Micronutrient deficiencies affect a significant portion of the global population, particularly women and children in low- to middle-income countries. Diets in these regions often rely on staple crops that are energy-dense but micronutrient-poor. Biofortification, through conventional breeding, aims to increase micronutrient concentrations in staple crops like maize, sweet potato, cassava, pearl millet, rice, beans, and wheat, improving micronutrient intake. While biofortified crops have higher baseline nutrient levels, post-harvest handling (PHH), including storage, processing, and cooking methods, significantly impacts micronutrient retention. This systematic review addresses the lack of a comprehensive, systematic analysis of micronutrient retention in conventionally bred biofortified crops, examining the effects of PHH on provitamin A, iron, and zinc retention across various processing and storage methods. The study aims to provide evidence-based recommendations for households, regulatory bodies, and program implementers to optimize micronutrient retention from biofortified crops and identify key knowledge gaps.

Previous research has individually examined micronutrient retention in biofortified crops, but a systematic review integrating this information across multiple crops and processing methods was lacking. Existing studies have highlighted the impact of processing and storage on the concentration of various vitamins and minerals. For instance, storage and cooking can affect vitamin concentrations due to oxidation and heat, while milling leads to mineral losses. The objective of this systematic review is to comprehensively evaluate this knowledge gap by systematically examining the impact of post-harvest handling on micronutrient retention in conventionally bred biofortified crops.

This systematic review followed a pre-registered protocol (PROSPERO CRD42021254461). A comprehensive literature search was conducted across multiple databases (MEDLINE, AGRICOLA, AgEcon, CABI Abstracts), organizational websites (HarvestPlus, CGIAR), and through citation searching and consultation with subject matter experts. The initial search yielded 5161 records. After removing duplicates and screening based on inclusion/exclusion criteria (studies focusing on conventionally bred biofortified crops, excluding agronomic or genetically engineered biofortification and animal products, and focusing on micronutrient retention after post-harvest handling), 67 studies were included in the final analysis. The primary outcome was micronutrient retention (apparent and true retention), considering factors such as storage, processing, and post-processing storage. Data extraction was performed using Microsoft Excel, FileMaker Pro, and PlotDigitizer software. Micronutrient retention was categorized as high (>70%), moderate (50–70%), or suboptimal (<50%). The data were compiled in an online, interactive Micronutrient Retention Dashboard.

The review found that provitamin A biofortified crops generally maintained high levels of provitamin A compared to non-biofortified counterparts even after processing. However, iron and zinc retention showed more variability across different processing methods. Specific findings for each crop include: **Maize:** Storage of unprocessed maize resulted in significant beta-carotene degradation, particularly within the first 15 days. However, pre-cooling at 4°C before freezing improved retention. Cooking or grinding did not negatively impact provitamin A retention; boiling or drying increased zinc retention. Aluminum packaging may be beneficial for long-term storage. **Orange Sweet Potato:** Drying methods retained at least 60% of beta-carotene, with solar drying showing the highest retention. Packaging and storage conditions are critical to maximize beta-carotene retention. Deep freezing was favorable for cooked sweet potato. **Cassava:** Boiling whole cassava and making porridge-like foods resulted in variable micronutrient retention. The dashboard provides more detailed information on intermediate steps. **Pearl Millet:** Parboiling and oven drying maximized iron and zinc retention. Soaking in a 1:5 grain:water ratio for 12 hours also improved retention. Malting and germination reduced zinc retention. **Beans:** Iron and zinc were well retained across various processing methods (boiling, processing into flour, extrusion). Malting/roasting may be slightly preferable to extrusion for zinc retention. **Rice:** Rice variety and processing method impacted iron and zinc retention. Polishing reduced iron and zinc content; brown rice retains maximal micronutrients. **Wheat:** Milling at 95% extraction yielded better zinc retention than 80% extraction. The study also compared micronutrient retention in biofortified crops to non-biofortified and fortified crops. Biofortified crops generally retained higher micronutrient levels even after processing, although processing methods significantly impacted retention levels for iron and zinc. Fortification methods, while capable of increasing micronutrient content, presented challenges regarding long-term vitamin stability, especially in stored flour.

This systematic review provides valuable insights into micronutrient retention in conventionally bred biofortified crops, highlighting the importance of considering post-harvest handling practices. The findings demonstrate that while biofortification significantly increases micronutrient content compared to non-biofortified crops, optimal processing and storage methods are crucial to maximize nutritional value in the final product. The variability in iron and zinc retention across processing methods emphasizes the need for tailored recommendations for each crop and the importance of educating consumers and stakeholders about best practices. The results inform household-level best practices, national-level regulations, and considerations for processors and retailers to maximize micronutrient retention across the biofortified crops value chain. This study also underscores the need for future research to address identified gaps, particularly in understanding the impact of storage and shelf life, considering regional variations and genotype-environment interactions, and integrating bioaccessibility and bioavailability data.

Provitamin A biofortified crops consistently maintained high provitamin A levels compared to their non-biofortified counterparts. Iron and zinc retention in other biofortified crops varied substantially based on processing, with whole grain products generally exhibiting higher retention. Further research is needed to address gaps in understanding storage and shelf life, regional variations, and to incorporate bioaccessibility and bioavailability data into future studies. Household-level best practices, national-level regulations, and education across the supply chain are crucial for maximizing micronutrient retention.

The review acknowledges limitations including the inconsistent reporting of contamination levels across studies. Iron and zinc retention values might be inflated due to contamination from cooking utensils or other food ingredients. Variability in laboratory methods and measurements of micronutrient concentration may also influence results. Furthermore, the lack of bioaccessibility and bioavailability data limits the complete understanding of the nutritional benefits of biofortified crops after processing.