Fruit dehydration is the earliest form of food preservation, with the earliest written record of dried fruits dating back to ancient Mesopotamian tablets, which contain the world’s earliest known recipes. Today, some of the most popular dried fruit snacks are raisins, dates, mango, pineapple, prunes, figs and apricots. Dried fruit processing equipment helps preserve the nutrients and sweetness of the fruit and prolongs its shelf life.
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Dehydration methods and dried fruit processing equipment
Many techniques of dehydrating fruit/vegetables developed throughout centuries, including sun drying, vacuum drying, spray drying, fluid bed drying, microwave and freeze-drying.
Sun drying is undoubtedly one of the oldest techniques of fruit drying. It relies on the sun’s thermal power and dry air. This process of food drying is usually lengthy and exposes the fruit to various contaminants like insects and microorganisms. To speed the process up and maximize solar energy, manufacturers use black-painted trays, solar trays, mirrors and collectors. As well as other stainless steel food grade pieces of equipment.
Atmospheric dehydration is a more modern method and is widely used in dried fruit manufacturing. It incorporates the use of hot, dry air in a stationary or continuous process. For this method, you need dried fruit processing equipment like cabinet driers or tunnel driers. Freeze-drying, or lyophilization is based on the sublimation principle. It applies low temperature and pressure to vaporize the water/ice in the fruit. Freeze-dried fruit retains all its nutrients as well as its structure. This cooled dried freezer method is mostly used on sensitive fresh fruit with a weaker structure and high water content, like strawberries and raspberries.
Prepping and sulfuring the fruit
Fruit needs to be prepped before drying and the basic steps of the pre-drying process are selection, sorting, washing and, in some cases, peeling and cutting the fruits. Another important step in the pre-drying process is color preservation, commonly known as sulfuring. Most fruits are treated with sulfur dioxide (SO₂), which slows the deterioration of the fruit’s browning, which happens when enzymes are not inactivated by the heat used during drying. Moreover, it helps preserve the carotene and ascorbic acid found in the fruit.
Sulfur dioxide is used widely in dried fruit manufacture, but it is good to note that it can negatively affect people with asthma if they ingest it in large quantities. The FDA, for example, requires food and snack manufactures to disclose the presence of sulfiting agents on the label if it is in concentrations of at least 10 parts per million.
Freeze-dried vs dehydrated fruit
While dehydrated fruit still has about 30% of its initial moisture, freeze-dried fruit retains only 1 to 4% of its original moisture. The water content gives the dehydrated fruits a chewy and sweet texture. On the other hand, the freeze-drying method leaves tiny air pockets in the fruit creating a light, crispy texture in it. This happens because the water escapes from the fruit in the form of gas.
The difference in water content also impacts the shelf life of dehydrated fruit and manufacturers usually add preservatives to prolong it. Contrastingly, freeze-dried fruit does not need additives because of its low moisture content and can last longer. Due to the heating process, dehydration can remove the nutritional content from the fruit. However, although the freeze-dried fruit process preserves all of the fruit’s original, it is a more expensive process and requires special packaging to minimize oxidation
Regulations and international standards for dried fruit
There are important regulations and international standards all manufacturers of dried fruit must comply with in order to export and sell their products.
Strict controls are implemented on the amount of aflatoxins. The FDA requires that levels of aflatoxins for nuts and dried fruit be below 2 μg/kg for aflatoxin B1, and below 4 μg/kg for the total aflatoxins content.
In Europe, all additives must be approved and contaminants like mycotoxins and pesticide residues are banned. The European Commission also has regulations on the maximum residue levels for chlorate and perchlorate. The maximum allowed level of chlorate is 0.3 for dates and figs and 0.05 mg/kg for most other fruit and vegetables. For perchlorate the maximum is 0.05 mg/kg.
In both the US and the EU deem sulfur dioxide safe for human consumption, but it is considered an allergen in the EU. Sulfur dioxide and sulfites must be clearly labeled and highlighted.
Conductive multi-flash drying method
Food scientists Jade Varaschim Link, Giustino Tribuzi, João Borges Laurindo published in the Journal of Food Processing and Preservation a new study for fruit drying in 2017. The study stems from the fact the short shelf life of some fruits limits their commercialization as fresh products. Their study focused on the drying process as well as the physical and structural properties of their samples and how they reacted to different drying methods. They also compared nutritional and rehydration properties of dried fruit.
KMFD, or conductive multi-flash drying, uses multiple cycles of heating-vacuum pulse to dry fruit. This method heats the fruit at atmospheric pressure to 60°C before applying a vacuum pulse (sudden decompression). The scientists used a vacuum drying chamber and connected it to a vacuum pump, while samples were laid on plates that were heated with electrical resistances.
KMFD-treated fruit developed a crispy-and-dried texture and retained its color, shape and nutrients in shorter time compared to freeze-drying, air-drying and vacuum drying.