Hi. Not all foods should cause cough n congestion. If its regular and as u mentioned theres white expectoration(in cough) and No fever headache n all, then there is possibility of Allergies. U need to Stay away from Dust, Chemicals, Pollutants n also avoid eating Cold Food Stuffs n Cold Beverages like Banana, Guava, Grapes, Cold drink, Icecream etc.
Theres not much treatment for Allergy in Modern medicine. U Should opt for Ayurvedic treatment.
Next Steps
For treatment n any other advice feel free to consult me online.
Health Tips
As any Allergy is due to weak immunity, U need to boost ur immunity. Exercise helps in building up Stamina, thereby reducing many symptoms, even Allergies . Do Pranayam regularly and as said avoid food Stuffs that causes throat irritation n cough . Take care.