# Powdered Foods: Structure, Processing, and Challenges: A Review
## 1 Introduction  
With exponential population growth, the consequent demand for food and, above all, a notable increase in diet-related diseases, there is a demand for healthier and more nutritious foods. Likewise, food waste is also increasing, making it essential to seek methods to extend shelf life. Examples of existing alternatives include the use of natural or synthetic additives, heat and cold treatments, coatings, and the production of powdered foods [1; 2].  
Powdered products are commonly found in markets, either by using the product itself for reconstitution with water, such as powdered milk, or as ingredients and dietary supplements. Powdered foods are practical, whether for easy handling, transport, or storage, and have an extended shelf life compared to fresh products. However, they have a disadvantage in terms of stability in relation to hygroscopicity, having a high degree of moisture absorption [3; 4].  
Although low-moisture foods are generally considered safer due to their low water activity (less than 0.85), and pathogens cannot grow under these conditions, this does not mean that they cannot survive and pose potential health risks, remaining viable during storage [5; 6]. Pathogens commonly associated with low-moisture foods include _Salmonella_ spp., _Clostriidium_ spp., _Listeria monocytogenes_, _Bacillus cereus_, _Staphylococcus aureus_, and _Cronobacter_, which can contaminate a range of products such as flour, powdered infant formula, grains, seeds, herbs, and spices [6].  
The safety of powdered foods can be compromised via cross-contamination, storage under inadequate temperature and humidity conditions [6], and contamination during processing (e.g., drying towers) [7]. Therefore, the production of powdered foods is not intended to replace processes that guarantee food safety, such as thermal processing and other emerging technologies [6].  
Changes in water activity can also impact enzymes; their reduction can reduce the enzymatic activity of lipoxygenase, for example, which is responsible for imparting undesirable characteristics to food, such as negative impact on color, flavor, and antioxidant activity [8; 9].  
Powdered products are defined as solid particulate substances with a size ranging between 50 and 1000 \(\upmu\)m, which can be classified based on their primary (individual or inherent properties) or secondary characteristics (bulk properties) [10]. Particle density, porosity, shape, diameter, hardness, and stickiness are considered primary properties, while secondary characteristics are related to bulk density and porosity, particle size distribution, and moisture content. Additionally, powdered products can also be classified regarding their physical (size, shape, density, and porosity) or chemical characteristics (composition, interactions with other substances, and instant properties) [10].  
This review paper aims to establish new topics related to powdered foods via a brief bibliometric analysis, in addition to presenting in a comprehensive way the process of production and rehydration of powdered foods, such as the influence of intrinsic parameters on fresh food when transformed to powder. Furthermore, recent case studies related to powdered foods are also discussed in detail within this review paper.