric tons in 2019/2020 . In particular, table dates, which accounted for 35% of world dried fruits production, showed the most significant increase over the last decade . In California, dried fruits are economically valuable specialty crops and 1,174,000 tons of raisins, 325,500 tons of dried prunes, 11,000 tons of dried apricots, and 6,900 tons of dried freestone peaches were produced in 2017 .Survey for dried fruit processors.
To gain insight into current practices for dried fruit production, a survey was developed for dried fruit processors . The survey questionnaire and protocol was submitted to the UC Davis Institutional Review Board for approval of human subject research. A contact list was developed from a public records request to the California Department of Public Health Food and Drug Branch. The list included all contacts holding a Processed Food Registration in California and listed the types of products made, including dried fruits and vegetables. Additionally, a list of registered Farmers’ Market managers was collated with the aim of these managers distributing the survey to vendors selling dried fruits. Upon approval the survey was built in Qualtrics and distributed via newsletters, social media channels, and email. The survey’s target demographic was dried fruit processors in California, however, the survey was publicly available for anyone to take. The following is a summary of insights gathered from the 47 responses to the survey: There are a wide variety of fruits being dried in California. While some of the major commodities are peaches and apricots, others include plums, pluots, various berries, tomatoes, various citrus fruits, apples, dates, nectarines, bananas, pineapples, and watermelons. In mostcases, these crops are grown and harvested by the same people who dry them, rather than sourcing from retailers or other distributors. However, some will purchase fruit to fill orders if they cannot meet the demand. The harvesting seasons, dependent on type of fruit, occur most often in early and late summer. After harvest, most processors wash their fruit in potable water and store them at refrigerated temperatures. Typically, pre-dried storage does not last longer than 1 to 3 days, but in some cases can last as long as 28 days. Over 50% of the processors that were surveyed add some sort of preservative to their fruits before drying. Preservatives include citric acid, lemon juice, Fruit Fresh®, and sulfur. While not everyone knew exactly what kind of dryer they were using, tunnel dryers, cabinet dryers, and ovens were most commonly reported.
A few processors also use sun drying, while one processor reported the use of freeze drying. About 60% of the processors measure temperature during drying either using the built-in sensor of the dryer or a standard thermometer. For sun drying, the ambient temperature is important, as that can change the drying time by a matter of days. Along with temperature, a small number of processors check for the aw of their drying fruits for a target value dependent on the type of fruit . Other processors mentioned validating the drying process by checking moisture content, brix, crispness, a pinch test, and wind feel. By far the most common way to check that fruits are done drying, was to check the visual appearance. Most processors did not apply any post-drying treatments to their fruit, but one did use ozone fumigation.Once the fruits are dried, the most common storage method is bulk refrigeration . Those not stored in bulk are either packaged by hand directly into their retail bags or stored in airtight containers or jars until selling. Many of these processors have use historical shelf lives for their products. Few mentioned using microbial tests to determine shelf life, though dried fruits should not spoil from microbial growth if properly dried. Andress and Harrison suggest that dried fruits can be introduced to moisture once its packaging is opened, which could lead to spoilage. None of the processors stated what kind of microbial tests they used, garden grow bags but an increase in moisture after opening packaging may be a reason why they performed those tests. Most processors relied on appearance/taste of the fruits, as well as what they called “experience with the product” to determine shelf life. The shelf life for many of these products in cold storage is years, but many claimed that their products are sold well before that. Using the survey as a guide, the following reviews the literature of common practices in dried fruit processing. Drying. Dryers, or dehydrators, are common devices used for fruit drying. A dehydrator can be as simple as using a home oven, or a more dedicated device can be used such as a cabinet or tunnel dryer. Both cabinet and tunnel dryers use convective air drying, where hot air is circulated to pull moisture out of the fruit and create water vapor . In cabinet dryers the fruits are placed on shelves and remain stationary through the drying process; in tunnel drying the fruit is continually moved through the tunnel on some sort of conveyor. A constant temperature can be selected on these devices . The heat allows for loss of water to occur from the fruits . Sun drying can be much more variable than using a convective dryer as it is dependent on ambient conditions . A change in temperature, UV radiation or humidity can change drying times by a magnitude of days . Other weather qualities, including precipitation and wind speed, can affect drying as well. However, one processor who participated in the survey suggested that sun drying has advantages compared to convective drying as no special equipment or monitoring is required. Different drying methods have different impacts on the microbial populations present on dried fruits. Compared to the use of convective drying instruments, sun drying does not show reduction in native microbial populations . During sun drying of apricots at 38 °C for 182 h, the total aerobic mesophilic bacteria counts increased from 2.75 log CFU/g to 5.16 log CFU/g.
When the apricots were dried using convective air drying at 70 °C for 20 h, the mesophilic bacteria counts decreased to less than 2 log CFU/g . Sun drying of unpeeled ginger at 30.8 °C for 11 d showed no reduction from the initial 3 log CFU/g in total aerobic counts. When using a convective dryer, the unpeeled ginger dried at 43.2°C for 11 d decreased the total aerobic count by 1 log CFU/g . Several studies evaluate the impact of drying on pathogenic bacteria. DiPersio et al. observed that a 1.6-1.7 log CFU/g reduction of Salmonella Typhimurium in carrots was achieved when the inoculated carrots were dried using convective air drying at 60 °C for 6 h. Phungamngoen et al. showed that Salmonella Anatum declined from approximately 6 log CFU/g to 3 log CFU/g during convective drying ofcabbage at 70 °C for 6 h. Yoon et al. observed that a 3.2-4.5 log CFU/g reduction of Salmonella Typhimurium in Roma tomatoes was achieved when the inoculated tomatoes were dried using convective air drying at 60 °C for 14 h. Wachuku et al. showed that Salmonella Typhimurium declined, from an approximate initial level of 4 log CFU/g, by 2.5 log CFU/g during sun drying of cowpeas at 33 °C for 3 d.Pre-drying and post-drying treatments. Dried fruits can be treated in different ways before or after drying. The purposes of applying such treatments include preventing discoloration, texture retention, and reduction of the microbial populations in the final product . Pre-Drying treatments. One common pre-drying treatment is to expose fresh fruits to sulfur dioxide gas. In dried fruit processing, sulfur dioxide is not usually used as an antimicrobial but rather is used as an antioxidant to prevent browning. However, the molecular form of SO2 can penetrate the cell membranes of microbes and disrupt enzymatic activities . Sulfur dioxide’s efficacy as an antimicrobial is dependent on pH, as there is more molecular SO2 present at lower pH . Before drying, SO2 can be added through a process called sulfuring, where sublime sulfur is burnt and the sulfur dioxide fumes are able to enter the fruit . During this process, fruits to be dried are placed on trays inside a large box or container. The trays are placed at least 5 cm away from each other and at least 15 cm away from the sublimed sulfur, which is placed underneath the trays. The sulfur can be wrapped in paper so that it is easily lit. Once all the sulfur is done burning, the container is closed, giving the sulfur gas a chance to penetrate the fruit. The time for sulfuring depends on the type of fruit. For example, 0.5 kg of apples should be sulfured for 30 min, while 0.5 kg of peaches should be sulfured for 1 h . On average, 5 mL of sublimed sulfur is used per 0.5 kg of fruit . While specifically used to preserve the appearance of dried fruits for commercial appeal, SO2 treatment also can act as an antimicrobial similar to its use in wine making . Sulfur dioxide treatments have been shown to inactivate pathogenic bacteria on table grapes . Table grapes inoculated with low or high of either Escherichia coli O157:H7, Salmonella Thompson, or Listeria monocytogenes were fumigated with different concentrations of sulfur dioxide .