Food, more essential than shelter or clothing, provides nutrients for growth, maintenance, repair, and reproduction. Food security remains a major concern for developing countries like Nigeria, where diverse agro-climatic conditions allow varied cultivation and crop protection approaches.
With limited land expansion, increased agricultural productivity relies on fertilizers and pesticides. Crop losses from pests and diseases range from 10–30% in developed countries and 40–75% in developing ones (Roy 2002). Greater losses occur post-harvest due to pests attacking stored products, especially in the tropics.
Chemical pest control, aimed at minimizing these losses, is a primary tool worldwide. Approximately 70% of global pesticide use occurs in developed countries, with 30% in developing ones (Usha and Sandhu, 2014).
Pesticides begin dissipating after application, requiring a waiting period before harvest, which varies by pesticide and crop.
Harvesting before this period ends results in higher residue levels, posing health risks like blindness, cancer, liver and nervous system diseases, reduced sperm count, infertility, elevated cholesterol, high infant mortality, and metabolic or genetic disorders (Gupta 2006).
Understanding Pesticide Residues in Agriculture
Pesticide residues refer to detectable pesticides in or on non-target locations, such as freshwater reservoirs, streambed sediments, and harvested food. High residue levels in these areas are undesirable.
Residues are measured in parts per million (ppm) to parts per billion (ppb) on a weight basis, where one ppm is one milligram per kilogram, equivalent to one ounce of salt in 62,500 pounds of sugar or one pound of pesticide in one million pounds of raw agricultural commodity.
Modern analytical techniques can detect residues below one ppb. Federal and state regulatory agencies set legal tolerances, the maximum pesticide amounts permitted in raw agricultural commodities, ensuring consumer safety. Tolerances vary by pesticide and crop.
Read Also: Pests of Stored Products and Damages Caused
Factors Influencing Pesticide Residue Levels in Crops
Organochlorine pesticides (e.g., DDT, chlordane) are highly persistent, while organophosphates (e.g., parathion, malathion), pyrethroids, pyrethrins, and carbamates are less or non-persistent.
Factors affecting chemical persistence and residue levels include:
- Amount applied
- Formulation
- pH of water diluent, target tissue, soil, or water
- Surface nature
- Weathering from wind, rain
- Chemical breakdown from high temperatures and humidity
- Photochemical reactions from sunlight
- Biological reactions
Proper application of public health pesticides per label restrictions ensures residues on crops remain safe.
Mechanisms of Pesticide Residue Decline in Food Commodities
Food commodity composition and properties vary by type, affecting pesticide absorbance, penetration, and degradation. The rate of pesticide movement and dissipation depends on the pesticide’s physico-chemical properties and environmental conditions.
Persistence is measured by half-life, the time for half the pesticide to degrade or disappear, ranging from hours to years. Half-life varies with environmental conditions and initial application amount.
Pesticides degrade via photolysis, hydrolysis, oxidation, reduction, metabolism (by plants, animals, or microbes), temperature, and pH. Half-life values differ by pesticide, crop, dose, and conditions.
In fruits and vegetables, residues are mostly on the peel, removable by washing, peeling, or chemical treatments (e.g., vinegar, turmeric, sodium bicarbonate, salt, alcohol).
Pesticides are applied at various production stages, with systemic pesticides minimally absorbed into flesh. Water solubility aids residue leaching from surfaces.
Insecticide effectiveness, particularly for synthetic pyrethroids, may decrease due to strong bonding with fruit skin’s waxy layer and non-systemic properties.
In cereal grains, residues concentrate in the bran, reduced by milling. Grains are sprayed post-harvest for storage, with lipophilic pesticide residues persisting on seed coats or moving to bran and germ.
In cowpea grains, cypermethrin and deltamethrin residues penetrate interiors, resisting removal by washing or cooking. In milk, insecticides from contaminated feed concentrate in fat, with higher levels in butter and cheese.
In meat, lipid-soluble pesticides accumulate in fat, and in eggs, they concentrate in yolk. Organochlorines in water adhere to suspended organics, consumed by invertebrates and accumulated by fish, especially in fat tissues.
Read Also: Collection, Handling, Storage and Pre-Treatment of Seeds
Impact of Handling and Processing on Pesticide Residues
Post-harvest or post-slaughter, foods undergo handling and processing (washing, peeling, blanching, juicing, cooking, milling, baking, pasteurization, canning) to extend shelf-life, enhance palatability, and increase nutrient availability.
These processes reduce residue levels, with effectiveness depending on pesticide molecule, location, and processing steps.
Loosely held residues are efficiently removed by washing, as most pesticides remain on outer surfaces with limited cuticle penetration.
Microorganisms or fermentation further degrade residues. Processes like drying, dehydration, milling, baking, malting, brewing, and canning reduce residues.
Washing Techniques for Pesticide Residue Reduction
1. Water Washing: Washing fruits and vegetables removes residues significantly. For example, one minute of washing okra with 15.20 ppm malathion reduced residues to traces, and 11.83 cm rainfall eliminated carbaryl residues completely.
Washing treated okra for 30 seconds with tap water significantly reduces malathion. Washing cabbages with cold water removes 96% of malathion residues.
2. Salt Solution Washing: Dilute sodium chloride solution effectively lowers contaminants from food surfaces, particularly fruits and vegetables, and is practical for household use.
3. Kitchen Processing (Trimming, Washing, Peeling, Cooking): These techniques remove 40–77% of diazinon and 37–82% of dimethoate in green beans and cauliflower. Washing alone reduces dimethoate by 25–80%, while washing and cooking cauliflower curds reduces residues by 52–91%.
Peeling is most effective, followed by frying, with boiling reducing water-soluble pesticides.
Thermal Treatment for Pesticide Residue Reduction
Heat treatments (pasteurization, boiling, cooking) cause residue loss through evaporation, co-distillation, or thermal degradation, varying by pesticide. Pasteurization reduces HCH in milk by 65–73%, making heat-treated dairy safer.
Pesticide residues persist in most food commodities due to pre- or post-harvest applications, varying by molecule, commodity, and environmental conditions.
Degradation occurs via photolysis, hydrolysis, oxidation, reduction, metabolism, temperature, and pH. Residue levels are reduced by washing, preparatory steps, heating, cooking, processing, and post-harvest handling.
Washing raw materials is the simplest reduction method, with chlorine water or dilute chemical solutions as effective alternatives, depending on the commodity. Special precautions are needed for concentrated or dehydrated products.
A systematic approach to pre- and post-harvest practices is essential to minimize residues, ensuring safe food, economical production, and reduced health risks.
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